- Veterinary Helminthology 1st Edition
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Author: J BOOMKER
Nematodes are spindle-shaped worms, usually cylindrical and pointed at both ends, and without appendages. The worms are unsegmented and the cuticle or integument may be smooth or adorned with annulations, longitudinal striations or cuticular bosses.
The mouth is situated at the anterior end of the worm – terminally, subdorsally or subventrally. It is surrounded by lips; 3 in the more primitive forms, and 6 in the higher forms. In some cases, lips may be completely absent and then additional structures such as leaf crowns may be present (Strongylidae) or absent (Filariidae). The mouth leads to the buccal capsule, which may be well developed in some worms, or very small and weakly developed in others. Teeth may or may not be present in the buccal capsule. The oesophagus is a muscular tube with a triradiate lumen. The wall is thick and muscular and may contain glands that secrete digestive enzymes. There are marked variations in the shape of the oesophagus – the strongyliform and the rhabditiform types being the commonest seen in parasitic nematodes. The intestine is a simple, straight tube; in the female it ends in the anus and in the male opens into the cloaca (Figure 121).
Nematodes lack both circulatory and respiratory systems and oxygen is absorbed from the surroundings or produced by the worm’s own metabolic processes.
The ‘brain’ is the nerve ring around the posterior 1/3 of the oesophagus. A few ganglia are present and these may send ramifications to different parts of the body. The ‘nerves’ usually run along the dorsal and ventral lines of the worm.
Tactoreceptors occur as cervical papillae or pre-bursal papillae in males, and as pre and post-cloacal papillae in the females near or posterior to the vulva.
The male reproductive organs consist of a testis, a vas deferens that is dilated for part of its length, and a muscular ejaculatory duct. In most species there are two spicules, which are situated dorsally immediately in front of the cloaca. They serve as organs of attachment and to dilate the vulva of the female during copulation, directing the sperm into the vagina. Frequently there is a small sclerotised organ dorsal to the spicules. This is the gubernaculum, which guides the spicules. A thickening on the ventral wall is known as a telamon. In some species both are present, in other species only the gubernaculum, and in still other species both are absent. The males of many of the worms possess a copulatory bursa supported by rays (Figure 122). In other species there are cuticular thickenings known as caudal alae. These have the same function as the copulatory bursa. They are not supported by rays but may be supported by pedunculated genital papillae (Figure 123).
The female reproductive organs (Figure 126) consist of 2 ovaries leading to oviducts, dilated seminal receptacles, uteri, an ovejector, a vagina, and a vulva. The position of the vulva in relation to the body length is an important morphological character distinguishing families and genera and will be discussed under the different families. Uteri that run in opposite directions are known as amphidelph uteri, those that run parallel from the front to the rear as opisthodelph, and those that run from the rear to the front as prodelph. The morphology of the ovejector is also a diagnostic tool and it may be opposed, Y-shaped, or a single muscular organ. The different types will be discussed under the families. Females can be oviparous (egg-laying), ovoviviparous (eggs that contain a larva that will hatch immediately upon being laid), or viviparous (live-bearing).
Few or no male caudal papillae. Phasmids (a pair of specialised sensory organs on the tail) absent. Oesophagus cylindrical or with glands free in the pseudocoelome forming a stichosome or trophosome. Eggs with polar plugs. Eggs may hatch in utero; L1 often have a stylet and are infective to the host.
The common morphological feature of ‘Trichinelloids’ is the stichosome oesophagus, which comprises a capillary-like tube surrounded by a single column of cells. The male has one or no spicule. Female reproductive system monodelphic (= single reproductive system). L1 usually infective in egg or free. In small or large intestine of mammals and birds.
Male with single spicule enclosed in a spicular sheath sometimes referred to as a cirrus. Spicular sheath may be armed with spines. Vulva opens near posterior end of oesophagus. Eggs oval with polar plugs.
Body with slender anterior region and thicker, shorter posterior region. Called ‘whipworms’ (trichos = hair). Single bacillary band present, lateral to oesophagus, terminating near its posterior end. Stichosome with single row of 40-200 stichocytes. Male cloaca terminal or subterminal. Female vulva opens near junction of narrower and wider parts of body. Thick muscular vagina present – generally with spinose lining – may evert through vulvar opening. Eggs lemon-shaped with thick polar plugs. Anus subterminal. Female tail short. In caecum and large intestine of mammals.
T. ovis (Figure 127)
Male 48-80 mm long, spicule 4.8-6.0 mm long, sheath spinous with distal bulb; spines on bulb smaller than on rest of sheath; female 40-70 mm long. In sheep, goat, camel, ox and deer.
Male 23-55 mm long, spicule 1.7-2.6 mm long, sheath bell-shaped at tip when fully everted; female 39-53 mm long. In pigs and wild boar.
Male and female 45-75 mm long, male tail is coiled and one spicule in the male is 9- 11 mm long. In dogs in wet, warm areas of the country e.g. KwaZulu-Natal.
Body long and slender without marked difference between the anterior and posterior regions. Oesophagus with 20-60 stichocytes in 1-3 rows. Male with single spicule. Male tail may have membranous caudal alae. Pedunculate papillae may be present and joined by a membrane. Vulva opens near end of oesophagus. Barrel-shaped eggs with parallel walls and polar plugs. Anus subterminal. Tail short. In intestines of birds, fish, and in liver of carnivores and rodents. Direct and indirect life cycles.
Male 22 mm long, spicules difficult to see, female 52-104 mm, eggs with striated appearance.
Body uniform in width. Anterior oesophagus forms a simple tube; posterior oesophagus surrounded by single row of stichocytes. Male tail has a conical projection on either side of the anus – referred to as copulatory appendages. Pair of papillae present between appendages. Spicule and spicular sheath absent. Vulva opens near middle of oesophagus in anterior 1/5 of body. Viviparous. Anus terminal.
T. spiralis (Figure 128)
Male 1.0-1.6 mm long; female 2.4-4.0 mm long. In small intestine of humans, pigs, rats and other mammals.
Numerous male caudal papillae. Phasmids present on tail. Oesophagus without stichosomes. Eggs without polar plugs; rarely operculate.
Females with weakly developed ovejectors. Vulva a transverse slit without well-developed sphincters. Parasitic females may be parthenogenic. Parasitic generation may alternate with free-living generations. Generally free-living, but some parasitic in plants and animals.
Superfamily Rhabditoidea (Figure 129)
Rhabditoids: Primitive group of nematodes which are mostly free-living, or parasitic in lower vertebrates or invertebrates. A few normally free-living genera (i.e. Micronema) cause problems in animals. Strongyloides is the only important genus from the veterinary point of view.
Genus Strongyloides (Figure 129)
Parasitic form with long, slender body. Oesophagus long and slender. Excretory pore does not open in a notch or depression in the body wall. Only females found in parasitic state. Vulva opens in posterior part of the body. Tail short, bluntly rounded. Complex life cycle takes three forms: 1. homogonic (parasitic); 2. heterogonic (free-living); and 3. transmammary infection.
Parasitic female, 3.33-4.49 mm long. In domestic pigs.
Parasitic female, 9 mm long. In equids.
Parasitic female, 3.5-6.0 mm long. In sheep.
Male tail modified into a bursa copulatrix with two lateral lobes and a median dorsal lobe. Each lobe contains elongate sensory organs called bursal rays. Surrounding the cloaca of the male tail is a group of structures called the genital cone.
‘Hookworms’, small to medium-sized, thick-bodied, bursate nematodes. With large globular buccal capsule armed with teeth or cutting plates, dorsally bent, which causes the characteristic ‘hook’ posture. Life cycle direct. Blood-sucking parasites of small intestines of mammals.
Duct of dorsal oesophageal gland in dorsal gutter on inner surface of buccal capsule. Gubernaculum present. Dorsal ray only divided near its distal end. Vulva opens posterior to middle of body. Terminal spine present on distal tip of female tail.
Genus Ancylostoma (Figure 130)
Mouth opening dorsal, armed with three pairs of teeth. Duct of dorsal oesophageal gland opens in notch on margin of globular buccal capsule, at apex of dorsal gutter.
Triangular subventral lancets at base of buccal capsule. Bursa with two large lobes and a shorter dorsal lobe. Bursal rays: ventral rays with a common stem remain close together; lateral rays with common stem diverge distally; externo-dorsal arises from main stem of dorsal ray; dorsal ray divides distally, each branch splitting into three. Spicules equal and filiform. Life cycle direct.
One pair of unequal ventral teeth on anterior rim of buccal capsule, lateral pair larger than median pair. Male 5-8 mm long, spicules 0.7-1.0 mm long; female 6-9 mm long. In small intestine of dog, cat and various wild carnivores.
Three pairs of ventral teeth on anterior rim of buccal capsule. Male 11-13 mm long, spicules 0.73-0.96 mm long, female 14-21 mm long. In small intestine of dogs, foxes, coyotes, wolves and other wild carnivores.
Male 9.5-11.0 mm, female 12-15 mm, buccal capsule armed with three pairs of well-developed teeth ventrally, very similar to A. caninum, two spicules 1.23-1.40 mm long.
Large dorsal cone and two sub-ventral lancets present in buccal cavity. Two semilunar cutting plates present on anterior rim of buccal capsule. Bursal lobes of equal size. Bursal rays: dorsal ray asymmetrical, externo-dorsal rays arising at different levels from main stem; lateral rays with common stem, antero-lateral diverges from other two lateral rays; ventral rays with common stem, remain close together.
B. phlebotomum (Figure 131)
Male 10-12 mm long, filiform spicules 3.5- 4.0 mm long; female 16-19 mm long. In the small intestine of cattle.
Male 12-17 mm long, spicules stout, 0.6- 0.64 mm long; female 19-26 mm long. In the small intestine of sheep and goats.
Buccal capsule funnel-shaped. Two ventral cutting plates. Duct of the dorsal oesophageal gland in a short dorsal cone in buccal capsule. Sub-ventral lancets present in buccal capsule. Bursa with two short lateral lobes and a very large dorsal lobe. Bursal rays: ventral rays with a common stem, antero-lateral short and separate; dorsal ray large, externo-dorsal arises part way along stem of dorsal; dorsal bifurcates, each branch divides at its distal tip. Vulva opens in front of middle of body. Female tail bluntly rounded.
G. pachyscelis (Figure 132)
Male 20 mm long, spicules 1.25-1.33 mm long; female 30 mm long. In small intestine of sheep and goats in Africa.
Superfamily Strongyloidea (Figure 133)
‘Strongyloids’. Mainly in the caecum and large intestine, rarely in the respiratory and urinary systems of birds and mammals. Large buccal capsule, often surrounded by corona radiata. Bursate males.
Well-developed corona radiata or leaf crown present. Dorsal ray bifurcates – with three branches on each bifurcation. Female reproductive system Y-shaped or T-shaped. Sphincter of ovejectors elongate. In caecum and large intestine.
Large stout worms (1.5-5.0 cm), referred to as the large strongyles. Globular or subglobular buccal capsule.
Figure 131 Bunostomum phlebotomum. dorsal and right lateral views of the head (a, b). The head is bent dorsally (b). note the teeth in the buccal capsule. The asymmetrical bursal rays are illustrated in (c) (redrawn from reinecke (1983) and Gibbons, Jones & Khalil (1996)).
Genus Strongylus (Figure 133)
Mouth opening terminal. Buccal capsule sub-globular, deeper than wide. Teeth in buccal capsule present or absent. Internal and external leaf-crowns present with needle-like elements. Dorsal gutter present, containing duct of the dorsal oesophageal gland. Bursal rays: dorsal ray characteristic of the family; externodorsal ray arising from base of dorsal; ventral rays with common stem remain close together; lateral rays with common stem. Spicule tips straight. Ovejectors form a T-shape. Vulva opens in posterior third of body. Female tail cone-shaped. In equids (horse, donkey, mule and zebra) worldwide.
In caecum and right ventral colon. Two sub-ventral teeth and a dorsal tooth divided at distal tip in buccal capsule.
In caecum. Two rounded teeth in buccal capsule.
In right ventral colon. No teeth in buccal capsule.
Subfamily Cyathostominae (Figure 134)
Subfamily Cyathostominae (Figure 134) Referred to as the small strongyles (cyathostomes, cyathostomins) (5-12 mm). Cylindrical buccal capsule present. Spicules with pick-shaped tips. Gubernaculum with longitudinal groove and flanged transverse ventral notch. Primarily in the colon and caecum of equids.
Mouth collar high. Buccal capsule as wide as or wider than deep. External leaf-crown elements few, large and broad. Internal leaf crown elements inserted deep in buccal capsule. Additional sclerotised supports for inner leaf-crown present near anterior edge of buccal capsule. Dorsal gutter absent from buccal capsule. Dorsal ray divided to proximal branch or origin of externo-dorsal ray. Spicules with pick-shaped tips.
Male 7-8 mm long; female 10-12 mm long.
Mouth collar high. Buccal capsule thinwalled, tapering anteriorly, wider than deep. External leaf-crown broader, larger and fewer than that of internal leaf-crown. Internal leaf-crown elements short, at or near edge of buccal capsule. Dorsal ray divided to origin of externo-dorsal rays. Spicules with pick-shaped tips.
Male 13 mm long; spicules 3.24 mm long; female 15 mm long; vulva opens 0.49 mm from tail tip.
Dorsal ray bifurcates, two branches or rami. Female reproductive system with opposed ovejectors, but posterior muscular arm turns anteriorly.
Genus Chabertia (Figure 135)
Large, thick-walled, subglobular buccal capsule present. Mouth opening directed ventrally. Internal and external leaf-crowns very reduced. Bursal rays: ventral rays close together with common stem; lateral rays with common stem, antero-lateral diverges from other lateral rays; dorsal ray characteristic of the family, externo-dorsal arises from its base. Vulva opens near anus.
Figure 134 (a) Cyathostomum, (b) Cylicostephanus, (c) Cylicocyclus and (d) Cylicodontophorus (redrawn from Reinecke, 1983).
Males 14-18 mm long; spicules 1.3- 1.8 mm long, with bluntly rounded tips; gubernaculum 0.18-0.2 mm long; female 14-26 mm long. In the large intestine of sheep, goats and cattle worldwide.
‘Nodular worms’ of the large intestine. So named because of the lesions they produce.
Genus Oesophagostomum (Figure 136)
Distinct buccal cavity present. External and internal leaf-crowns well developed. External leaf-crown with more than eight elements. Cephalic vesicle. Excretory pore at level of ventral groove. Bursal lobes equal in size. Bursal rays: ventral rays with a common stem, remain close together; lateral rays with common stem, antero-lateral ray shorter and separate from the others; externo-dorsal ray arises part way along dorsal ray, which is characteristic of the family. Spicules alate. Gubernaculum present. Vulva opens near anus. Parasites of ruminants, pigs, primates and rodents.
38-40 elements in external leaf-crown, internal leaf-crown poorly developed.
Males 14-16 mm long, spicules 0.7-0.8 mm long; female 17-20 mm long, vulva opens 1.4 mm from tail tip. In large intestine of cattle and buffalo.
20-24 elements in external leaf-crown, 40-48 elements in internal leaf-crown. Male 12-14 mm long, spicules 0.78-0.85 mm long; female 15-18 mm long, vulva opens 1.25-1.4 mm from tail tip. Primarily in the large intestine of sheep and cattle.
Ten elements in external leaf-crown, internal leaf-crown poorly developed. Male 8.8 mm long, spicules 0.9 mm long; female 7.5-13.4 mm long, vulva opens 0.32-0.37 mm anterior to anus. In large intestine of pigs.
Morphology similar to O. dentatum. In large intestine of pigs.
Eighteen to twenty elements in external leaf-crown, 36-40 elements in internal leaf-crown. Male 12-14 mm long, spicules 1.1-1.2 mm long; female 16-20 mm long, vulva opens 0.47 mm from tail tip. Primarily in large intestine of sheep and goats.
‘Trichostrongyloids’. Male tail with well-developed bursa. Buccal capsule small, poorly developed or absent. Cephalic vesicle often present. Longitudinal cuticular ridges generally present on body surface. Female reproductive system single (monodelphic) or double (didelphic). Sphincters of ovejectors spherical. Parasitic in the gastrointestinal tract, mainly the abomasum and small intestine – except in Dictyocaulus which is in the lungs. Life cycle direct.
Genus Trichostrongylus (Figure 137)
Body small and filiform, cephalic vesicle absent. Buccal capsule absent. Cervical papillae very small and difficult to see. Excretory pore opens in a notch. Transverse striations present; longitudinal cuticular ridges absent in adults. Bursa with two large lateral lobes, with a symmetrical dorsal lobe generally shorter than lateral lobes. Bursal rays: ventral rays widely separated, antero-ventral smaller than postero-ventral; lateral rays separate; externo- dorsal ray short and curved; dorsal ray divides near its distal end. Genital cone simple with simple ventral raylet and a pair of short dorsal raylets. Spicules short and generally twisted, and with barbs. Gubernaculum boat-shaped. Females didelphic. Female tail short and pointed. Mainly in small intestine of artiodactylids, more rarely in birds and humans.
Spicules unequal. Male 5.5-7.0 mm long; female 6-8 mm long. In abomasum of artiodactylids, stomach of equids, and, more rarely, in pigs and humans.
Spicules equal, each with a single barb near the distal tip. In small intestine of artiodactylids, humans, and other animals.
Figure 137 Trichostrongylus head, lateral view, showing the ventral notch into which the excretory pore opens (left), ventral view of the opened male bursa (middle) and the spicules of T. axei, T. colubriformis and T. rugatus (right) (all figures del J. Boomker).
Cephalic vesicle absent. Buccal capsule small with a single lancet-shaped tooth attached to dorsal wall. Prominent transverse striations at anterior end of body, followed by numerous longitudinal cuticular ridges of more or less even height, restricted to anterior part of the body. Cervical papillae large, more or less equidistant from anterior end, project above body surface. Bursal rays: ventral rays with common stem, separated at distal tips; lateral rays with common stem, antero-lateral ray more or less straight, medio-lateral and postero-lateral curve dorsally; externodorsal ray thin; dorsal ray asymmetrical and Y-shaped. Spicules with barbs at distal end. Gubernaculum present. Vulva opens in posterior part of body. Vulvar flaps, knobs or linguiform process present or absent. Females didelphic, vagina very short. Female tail pointed. In abomasum of hosts.
H. contortus (Figure 138)
Male 10-22 mm long, female 18-30 mm long. Spicule barbs to spicule tips: left 0.02-0.04 mm, right 0.04-0.05 mm. Female tail 0.25-0.53 mm long, mean 0.40 mm. Primarily in sheep.
Spicule barbs to spicule tips: left 0.02-0.03 mm; right 0.05-0.06 mm. Female tail 0.38- 0.72 mm, mean 0.53 mm. Primarily in cattle.
Genus Cooperia (Figures 139 and 140)
Large cephalic vesicle present. Buccal capsule absent. Prominent transverse striations on cephalic region followed by a reduced number of longitudinal cuticular ridges. Cervical papillae tiny and difficult to see. Bursa with two lateral lobes and a symmetrical dorsal lobe either the same length, or reduced. Bursal rays: ventral rays widely separated; postero-lateral ray separated from other laterals; dorsal ray lyre-shaped. Vulva opens in posterior part of body. Female didelphic. Female tail pointed. In abomasum and small intestine of their hosts worldwide.
Spicule 0.24-0.32 mm, well striated with large median projection. Primarily in cattle.
Spicule 0.13-0.21 mm, with median circular projection. Primarily in cattle.
Figure 139 Cooperia pectinata. note the spicules (left) with the pectinate expansion and the arrangement of the branches of dorsal ray (right) (redrawn from Gibbons, 1981).
Spicule short, 0.140-0.171 mm, with median striated bulge. Primarily in sheep and goat.
Spicule 0.24-0.30 mm, broad in width. Primarily in cattle.
Genus Teladorsagia (FFigure 141)
Small cephalic vesicle present. Buccal capsule absent. Cervical papillae small, project above body surface. Prominent region of transverse striations followed by numerous longitudinal cuticular ridges. Prebursal papillae large. Bursa with two lateral lobes and a dorsal lobe symmetrical and not very reduced. Bursal rays: 2-2-1 type, ventral rays close together at tips; anterolateral and medio-lateral close together, postero-lateral separate; dorsal ray divides in its distal half. Spicules with a main stem and two sharply-pointed branches (i.e. like claws of a crab). Gubernaculum present. In abomasum of artiodactylids – particularly sheep and goats.
Slender spicules, genital cone with dorsal raylets and a simple accessory bursal membrane.
Thicker spicules, slender gubernaculum and distinct spines on the dorsal and lateral aspects of the spicules, genital cone with dorsal raylets incorporated in a Sjöberg organ. Not often encountered, and then only in conjunction with T. circumcincta.
Genus Ostertagia (Figure 142)
Small cephalic vesicle present. Buccal capsule absent. Cervical papillae small, project above body surface. Prominent region of transverse striations followed by numerous longitudinal cuticular ridges. Pre-bursal papillae large. Bursa with two lateral lobes and a dorsal lobe symmetrical and not very reduced. Bursal rays: 2-1-2 type, ventral rays together, antero-lateral ray separate, medio- and posterolateral rays together. Gubernaculum present. Parasites of mainly cattle – rarely in sheep and goats.
Figure 141 Teladorsagia circumcincta. note the claw-like tip of the spicule (left) and the arrangement of the bursal rays (2-2-1) (middle). The genital cone is illustrated on the right (redrawn from Gibbons, Jones & Khalil, 1996).
Males 7-9 mm long, spicules 0.22-0.23 mm long, ending in three branches. Females 8-12 mm; vulva may be covered by a small flap. Eggs 0.074-0.090 mm x 0.038-0.044 mm, oval and symmetrical.
Genus Libyostrongylus (FFigure 143)
Cephalic vesicle sometimes present. Bursa with two large lateral lobes and a large symmetrical dorsal lobe. Vulva opens in posterior part of the body. In stomach of birds (Struthioniformes, Rheiformes) and hyracoids.
In proventriculus of ostrich.
Genus Dictyocaulus (Figure 144)
Small buccal capsule with strongly chitinised walls. Single tooth at base of buccal capsule. Bursa bell-shaped, not divided into lobes. Bursal rays: ventral rays close together, antero-lateral separate, medio-lateral and posterolateral fused for part or all of their length. Spicules short with porous texture and large, lateral alae or 1-2 small processes or branches. Gubernaculum present. Vulva opens in middle of body and may have raised lips. Female tail pointed. Life cycle direct. In respiratory tract of hosts.
Male is 25-43 mm long; female 43-68 mm long. In equids, primarily donkeys.
Male is 30-80 mm long, spicules 0.4-0.6 mm long; female 50-100 mm long. In sheep and goats.
Male is 40-55 mm long, spicules 0.195- 0.215 mm long; female 60-80 mm long. In cattle and buffaloes.
Figure 143 Libyostrongylus douglassi. Bursa showing the complex dorsal ray (left) and the sharp-tipped spicules with associated spines (right) (del J. Boomker).
Genus Nematodirus (Figure 145)
Large cephalic vesicle present. Anterior region with prominent transverse striations followed by longitudinal cuticular ridges, restricted to anterior half in female. Bursa with two large lateral lobes and reduced symmetrical dorsal lobe. Spicules long and filiform, distal tips joined and ornamented. Gubernaculum absent. Vulva opens in posterior third of body. Female didelphic with two functional uteri. Female tail with spine on distal tip. Egg is large and distinct. All developing stages inside the egg (i.e. to L3). In small intestine of its hosts.
Spicules 0.90-1.25 mm, tip pointed. Primarily in cattle; less common in sheep, goats and camels.
Spicules 0.90-1.25 mm, tip rounded and spatulate. Primarily in sheep and goats, also in cattle and other artiodactylids.
‘Metastrongyloids’, parasites of respiratory system, life cycle usually indirect.
Gubernaculum present. Dorsal ray only divided near its distal end. Vulva opens posterior to middle of body.
Figure 145 Nematodirus sp. note the split dorsal ray (a), the spine on the female tail (b), the egg (c) is large and the shape of the tips of the spicules of N. spathiger (d) and N. helvetianus (e) are distinctive (redrawn from Gibbons, Jones & Khalil, 1996).
Genus Oslerus (Figure 146)
Male tail with ill-defined lateral and ventral rays, reduced to small single and sessile papillae. Spicules short and stout. Small gubernaculum present. Female tail bluntly rounded, with anus and vulva terminal or sub-terminal. Parasitic in canids and felids.
Male 4-7 mm long, spicules 0.099- 0.113 mm long. Eggs 0.08 x 0.05 mm, contain a first stage larva with a ‘S’- shaped tail when laid. Unlike the other metastrongyloids, the life cycle is direct.
Male bursa reduced or rudimentary and the ventral and later rays are atypical. Gubernaculum absent. Cephalic vesicle absent. Lips large, trilobed.
Metastrongylus (Figure 147)
Long filiform spicules. Dorsal ray reduced, other rays atypical, with some absent. Parasitic in the trachea, bronchi and bronchioli of wild and domestic pigs. Indirect life cycle.
Male 11-26 mm long, spicules 3.9-5.5 mm. Female 28-60 mm, vulva close to tail. Eggs 0.045-0.057 x 0.038-0.041 mm, with a corrugated surface. Intermediate hosts are various species of earthworm. Occurs worldwide.
Figure 147 Metastrongylus apri. Head (left) showing the large lips and male burs and the rather abnormal bursal rays (right) (redrawn from Gibbons, Jones & Khalil, 1996).
Superfamily Oxyuroidea (Figure 148)
‘Oxyuroids’. Parasites of mammals, rarely birds as well as cold-blooded vertebrates. Double bulb oesophagus and direct life cycle. Female with pointed tail. Tail of male irregular or regular in shape.
Mouth opening hexagonal. Oesophagus with corpus, isthmus and bulb. Buccal capsule with cuticular bristles, and, in the female, three teeth. Male posterior end truncate. Male caudal alae supported by pair of pedunculate papillae and two other large papillae. Single spicule present. Gubernaculum absent. Female body narrow and long behind anus. Vulva opens in anterior part of body. Eggs large and asymmetrical, embryonated when laid. Life cycle direct. In large intestine of equids and rhino.
Male 9-12 mm long, spicule 0.12-0.17 mm long; female 40-150 mm long. In horses.
Mouth cylindrical or short, oesophagus usually has bulb, female has two ovaries.
Minute worms, 2-3 mm long, only visible microscopically. Long, pointed tail. Male 2 mm long, female 3 mm long.
‘Heterakoids’, pre-cloacal sucker present, surrounded by a cuticularised ring in the male.
Genus Heterakis (Figure 149)
Small worms. Large lateral alae extend some distance down sides of body. Three well-defined lips with interlabia. Oesophagus with prominent posterior bulb with valve. Male caudal alae well-developed, pedunculated papillae present. Spicules similar, equal or unequal. Gubernaculum absent. Female vulva near middle body. Uteri are opposed and eggs have thick shells. Generally in caecum of birds and mammals.
Male 4-13 mm long, spicules unequal, 0.85-2.8 mm and 0.37-1.1 mm long; female 8-15 mm long. In chickens, turkeys, guinea fowl, ducks, geese and other birds. Vector of the protozoan Histomonas meleagridis, the causal agent of ‘blackhead’ in turkeys.
Genus Ascaridia (Figure 150)
Medium-sized worms. Small lateral alae present. Three large lips with no interlabia. Oesophagus club-shaped without posterior bulb. Caudal alae of male tail narrow, with relatively large papillae. Spicules equal or subequal. Gubernaculum absent. Female vulva near middle of body. Uteri opposed and eggs with thick shells. In small intestine of birds.
Male 30-80 mm long, spicules 2-4 mm long; female 60-120 mm long. In chickens, turkeys, guinea fowl, ducks, geese, pheasants and other birds.
Male 16-31 mm long, spicules equal 1.2- 1.9 mm long; female 20-37 mm long. In pigeons and doves.
These nematodes can have zoonotic implications. Humans can be a paratenic or a final host. The infective stage is an egg containing an L2. Life cycle direct. ‘Ascarids’, medium to large, nonbursate, no buccal capsule, and mouth surrounded by three large lips, tail of male coiled, tail of female blunt. Parasites of the small intestine of mammals and birds
Genus Toxocara (Figure 151)
Cervical alae present. Oesophagus with a small ventriculus or bulb. Male tail with a terminal appendage. Spicules subequal and alate. Gubernaculum absent. Female vulva in anterior fourth of body. Eggs globular or subglobular with pitted surface.
Male 40-100 mm long, spicules 0.75-1.3 mm long; female 50-180 mm long. Cervical alae narrow. In dog, fox and other carnivores.
Male 30-70 mm long, spicules 1.6-2.1 mm; female 40-120 mm long. Cervical alae very large, narrow anteriorly, very broad posteriorly. In cats, lions and leopards.
Male 15-26 cm long, female 22-30 cm. In cattle and water buffalo.
Interlabia absent. Cervical or caudal alae absent. Ventriculus absent. Male tail conical with numerous pre-cloacal papillae and few post-cloacal papillae. Spicules equal. Gubernaculum absent. Vulva anterior to middle of body, uterine branches parallel. Oviparous, eggs with thick shells surrounded by albuminous and mammilated coat.
Male 150-250 mm long, spicules 2-3 mm long; female 200-500 mm long. In pigs and humans.
Cervical alae present. Oesophagus without posterior ventriculus or bulb. Male tail conical, without terminal appendage. Spicules subequal. Gubernaculum absent. Female vulva near anterior third of body. Eggs with a smooth, thick shell.
Figure 151 Toxocara canis anterior end (left), male posterior end (middle), and egg with pitted shell (right) (redrawn from Yorke & Maplestone, 1926).
T. leonina (Figure 152)
Male 20-70 mm long, spicules 0.7-1.5 mm long; female 20-100 mm long. Cervical alae long and narrow. In cats, dogs, lions and other carnivores.
Cervical alae absent. Male tail blunt and conical with small caudal alae. Spicules equal; not alate. Gubernaculum absent. Female tail ends in a short conical process. Vulva posterior to middle of body. Eggs finely punctate.
Male 150-280 mm long, spicules 2.0-2.5 mm long; female 180-500 mm long. In horses, donkeys and mules.
Oesophagus divided into short anterior muscular part and longer, posterior glandular part. Pre-cloacal sucker absent. Indirect life cycle. In anterior part of gastro-intestinal tract, and tissues or tissue spaces.
‘Habronematoids’. Pseudolabia. Parasites of birds and mammals.
Genus Habronema (Figure 153)
Mouth with two lateral lips, usually trilobed. Lateral alae may be present, but on one side only. Buccal capsule strongly chitinised. Male caudal alae well developed.
Spicules unequal. Gubernaculum present. Female tail conical. Vulva opens near middle of body; oviparous. In mammals and birds. Muscid flies serve as intermediate hosts. In stomach as adults.
Male 9-22 mm long, spicules 0.76-0.8 mm and 0.35-0.38 mm long; female 15-25 mm long. In horses, donkeys and mules. Intermediate hosts are Musca spp. and Stomoxys calcitrans.
Male 8-14 mm long, spicules 2.5 mm and 0.5 mm long; female 13-22 mm long. In horses, donkeys, mules and zebras. Intermediate hosts are Musca domestica and Musca spp.
Genus Draschia (Figure 153)
Similar to Habronema, but differs in having the head separated from the body by a transverse groove. Buccal capsule funnel-shaped. In nodules in stomach wall of mammals.
Male 7-10 mm long, spicules 0.46 mm long; female 10-13 mm long. In horses, asses and mules. Intermediate hosts are Musca domestica and Musca spp.
Two subgenera, one in birds and the other in the conjunctival sacs of mammals.
‘Spiruroids’, posterior end of male usually spirally coiled and bears lateral alae, spicules unequal and dissimilar. Eggs are thick-shelled and contain larvae when laid. Adults are parasites of vertebrates and frequently live in the lumen or wall of stomach. Life cycle indirect – with arthropods acting as intermediate hosts.
Genus Spirocerca (Figure 154)
Mouth opening hexagonal, without lips. Buccal capsule with thick walls. Male tail spiral with caudal alae. Spicules very unequal and gubernaculum rudimentary. Vulva opens near posterior end of oesophagus. In nodules of oesophagus and stomach of carnivores and marsupials. Beetles serve as intermediate hosts.
Male 30-54 mm long, spicules 2.45 mm and 0.75 mm long; female 54-80 mm long. In dogs, foxes, wolves and jackals.
‘Filarids’. Long, relatively thin worms. Parasites of tissues and tissue spaces of all classes of vertebrates – except fish. Indirect life cycle. Transmitted by haematophagous insect and acarine arthropod vectors. Morphologically dissimilar spicules in males. Females oviparous or viviparous.
Viviparous; microfilariae (mff) are L1 that circulate in blood and lymph, mff sheathed or unsheathed.
Slender body with smooth cuticle. Caudal extremity of males and females with two well-developed petaloid appendages. Male posterior end spirally coiled; four pairs of precloacal papillae. Spicules very unequal and dissimilar. Female vulva opens in middle of glandular oesophagus. Microfilariae sheathed or unsheathed. In connective tissues of mammals.
Male 9-17 mm long, spicules 0.22- 0.30 mm and 0.092-0.104 mm long; female 20-33 mm long. In body cavity, connective tissues and kidneys of dogs, jackals and hyaenas in Europe, America and Africa.
Male 13-31 mm long, spicules 0.32-0.39 mm and 0.120-0.165 mm long; female 32- 60 mm long. In peritoneal cavity of dogs and hyaenas in Africa and India. Microfilariae develop to L3 in hippoboscid flies.
Cervical papillae small. Oesophagus short and divided. Male tail spirally coiled. Male caudal alae present, with large pedunculated papillae. Spicules unequal. Gubernaculum absent. Female posterior end rounded. Vulva opens behind oesophagus. Microfilariae unsheathed in blood. In heart and connective tissues of carnivores, primates, rodents and marsupials.
D. immitis (Figure 155)
Male 120-200 mm long, spicules 0.3- 0.375 mm and 0.175-0.229 mm long; female 250-310 mm long. In right ventricle and pulmonary artery of dogs, cats, foxes, tigers, and, occasionally, in lung tissue of humans. Worldwide (includes East Africa, Northern Africa and Mozambique). Microfilariae unsheathed in blood transmitted by mosquitoes.
Male 50-70 mm long, spicules 0.46-0.59 mm and 0.18-0.21 mm long; female 100- 170 mm long. In subcutaneous tissues of dogs, cats and foxes in Europe, Africa and Asia. Occasionally subcutaneous nodules in nose, arm and eye of humans.
Genus Parafilaria (Figure 156)
Cuticle transversally striated, except anterior end which is covered with numerous elliptical and circular papilliform thickenings. Male caudal alae present with large pedunculated pre- and post-cloacal papillae. Spicules unequal. Gubernaculum present. Female anus sometimes terminal. Vulva opens close to mouth. Females lay microfilariae. Adults are visible in haemorrhagic exudate of bleeding spots. In equids and ruminants.
Male 20-30 mm long, spicules 0.35 mm and 0.15 mm long; females 40-50 mm long. In subcutaneous tissues of cattle, African buffalo and water buffalo in parts of Africa, Asia and Europe.
Mouth surrounded by cuticular ring. Males have four pairs of post-cloacal papillae. Tail of female may or may not bear spines or a few large conical projections. These white worms are found in the peritoneal cavity of horses, cattle, antelope, wild pigs, and hyrax. Several centimetres long and body tapers to a tail that is usually coiled in the male.
Male 50-80 mm long; right spicule 140- 210 mm long, ending in a claw through which left spicule slides; left spicule 610- 660 mm long; female 70-130 mm long. Sheathed microfilariae present in host’s blood.
Male 40-51 mm long; right spicule 120- 160 mm long, left spicule 370-420 mm long; female 60-94 mm long, characteristic tail which terminates in a number of spikes. In abdominal cavity of cattle; microfilariae in blood of host.
Figure 155 Dirofilaria immitis, head (left), lateral view of female (middle) and male posterior end (redrawn from Yorke & Maplestone, 1926).
Morphology (Figures 157 - 159)
Cestodes are hermaphroditic helminths with an elongate, flat body without a body cavity or alimentary canal. Their size varies from a few millimetres to several metres in length. The body consists of a head or scolex followed by a short unsegmented portion called the neck and the remainder of the body or strobila, which consists of a number of segments or proglottids. The proglottids are separated by transverse constrictions, and each proglottid contains one or two sets of reproductive organs.
Usually globular; provided with suckers (acetabula, which may be armed with hooks) or grooves (bothria); a protrusible rostellum, often armed, may be present (Figure 157).
Proglottids are formed from the neck by asexual budding, and undergo maturation as they are pushed away from the scolex (Figure 158). Mature proglottids are those in which the reproductive organs are fully mature and functional. Following fertilisation of the eggs, the reproductive organs degenerate leaving a gravid proglottid filled with eggs. Eggs are embryonated (as is the case with most cestodes of veterinary importance) or un-embryonated when passed from the final host. Each fully embryonated egg contains an oncosphere.
The body is covered by a tegument that absorbs nutrients.
They are arranged as either a compact vitellarium or scattered as follicles, which contribute to eggshell formation and nutrition for the developing embryo, and are associated with the ovary.
The ovary discharges into the oviduct where fertilisation takes place. Cells from the vitelline glands pass through a common vitelline duct and join with the zygote. Together they pass into a dilated area of the oviduct (ootype) surrounded by Mehlis’ gland. The Mehlis’ gland secretes a membrane around the zygote and associated vitelline cells. Eggshell formation is then completed from within, by the vitelline cells. On leaving the ootype, the embryonating eggs pass into the uterus where embryonation is eventually completed. The vagina is a tube that leads from the genital pore to the ootype. Near the proximal end, there is usually a dilated part (receptaculum seminis) that stores sperm received during copulation. The shape of the uterus varies considerably between the different taxonomic groups. In some groups, the uterus disappears and the eggs are – either singly or in groups – enclosed within hyaline egg capsules imbedded within the parenchyma. In other groups one or more fibro-muscular structures, the par-uterine organs, form attached to the uterus. In this case, the eggs pass from the uterus into the par-uterine organs, which assume the function of a uterus. The uterus then usually degenerates. In many groups, eggs are released from the proglottid through a preformed uterine pore. In others, the proglottid splits or fragments, releasing the eggs. In many apolytic species, the gravid proglottids detach from the strobila and are passed from the host, where they crawl on faeces or the ground, scattering eggs as they move around. In most anapolytic species, the eggs are first discharged, and then the senile segments are released.
The male and female genital pores usually open into a common sunken chamber, the genital atrium. Self-fertilisation or crossfertilisation between proglottids may occur.
Figure 158 Schematic representation of a cestode, indicating the various body parts (redrawn from Schmidt, 1982).
This is the largest order of tapeworms, with more species than all other orders put together.
Scolex usually with four suckers, rostellum present or not, armed or not. Neck present or absent. Strobila variable, usually with distinct metamerism. Genital pores lateral (ventral in Mesocestoididae). Vitelline gland single, compact, usually posterior to ovary. Uterus variable. Parasites of amphibians, reptiles, birds, and mammals.
Mostly found in mammals. Scolex without rostellum, suckers unarmed. Proglottids are usually numerous and wider than long. Reproductive systems are single or double. Eggs commonly have an outer shell elongated and crossed at one pole (pyriform apparatus). Metacestode a cysticercoid, parasitic in arthropods. Final hosts: marsupials and placental mammals (predilection site: intestine or bile ducts) or birds (predilection site: intestine).
Scolex with or without a pair of lappets on either side. Strobila large; proglottids wider than long, craspedote. Genitalia single; genital pores unilateral. Genital ducts dorsal to osmoregulatory canals. Internal and external seminal vesicles present. Testes numerous throughout medulla. Ovary multilobate, filling most of medullary width. Vitellarium post-ovarian. Seminal receptacle present. Vagina posterior to cirrus sac. Uterus a transverse sac. Eggs with pyriform apparatus. Final hosts: perissodactyls, hyracoidea, gorilla.
Length up to 80 cm; width 2.5 cm. Scolex 4-6 mm in diameter, with four suckers lacking lappets. Eggs 70-80 μm in diameter with relatively small oncosphere (8 μm). Intermediate hosts: oribatid mites in which metacestode (cysticercoid) develops. Final hosts: equids; predilection site: small intestine, particularly jejunum.
A. perfoliata (Figure 160)
Length 2.5-8.0 cm; width 8-14 mm. Scolex 2-3 mm in diameter, with a lappet behind each sucker. Egg 60-85 μm in diameter with large oncosphere (16 μm). Intermediate hosts: oribatid mites in which metacestode (cysticercoid) develops. Final hosts: equids; predilection site: caudal ileum, ileo-caecal valve, caecum, proximal colon (rare). Cosmopolitan.
Genus Anoplocephaloides (Figure 161)
Scolex usually wider than neck. Strobila variable, from small to medium; proglottids wider than long, craspedote. Genitalia single; genital pores usually unilateral. Genital ducts dorsal to osmoregulatory canals. Internal and external seminal vesicles present. Testes numerous.
Vagina posterior to cirrus sac. Eggs with pyriform apparatus. Final hosts: rodents, lagomorphs, perissodactyls.
A. mamillana (syn. Paranoplocephala mamillana)
Length 1-4 cm; width 4-6 mm. Scolex 1 mm in diameter with slit-like suckers. Egg 50-60 μm in diameter. Intermediate hosts: oribatid mites in which metacestode (cysticercoid) develops. Final hosts: equids; predilection site: small intestine.
Scolex simple. Four unarmed suckers present. Strobila very long. Proglottids wider than long, craspedote. Interproglottidal glands present or absent. Reproductive glands double. Genital pores marginal, equatorial. Genital ducts cross osmoregulatory canals dorsally.
External seminal vesicles absent. Testes numerous, scattered throughout medulla. Ovaries poral. Vagina posterior to cirrus sac. Eggs with pyriform apparatus. Cysticercoids in oribatid mites (intermediate hosts). Final hosts: ruminants, suids, rodents, primates and ratite birds. Cosmopolitan.
M. benedeni (Figure 162)
Length 0.5-2.5 m, width 26 mm. Interproglottidal glands limited to the centre of each proglottid. Egg almost cuboid in shape, 80-90 mm in diameter. Final hosts: mainly cattle, other ruminants. Predilection site: small intestine. Cosmopolitan.
Figure 160 Anoplocephala perfoliata. note the lappets (proglottids redrawn from Khalil, Jones & Bray, 1994, scolex after Gibbons, Jones & Khalil, 1996).
Length 1-6 m, width 16 mm. Interproglottidal glands extend over the full width of each proglottid. Egg somewhat triangular, 56-67mm in diameter. Final hosts: sheep, goat, cattle and other ruminants. Predilection site: small intestine. Cosmopolitan.
Four unarmed suckers present. Strobila large. Proglottids wider than long, craspedote. Reproductive organs single. Genital pores alternate irregularly. Genital ducts pass between osmoregulatory canals. Internal seminal vesicles present. Testes numerous in two lateral groups external to the osmoregulatory canals. Vas deferens convoluted anterior to cirrus pouch. Ovary lobed, poral. Vitelline gland compact, post-ovarian. Seminal receptacle present. Vagina posterior to cirrus sac. Uterus first a transverse undulating tube, replaced by approximately 300 par-uterine organs each with several eggs in mature segments. Cysticercoids in oribatid mites and psocids (intermediate hosts). Final hosts: ruminants. Cosmopolitan.
T. ovilla (= T. giardi) (Figure 163)
Length 2 m, width 12 mm. Eggs are found in groups of 10-15 (never singly) in elongated par-uterine organs (about 100 μm long), with a thick grey shell and a protuberance at one end; individual eggs 20-25 μm in diameter, without a pyriform apparatus. Final hosts: cattle, sheep, buffalo. Predilection site: small intestine. Almost cosmopolitan.
Four unarmed suckers present. Strobila large and narrow. Segmentation indistinct, except in the gravid region.
Figure 162 Scolex and mature proglottid of Moniezia benedeni. note the interproglotiddal glands in the centre (redrawn from Gibbons, Jones & Khalil, 1996).
Proglottids wider than long. Reproductive organs single. Genital pores alternate irregularly. Genital ducts lie dorsal to the osmoregulatory canals. Cirrus pouch small. Testes in two lateral groups, each subdivided by osmoregulatory canals. Seminal vesicles absent. Germovitellarium (combined ovary and vitellarium) present. Par-uterine organ single, containing several fibrous capsules, each containing several eggs. Pyriform apparatus absent. Cysticercoids suspected in oribatid mites, psocids and Collembola. Final hosts: sheep, goats, cattle, buffalo, and camels. Europe, Asia, Africa, North America.
A. centripunctata (Figure 164)
Length 3 m, width 3 mm. Egg 35 μm in diameter, without a pyriform apparatus. Final hosts: sheep, goat, dromedary, and rarely cattle. Predilection site: small intestine. In Europe, Asia and Africa.
Genus Stilesia (Figure 165)
Strobila large. Proglottids craspedote, wider than long. Reproductive organs single. Genital pores alternate irregularly. Genital ducts pass between osmoregulatory canals. Seminal vesicles absent. Testes few, in two lateral fields.
Germovitellarium (combined ovary and vitellarium) present. Vagina posterior to cirrus sac. Uterus first a bi-lobed sac with slender isthmus, then replaced by two paruterine organs. Cysticercoids suspected in oribatid mites (intermediate hosts). Final hosts: ruminants. Asia, Africa, Europe.
Length 20-50 cm, width 2 mm. Egg elliptical and transparent ± 25 μm x 55 μm; may appear isolated in the faeces. Final hosts: sheep, goat and rarely cattle. Predilection site: bile ducts. Africa and Asia.
Rostellum usually well developed, with two or one (rare) circles of hooks. Strobila usually medium to very large, rarely very small. Gravid proglottids longer than wide. Single set of genitalia per segment. Genital pores irregularly alternating. Testes numerous. Ovary posterior. Gravid uterus median, with lateral branches. Eggs are spherical, ± 40 μm in diameter, with a thick, striated embryophore, 4.5-5.6 μm thick (taeniid egg). Metacestodes develop in mammals. Final hosts: Mammals, usually carnivores.
Rostellum usually well developed, armed with two circles of hooks. Strobila large with many proglottids. Gravid proglottids are longer than wide (8-10 x 4-5 mm). Genitalia single. Genital pores alternate irregularly. Cirrus pouch pyriform. Testes numerous in single field anterior and lateral to female organs. Ovary bi-alate, posterior. Vitelline gland compact, post-ovarian. Vagina opening behind cirrus. Uterus with median stem and lateral branches. Metacestode a cysticercus, strobilocercus or coenurus, developing in mammals. Final hosts: mammals; predilection site: small intestine. Cosmopolitan.
T. solium (Pork tapeworm, Armed tapeworm) (Figure 166)
Length 1.5-2.0 mm long; scolex with two rows of 22-32 rostellar hooks; large hooks 160-180 μm long, small hooks 110-140 μm long. Gravid proglottids longer than wide (ca. 10-12 x 5-6 mm); uterus has 7-12 lateral branches on either side. Eggs typically taeniid. Intermediate hosts: pigs (all suids can act as intermediate hosts), occasionally humans, dogs and small ruminants; the metacestode (Cysticercus cellulosae, elliptical, semi-transparent, 5-20 mm long and 5-10 mm wide) may be found in all muscles (muscles of the sacrum, shoulder, diaphragm and tongue are favourite sites), or the CNS (humans and dogs) Final host: man only.
Figure 165 Scolex and mature proglottid of Stilesia globipunctata (redrawn from Gibbons, Jones & Khalil, 1996).
T. saginata (Beef tapeworm, unarmed tapeworm)
Length 10 m or more; scolex unarmed, no rostellum. Gravid proglottids longer than wide (16-20 x 4-7 mm); uterus has 18-32 lateral branches on either side. Eggs typically taeniid. Intermediate hosts: cattle; the metacestode (Cysticercus bovis) occurs in the skeletal and cardiac muscles. Final host: humans.
Length 0.75-5.0 m; scolex with two rows of 26-44 rostellar hooks; large hooks 170- 220 μm long, small hooks 110-160 μm long. Gravid proglottids longer than wide (ca. 10-15 lateral branches on either side). Eggs typically taeniid. Intermediate hosts: domestic and wild herbivores (e.g. sheep, goats, cattle), omnivores (pigs), and primates; the metacestode (Cysticercus tenuicollis; up to 6 cm long with a thin long neck) occurs in the peritoneal cavity, where it attaches to the sub-serosal tissue of the abdominal cavity (omentum, mesentery, and liver). Final hosts: dogs and cats.
Length 0.45-1.10 m; scolex with two rows of 24-36 rostellar hooks; large hooks 168-197 μm long, small hooks 111-138 μm long. Gravid proglottids longer than wide (ca. 15 x 3-3.5 mm); uterus with 20- 25 lateral branches on either side. Eggs typically taeniid. Intermediate hosts: sheep and goats; the metacestode (Cysticercus ovis, size: 4 x 2.5 to 9 x 4 mm, usually oval and whitish) in cysts in the heart, diaphragm and skeletal muscles. Final host: dogs.
Length 0.6-2.0 m; scolex with two rows of 34-48 rostellar hooks; large hooks 225-294 μm long, small hooks 132- 177 μm long. Gravid proglottids longer than wide (ca. 7-10 x 4.5 mm); uterus has 8-14 lateral branches on either side. Eggs typically taeniid. Intermediate host: lagomorphs (wild and domestic rabbits); the metacestode (Cysticercus pisiformis, pea-sized, oval, 6-12 x 4-6 mm, usually oval) is found attached to the sub-serosal tissue (omentum, mesentery, stomach, intestine, and liver); in heavy infections cysticerci are clustered like a bunch of grapes. Final hosts: dogs and cats (cats appear to shed the tapeworm before gravid proglottids have developed).
Length 0.15-0.60 m; scolex with two rows of 26-52 hooks; large hooks 380-420 μm long, small hooks 250-270 μm long. No neck, so segments begin directly behind the scolex, with 1st segment as wide as or wider than scolex. Gravid proglottids longer than wide, trapezoidal (8-10 mm x 5-6 mm). Eggs typically taeniid. Intermediate hosts: rodents, lagomorphs, occasionally humans; the metacestode (Strobilocercus fasciolaris, contained in oval cysts 8-10 mm in diameter, sometimes larger) occurs in the liver. Final host: cats.
Length 0.2-0.27 m; scolex with two rows of 26-32 rostellar hooks; large hooks 135- 175 μm long, small hooks 78-120 μm long. Gravid proglottids longer than wide (ca. 6-12 x 3-5 mm); uterus with 9-26 lateral branches on either side. Eggs typically taeniid. Intermediate hosts: principally sheep, other herbivores, omnivores, and occasionally humans; the metacestode (Coenurus cerebralis, round or oval, up to 10 cm in diameter, semi-transparent) is neurotropic and develops mainly in the brain, but sometimes also in the spinal cord. Final host: dogs.
Length 0.2-0.27 m; scolex with two rows of 26-32 rostellar hooks; large hooks 135- 175 μm long, small hooks 78-120 μm long. Gravid proglottids longer than wide (ca. 6-12 x 3-4 mm); uterus has 7-12 lateral branches on either side. Eggs typically taeniid. Intermediate hosts: lagomorphs and occasionally rodents, and humans; the metacestode (Coenurus serialis, oval, 3-4 cm in diameter) occurs in the subcutaneous and inter-muscular connective tissues. Final hosts: dogs and cats.
Rostellum with a double circle of hooks. Strobila very small with fewer than six proglottids. Neck absent. Genital pores irregularly alternating. Testes few, anterior and lateral to the female organs. Ovary bilobed, posterior. Vitelline gland compact, post-ovarian. Seminal receptacle present. Uterus with median stem and short, undivided lateral branches. Gravid proglottid longer than wide, gravid uterus occurs only in the last proglottid. Metacestode is a hydatid, developing in mammals. Final hosts: Canidae and Felidae. Predilection site: small intestine. Cosmopolitan.
E. granulosus (Figure 167)
Length 2-6 mm; scolex with two rows of 36-40 hooks; large hooks 32-43 μm, small hooks 20-36 μm. Strobila with 3 proglottids (2-7); penultimate proglottid is mature, terminal proglottid is gravid and usually about half length of the worm. Uterus extends through entire length of the mature segment, has lateral branches and protrusions – the form and number of which are inconstant. Eggs typically taeniid. Intermediate hosts: mainly herbivorous and omnivorous domestic and wild animals, and humans; the metacestode (hydatid cyst) is a monolocular fluid-filled cyst; size: commonly 5-10 cm in diameter; wall consists of two membranes (external cuticular and internal germinative); occurs in the liver, lungs and more rarely, also in other organs. Final hosts: dogs (never in cats), other canids (e.g. jackals, wolves, coyotes, foxes, and wild dogs) as well as hyaena, lions and leopards.
Rostellum armed with several rows of hooks; hooks usually thorn-shaped. Suckers unarmed. Strobila small to medium. Proglottids numerous; mature and gravid proglottids longer than wide. Two sets of genitalia per proglottid. Genital pores bilateral. Testes numerous. Uterus not persistent, replaced by thinwalled capsules containing one or several eggs. Metacestodes developing in Amphibia, reptiles, or insects. Final hosts: Carnivora.
Rostellum conical, armed with several circles of rose-thorn-shaped hooks. Suckers unarmed. Mature and gravid proglottids longer than wide, acraspedote, constricted at intersegments. Each proglottid with two sets of reproductive organs. Testes numerous, in entire intervascular field. Genital pores bilateral, postequatorial. Ovary bilobed. Vitelline gland post-ovarian. Vagina ventral or posterior to cirrus pouch. Uterus first reticular, then breaking into egg capsules, each with several eggs. Metacestode a cysticercoid, developing in insects. Final hosts: Carnivora, rarely humans.
D. caninum (Figure 168)
Length 14-45 cm. Gravid proglottids longer than wide, cucumber-shaped. Egg capsules (120-200 μm in length) each with 5-30 eggs. Individual eggs spherical (40-50 μm in diameter) containing an oncosphere which is covered by a very thin striated embryophore. Intermediate hosts: fleas (Ctenocephalides felis, C. canis, and Pulex irritans), and chewing lice species (Trichodectes canis). Final host: dogs, cats and occasionally humans; predilection site: small intestine. Cosmopolitan.
Rostellum conical, armed with several circles of rose-thorn-shaped hooks. Mature proglottids usually wider than long. Two sets of reproductive organs per proglottid. Genital pores bilateral, pre-equatorial. Cirrus pouch crosses osmoregulatory canals. Testes numerous, filling intervascular space. Ovary lobated. Vitelline gland postovarian. Vagina posterior to cirrus pouch. Uterus breaks into egg capsules, each with a single egg. Final hosts: Carnivora.
J. pasqualei (Figure 169)
Length 0.5 m. Rostellum conical, 8-18 alternating rows of rose-thorn-shaped hooks with blades shorter than bases. Gravid proglottids longer than wide (0.9-6.0 x 0.3-3.2 mm). Intermediate hosts: beetles act as first intermediate host and reptiles as second intermediate host. Final hosts: cats, wild felids, and dogs.
Rostellum usually present, with one to several circles of very small, very numerous, T- or hammer-shaped hooks. Suckers present, rarely absent, commonly armed with very small spines that are easily lost from dead specimens. Strobila small to large, rarely tiny. Reproductive systems single or double; when single, genital pores unilateral or alternate irregularly. Uterus sac-like or replaced by egg capsules or paruterine organ. Final hosts: birds and mammals.
Rostellum armed with two circles of very small, hammer-shaped hooks. Suckers small, armed or not. Neck absent. Strobila very small, proglottids very few. Genitalia single; genital pores irregularly alternating or unilateral. Cirrus pouch crosses osmoregulatory canals. Testes few, mainly post-ovarian. Ovary median. Vitellarium post-ovarian. Seminal receptacle present. Egg capsules each with single egg. Metacestode a cysticercoid developing in insects or gastropods. Final hosts: birds.
Length 0.5-4.0 mm, 0.6 mm wide. Rostellum armed with two circles of 86- 94 hammer-shaped hooks. Suckers armed with four circles of hooks. Strobila with 4-7 whitish, transparent proglottids. Genital pores regularly alternating. Egg capsules spherical, containing a single egg; size: 55 x 36 μm. Intermediate host: gastropod molluscs. Final hosts: fowl and other galliform birds; predilection site: small intestine.
Rostellum armed with two circles of very small hammer-shaped hooks. Suckers armed or not. Strobila with many proglottids. Proglottids craspedote. Genital pores unilateral or irregularly alternating. Testes numerous. Ovary median. Vitellarium post-ovarian. Uterus replaced by egg capsules, each containing one or several eggs. Metacestode a cysticercoid, developing in insects. Final hosts: birds and mammals; predilection site: small intestine.
Rostellum armed with two circles of 400- 500 hooks. Suckers inconspicuous and unarmed. Egg capsule containing single egg; size: 93 x 74 μm. Intermediate hosts: beetles. Final hosts: chickens, turkeys, pigeons, and many other domestic and wild birds.
Length 4-13 mm; width 1-4 mm. Rostellum with two circles of 200 hooks. Suckers rounded with 8-15 rows of hooklets. No neck. Egg capsule containing 8-12 eggs; size 93-74 μm. Intermediate hosts: ants. Final hosts: fowl, turkey, pigeon, and other galliform birds.
R. tetragona (Figure 170)
Length 10-25 cm. Rostellum with single row of 100 hooks, Suckers oval with 8-15 rows of hooklets. No neck. Egg capsule containing 6-12 eggs; size 93- 74 μm. Intermediate hosts: Musca spp. and ants. Final hosts: fowl and other galliform birds.
Rostellum large, with two circles of hooks followed by several circles of very small spines. Suckers unarmed. Strobila large; proglottids wider than long, craspedote. Genitalia single; genital pores unilateral. Testes numerous. Ovary bilobed. Vitellarium post-ovarian. Egg capsules numerous, with several eggs. Final hosts: Struthioniformes and Rheiformes. Africa and South America.
H. struthionis (Figure 171)
Length up to 0.6 m; width 9 mm. Rostellum with two circles of 160 hooks. Uterus of gravid proglottids breaking up into egg capsules, each containing 15- 25 eggs. Life cycle unknown. Final hosts: ostrich, and the South American rhea. Predilection site: small intestine.
Scolex with simple, unarmed suckers, no rostellum. Genitalia single, genital pores median and ventral. Uterus present or replaced by a paruterine organ. Vitellarium double. No complete life cycle is known within this family. Life cycle involves two intermediate hosts. 1st intermediate host and type of metacestode developing unknown. Second intermediate hosts are Amphibia, reptiles, birds, and mammals (e.g. rodents, dogs, and cats); the metacestode (tetrathyridium) occurs in the body cavities, liver and other sites. Final hosts: carnivorous mammals, birds, and humans.
Strobila up to 1.5 m in length. Proglottids craspedote.
M. lineatus (Figure 172)
Length 0.3-0.8 m. Gravid proglottids are barrel-shaped. Eggs oval with thin embryophore (40-60 x 34-43 μm). Final hosts: dogs, cats, wild carnivores, and occasionally humans; predilection site: small intestine.
The name “Trematoda” is derived from the Greek ‘trematodes’, meaning ‘having holes’. This refers to the suckers that form a characteristic feature of the group. All organs are embedded in a parenchyma, with no body cavity being present. A mouth and an alimentary canal are present, but usually there is no anus (blind alimentary tract). The life cycles are either direct (Monogenea) or indirect (Digenea). Trematodes occur in a wide range of host environments; most species are endoparasitic, but many are ectoparasitic.
Morphology (Figure 173)
Most trematodes are flattened dorsoventrally, but some have thick, fleshy bodies and some are long, round in section, and worm-like. There are typically two suckers, an anterior oral sucker surrounding the mouth, and a ventral sucker (= acetabulum) on the ventral surface.
Most trematodes have a blind alimentary tract. The digestive system consists of a mouth (surrounded by anterior sucker), pharynx, oesophagus and intestine – which usually divides into two blind caeca. Only a few trematode species have an anus.
The excretory system is of the protonephridial type – consisting of flame cells, collecting ducts, and a bladder. The number and arrangement of the flame cells is of taxonomic importance, particularly in the larval stages where they can be seen more easily than in the adults. The excretory bladder may be Vor Y-shaped – which is important in the classification of this group.
With the exception of the Schistosomatidae, the digenetic trematodes are hermaphrodites. The male reproductive system consists of usually two testes, vasa efferentia, a vas deferens, seminal vesicle, an ejaculatory duct, and a cirrus enclosed in a sac or pouch. So-called prostate glands surrounding the ejaculatory duct may be present.
The female reproductive system usually consists of a single ovary from which a short oviduct arises. The proximal part of the oviduct is provided with a sphincter, the ovicapt, which controls the passage of ova. In some species, a seminal receptacle forms an outpocket of the wall of the oviduct. At the base of the seminal receptacle, a slender tube – Laurer’s canal – occasionally arises. This canal sometimes ends blindly in the parenchyma, but may open on the dorsal surface in some species. It may be a vestigial vagina which is functional only in a few species. Generally there are follicular vitelline glands arranged in two lateral fields connected to the main right and left vitelline ducts.
These ducts carry the vitelline cells to a single median vitelline reservoir – from which a common vitelline duct arises to join the oviduct. After joining, the oviduct expands to form an ootype. Numerous gland cells surround the ootype forming Mehlis’ gland, and secrete through tiny ducts into the ootype. The female duct expands beyond the ootype to form the uterus whose length and form varies. The distal end of the uterus is often quite muscular and is called the metraterm; this functions as an ovijector and vagina. The female genital pore opens near the male genital pore usually in the genital atrium. Self-fertilisation or cross-fertilisation may take place.
Large flat distomes; suckers usually close to each other, cuticle spined or not. Caeca simple or with dendritic lateral branches, but without anterior diverticles. Testes usually branched, but may be without branches or lobes. Cirrus pouch present. Genital pore preacetabular. Ovary branched or entire; seminal receptacle reduced or absent. Laurer’s canal present. Vitellaria profusely developed laterally, confluent posteriorly. Uterus with relatively few coils; eggs large. Final hosts: herbivorous and omnivorous mammals, humans. Predilection sites: bile ducts, liver, intestine.
Body large, broad, flat, foliate, with distinct head cone. Cuticle with scalelike spines. Oral sucker subterminal, prepharynx short, pharynx well developed, and oesophagus short. Caeca long, reaching to posterior extremity – provided with numerous, long, dendritic, outer branches, and fewer, shorter, inner branches. Acetabulum at the base of the cephalic cone of nearly the same size as the oral sucker. Testes profusely branched, in tandem, occupying the greater part of the postovarian intervitellarian field. Cirrus pouch anterodorsal to the acetabulum, not extending further back. Genital pore at the intestinal bifurcation. Ovary branched, submedian, pretesticular. Seminal receptacle absent. Laurer’s canal present. Vitellaria very profusely developed, extending in the lateral fields both dorsal and ventral to the caecae, from behind the base of the cephalic cone to the extreme posterior end of the body. Uterus coiled in form of rosette between acetabulum and ovary. Eggs operculated, large, ovoid and light yellowish-brown in colour. Final hosts: herbivorous and omnivorous mammals, man. Predilection sites: bile ducts.
F. hepatica (Figure 174)
Length: 20-50 mm; width: 4-13 mm. Eggs operculated and yellowishbrown; size: 130-150 μm x 65-90 μm. Intermediate hosts: Lymnaea (Galba) truncatula, Lymnaea (Pseudosuccinea) columella. Infective stage for final host: metacercariae (about 0.2 mm in diameter) that encyst on vegetation. Final hosts: herbivorous and omnivorous mammals, humans. Predilection sites: bile ducts. Cosmopolitan.
F. gigantica (predominant species in Africa)
Length: 25-75 mm; width: 3-11 mm. Eggs operculated and yellowish-brown; size: 156-197 μm x 90-104 μm. Intermediate hosts: Lymnaea (Radix) natalensis, Lymnaea (Pseudosuccinea) columella. Infective stage for final host: metacercariae that encyst on vegetation. Final hosts: herbivorous and omnivorous mammals, humans. Predilection sites: bile ducts. Africa, Asia, and southern Europe.
Unisexual. Pharynx absent, oesophagus short. Caeca joining together to form a single slender limb, terminating near posterior extremity. Suckers present or absent. Acetabulum, when present, anterior to genital pore. Body of male may be widened to form gynaecophoric canal. Testes consisting of four or more follicles anterior or posterior to caecal union. Cirrus pouch present or absent.
Female more slender than male. Ovary elongated, sometimes spirally curved, anterior to caecal union. Laurer’s canal present or absent. Uterus more or less winding in the inter-caecal field. Vitellaria extending from ovarian, or pre- or postovarian zone to posterior extremity. Eggs not operculated, embryonated when passed from the final host, with terminal or lateral spine, or spineless. Final hosts: birds and mammals. Predilection sites: blood vessels.
Genus Schistosoma (Figure 175)
Male: Fore body (pre-acetabular portion) short, cylindrical or nearly so; hind body widened, with edges enrolled ventrally to form gynaecophoric canal. Cuticle with spines or tubercles. Testes less than 10 in number, situated in one or two rows at beginning of gynaecophoric canal. Seminal vesicle pretesticular. Cirrus pouch absent. Genital pore immediately post-acetabular.
Female: Body filiform, longer than male. Ovary median, elongate, usually caudad, rarely cephalad of equator of body. Laurer’s canal absent. Vitellaria extending along common caecum. Eggs oval or fusiform, with terminal or lateral spine, or with a rudimentary spine.
Final hosts: mammals. Predilection sites: veins. Cercaria furcocercous (infective stage for final host). No metacercarial stage.
Male 6-22 mm long; female 7-28 mm long. Egg spindle-shaped with terminal spine at one pole; size: 145-250 μm x 40-70 μm. Intermediate hosts: Bulinus globosus, Bulinus africanus. Final hosts: cattle, sheep, goat, man. Predilection sites: portal and mesenteric veins. Central, southern and East Africa.
Digenea with thick body; ventral pouch present or absent. Cuticle may be papillated but not spined. Acetabulum terminal or subterminal, occasionally almost ventral. Oral sucker terminal, exceptionally subterminal, with or without posterior diverticles. Oesophagus may or may not be provided with muscular bulb posteriorly. Caeca simple or more or less sinuous, terminating usually at or near posterior end of body, occasionally a considerable distance away from it. Testes usually in middle third of body, double, exceptionally single. Vesicula seminalis usually present. Cirrus pouch or hermaphroditic pouch present or absent. Genital pore anterior, midventral, mid-dorsal, with or without genital sucker. Ovary usually posttesticular. Laurer’s canal present, exceptionally absent. Vitellaria acinous or follicular, lateral. Uterine coils mostly intercaecal. Final hosts: vertebrates.
Body conical, broader posteriorly than anteriorly, almost round in cross section; ventral pouch absent. Acetabulum sub-terminal, moderate in size. Pharynx without pouch or diverticula; oesophagus with or without posterior sphincter or bulb. Caeca in lateral sides of the body, usually sinuous. Testes lobed, directly or obliquely tandem or side-by-side; seminal vesicle thin-walled and strongly coiled. Cirrus pouch absent. Ovary post-testicular; uterus coils dorsal and anterior to testes. Laurer’s canal crossing excretory vesicle. Vitellaria in lateral fields. Final hosts: ruminants. Predilection sites: stomach.
C. microbothrium (Figure 176)
Body conical. Length: 3.8-12.5 mm; width: 1.6-4.2 mm. Egg operculated; size: 150-190 μm x 68-100 μm. Intermediate hosts: Bulinus (Bulinus) tropicus. Infective stage for final host: metacercariae which encyst on vegetation and walls of containers. Final hosts: cattle, sheep and other ruminants. Predilection sites: rumen and reticulum. Africa, Europe, Middle East.
Genus Gastrodiscus (Figure 177)
Body divided into small, nearly cylindrical anterior portion and large, discoidal, ventrally excavated, posterior portion provided with numerous papillae. Ventral pouch absent. Acetabulum small, ventral, with raised margin. Oral sucker with paired evaginations. Pharynx with pharyngeal bulb and paired pharyngeal pouches. Oesophagus with muscular thickening. Caeca long, not wavy, terminating in front of acetabular zone.
Testes branched, diagonal, in middle third of body. Cirrus pouch present. Genital pore without sucker, at or near anterior margin of excavated posterior region. Ovary lobate, post-testicular, inter-caecal. Laurer’s canal opening anterior to excretory pore. Excretory pore posterior in acetabular zone. Uterus strongly winding in inter-caecal field dorsal to ovary and posterior testis and ventral to convoluted seminal vesicle. Vitellaria occupying whole extracaecal
Length: 9-17 mm; width: 8-11 mm. Egg operculated, yellowish-brown, size: 131-139 μm x 78-90 μm. Intermediate hosts: Bulinus forskalii. Infective stage for final host: metacercariae that encyst on vegetation. Final hosts: equines, pigs, and warthogs. Predilection sites: large and small intestine. Africa and Asia.
Snail morphology and identification
Examine a snail by placing it in the palm of the hand with the apex opening to the fingertips. All Lymnaea spp. have a body-whorl with the opening to the right. The whorls are clockwise and the tentacles are triangular (Figure 178).
The following species are intermediate hosts in South Africa:
(a) Lymnaea truncatula (intermediate host of F. hepatica)
- The body-whorl does not form the major part of the shell as in L. natalensis or L. columella.
- There are at least five whorls. In the other species, there are never more than four whorls.
- Field specimens are dark brown or black. The adult snails are rarely more than 45 mm long.
(b) Lymnaea natalensis (intermediate host of F. gigantica)
- The body-whorl comprises almost the whole shell.
- The spire is acute.
- Growth lines run parallel to the curvature of the shell.
- The parallel lines on the penultimate whorl are indistinct.
- Field specimens are lighter in colour than those of L. columella, but the shell is sturdy and adults can exceed 10 mm in length.
c) Lymnaea columella (intermediate host of both F. hepatica and F. gigantica)
- The body-whorl comprises most of the shell, but is narrower than that of L. natalensis.
- The spire is narrower than that of L. natalensis.
- The growth lines are superimposed on the reticulation.
- The distinct parallel lines on the penultimate whorl are not growth lines.
- Field specimens are darker than those of L. natalensis, but the shell is fragile. Adults can exceed 10 mm in length.
(d) Bulinus (Bulinus) tropicus (Intermediate host of Calicophoron spp.).
- If held on its back with its apex towards the fingertips, the opening of the shell is on the left.
- The whorls run anti-clockwise, with a fairly long spire.
- The shell is ridged.
- The foot and body fluid are red.
- The tentacles are long and the eggs are passed in a yellowish packet.
(e) Bulinus africanus (intermediate host of both S. mattheei and S. haematobium).
- The snail’s aperture (opening) is on the left side.
- The whorls run anti-clockwise and comprise most of the shell.
- The spire may be low to slightly elevated or prominent.The columella does not join the peristome (labium) in an evenly rounded curve as in . tropicus – but is conical at its base, forming a notch where it joins the peristome.
(f) Biomphalaria pfeifferi (intermediate host of S. mansoni)
- This snail’s shell is spirally coiled in one plane.
Host-parasite lists for domestic species
(N=nematodes, C=cestodes, T=trematodes)
N Gongylonema spp.
Rumen and Reticulum
T Calicophoron spp. adults
N Gongylonema spp.
N Haemonchus placei
N Ostertagia ostertagi
N Trichostrongylus axei
T Calicophoron spp. immatures
C Moniezia expansa
C Thysaniezia spp.
N Bunostomum phlebotomum
N Cooperia spp., including C. pectinata, C. punctata and C. oncophora
N Nematodirus helvetianus
N Strongyloides papillosus
N Toxocara vitulorum
N Trichostrongylus spp., including T. colubriformis and T. falculatus
Caecum and Colon
N Oesophagostomum radiatum
N Trichuris spp.
N Setaria labiatopapillosa
N Onchocerca gibsoni
N Elaeophora sagitta
T Fasciola gigantica
T Fasciola hepatica
C Echinococcus spp. larvae (Hydatid)
C Stilesia hepatica
C Echinococcus spp. larvae (Hydatid)
N Dictyocaulus viviparus
N Thelazia rhodesii
N Onchocerca linealis
T Schistosoma mattheei
C Taenia saginata larvae (Cysticercus bovis)
N Parafilaria bovicola
Sheep and goats
N Gongylonema spp.
Rumen and Reticulum
T Calicophoron spp. adults
N Gongylonema spp.
N Haemonchus contortus
N Teladorsagia circumcincta
N Teladorsagia trifurcata (rare)
N Trichostrongylus axei
T Calicophoron spp. immatures
C Avitellina spp.
C Moniezia spp.
C Stilesia globipunctata
C Thysaniezia spp.
N Bunostomum trigonocephalum
N Cooperia punctata (rarely) and
Cooperia pectinata (rarely)
N Gaigeria pachyscelis
N Nematodirus spathiger
N Strongyloides papillosus
N Trichostrongylus colubriformis
N Trichostrongylus falculatus
N Trichostrongylus rugatus
N Trichostrongylus vitrinus
Caecum and colon
N Chabertia ovina
N Oesophagostomum columbianum
N Oesophagostomum venulosum
N Trichuris spp.
C Taenia hydatigena larvae (Cysticercus tenuicollis)
C Taenia multiceps larvae (Coenurus cerebralis)
T Fasciola gigantica
T Fasciola hepatica
C Stilesia hepatica
C Taenia hydatigena larvae
C Echinococcus spp. larvae (Hydatid)
N Dictyocaulus filaria
N Muellerius capillaris
T Schistosoma mattheei
C Taenia ovis ovis larvae (Cysticercus ovis)
N Draschia megastoma
N Habronema majus
N Habronema muscae
N Trichostrongylus axei
C Anoplocephala magna
C Paranoplocephala mamillana (syn. Anoplocephaloides mammillana)
N Parascaris equorum
N Strongyloides westeri
Caecum and colon
T Gastrodiscus aegyptiacus
N Small strongyles (Cyathostomes)
N Large strongyles
N Oxyuris equi
N Probstmayria vivipara
N Setaria equina
N Strongylus vulgaris immatures
T Fasciola hepatica
C Echinococcus spp. larvae (Hydatid)
N Strongylus equinus and S. edentatus immatures
C Echinococcus spp. larvae (Hydatid)
N Dictyocaulus arnfieldi
N Strongylus edentatus larvae
N Draschia megastoma L3
N Habronema majus sp. L3
N Habronema muscae sp. L3
Tendons and ligaments
N Onchocerca cervicalis
N Onchocerca reticulata
N Ascarops strongylina
N Physocephalus sexalatus
N Trichostrongylus axei
N Ascaris suum
N Strongyloides ransomi
N Trichinella spiralis
Caecum and colon
N Oesophagostomum dentatum
N Oesophagostomum quadrispinulatum
N Trichuris suis
C Taenia solium larvae (Cysticercus cellulosae)
Liver and kidney
C Echinococcus spp. larvae (Hydatid)
T Fasciola hepatica (rarely)
N Stephanurus dentatus
C Echinococcus spp. larvae (Hydatid)
N Metastrongylus apri
Muscles and tendons
C Taenia solium larvae (Cysticercus cellulosae)
N Trichinella spiralis larvae
N Suifilaria suis
Dogs and cats
N Spirocerca lupi
C Dipylidium caninum
C Echinococcus spp. (Dog only)
C Joyeuxiella spp.
C Mesocestoides lineatus
C Taenia hydatigena
C Taenia multiceps (Dog only)
C Taenia ovis ovis (Dog only)
C Taenia pisiformis
C Taenia serialis
C Taenia taeniaeformis (Cat only)
N Ancylostoma braziliense
N Ancylostoma caninum
N Ancylostoma tubaeforme
N Toxascaris leonina
N Toxocara canis
N Toxocara mystax (Cat only)
N Trichuris vulpis
Heart and aorta
N Dirofilaria immitis
N Spirocerca lupi
N Dipetalonema spp.
N Dirofilaria repens
N Capillaria spp.
T Paragonimus sp.
N Oslerus osleri
N Trichinella spiralis
C Taenia solium larvae (Cysticercus cellulosae)
Poultry and ostriches
N Capillaria spp.
N Acuaria spp.
N Libyostrongylus douglassi (Ostrich)
C Amoebotaenia sphenoides
C Choanotaenia infundibulum
C Houttuynia struthionis (Ostrich)
C Raillietina spp.
N Ascaridia columbae
N Ascaridia galli
N Capillaria spp.
N Hartertia gallinarum
Caecum and colon
N Heterakis gallinarum
N Codiostomum struthionis (Ostrich)
N Syngamus trachea
N Contortospiculum spicularia (Guinea worm of ostriches)
Necropsy and helminth recovery technique in the ruminant
In order to make an accurate diagnosis of a helminthosis, a systematic necropsy must be done. Both the worms and the pathology they cause can be observed. The method given below is recommended by the Helminthology Department of the University of Pretoria, South Africa.
- Protective clothing, including rubber or disposable gloves.
- Bowel scissors.
- String for ligation of organs.
- Plastic trays or bins for organs.
- Buckets with at least a 10 litre capacity.
- Bottles containing 10-40% formalin and 70% ethyl alcohol.
- Sieves – with copper or stainless steel mesh – and with 0.038, 0.15 and 0.3 mm apertures.
- Fine-pointed forceps to collect the worms, or ‘fishing rods’ made out of a fine hypodermic needle attached to a disused ballpoint pen. The tip of the needle is slightly bent by dropping it on a hard surface.
- Wide-mouthed glass or plastic jars with screw caps, of about 1 litre capacity. Some small bottles or vials are also needed to store the worms in.
- A stereo and a standard microscope.
- Glass slides and coverslips.
- Lactophenol for clearing of nematodes.
- A counting chamber made out of Perspex or a glass Petri dish with parallel lines scored on the bottom.
- Salt, hydrochloric acid and pepsin powder.
- Waterproof pens for marking bottles
- Adhesive tape for labelling bottles.
Before attempting the autopsy or the helminth collection, the bottles should be marked with the animal number, the date of collection, the organ and the aliquot size. An example of how this is done is given below.
Usually, the organ names are abbreviated: ABO (abomasum), STOM (stomach), SI (small intestine), CC (caecum and colon), LI (large intestine), and RUM (rumen). Heart, lung and liver are written in full. As any collection of the g.i.t. comprises 2 parts, the ingesta and the digests, the bottles are labelled accordingly. The ingesta or residue is abbreviated RES and the digests DIG. An aliquot is a representative sample of a collection. For example, the content of the SI of a sheep is 2.5 l. To make a fifth aliquot of this volume, one takes 500 ml and it must be written on the label of the particular bottle which contains the SI RES.
Example: sheep, number 123 died on 20th February 1987, and a full helminthological collection has been made:
|Sheep 123, 20/2/87 |
ABO RES 1/10
|Sheep 123, 20/2/87 |
SI DIG 1/1
The serosal surfaces are to be examined for the presence of larval cestodes or adult or larval nematodes. When encountered, count them and place them in a marked bottle in 70% alcohol.
Blood and blood vessels
Make 3 smears of the peripheral blood of the animals. This is stained with Giemsa and examined for microfilariae. Examine the mesenteric, ileo-caeco-colic and jejunal blood vessels against the light. Collect all visible schistosomes by cutting the blood vessel above them and by expressing them into a marked bottle with 70% alcohol.
- Remove the heart, lungs, liver, pancreas, kidneys, adrenals and diaphragm from the g.i.t. by blunt dissection and light pressure, and place them in a separate tray. Report any abnormalities such as abscesses, adhesions, perforations.
- By blunt dissection and gentle pulling, remove the omentum from the rumen and separate the reticulum, omasum, abomasum and anterior part of the duodenum from the rumen and associated ligaments.
- Examine the lymph nodes and Peyer’s patches externally – noting any abnormalities.
- Carefully cut the small intestine loose from the mesenterium and do the same for the caecum-colon. Mark the spot where the terminal gyrus centripetalis reflects upon itself with a double ligature, 5 cm apart. The entire g.i.t. should now be free from any ligaments or the mesenterium.
- Tie double ligatures around the junction of the oesophagus with the rumen, as well as the junction of the reticulum and the omasum, the junction of the omasum and the abomasum, the pyloric valve, and below the caecum and above the colon to isolate the ileo-caecal valve. The ligatures are necessary to prevent the contents of the g.i.t. from flowing out.
- Cut between the ligatures to divide the g.i.t. into oesophagus, rumen and reticulum, omasum, abomasum, small intestine and anterior part of the large intestine. Place each part of the g.i.t. in a separate tray.
- Open the posterior part of the large intestine with bowel scissors and examine the contents for the presence of large worms. If these are found, place them in a marked bottle with 70% alcohol. Collect 50-100 g faeces for nematode egg counts from the rectum, and then place in a marked bottle.
- Carefully open the rumen and inspect the contents for the presence of Calicophoron spp. Remove the contents and inspect the wall of the rumen for the same. If flukes are found, place the wall of the rumen in a bucket with hot water and allow to stand for approximately 15 minutes. Remove the rumen from the bucket, remove all the remaining amphistomes and then place them in a marked bottle with 70% alcohol.
- Sieve the contents of the bucket over a 0.3 mm sieve and preserve the residue that contains flukes in a marked bottle in 10% formalin. Note any lesions on the rumen wall.
- Open the oesophagus with a pair of bowel scissors and examine for the presence of nematodes in and under the mucosa. If these are present, remove the worms or the part of the oesophagus containing the worms, and preserve in 10% formalin. Note any lesions.
- Open the omasum and inspect the wall for lesions, which should be noted. If none are present it can be discarded as no parasites occur here.
- Remove the ligaments on the greater and lesser curvatures of the abomasum and discard. Open the abomasum and drain the contents into the plastic tray. Rinse the abomasum twice in a bucket, with a small amount of water. Add the rinsing water to the residue (ingesta) and keep the clean abomasum for later processing. Mix the abomasal contents well and take an aliquot either by weight or by volume. Pour the aliquot into a glass or plastic jar of sufficient capacity, and add 40% formalin to a final concentration of 10%. Take specimens of the abomasal wall and the pyloric valve for histopathology.
- Small and large intestines: Follow the same procedures as described for the abomasum and retain the walls for later processing. Note abnormalities such as enlarged Peyer’s patches, intussusceptions, inflammation and abscesses, and take specimens for histopathology if necessary.
- Digests are prepared by separately scraping with a knife the mucosae from the abomasum, the small intestines and the large intestines (abomasum) or with a glass slide (SI and CC). The respective mucosae are placed in glass jars and digesting fluid comprising 35 ml technical hydrochloric acid and 10 g pepsin powder per litre of water or normal saline, is added in the ratio of 4 parts digesting fluid to 1 part mucosa. The jars are marked, vigorously shaken, and placed in a water bath at 38-42°C for a maximum of 3 hours. The digests should be shaken every 20 minutes, or continuously if a mechanical stirrer is used, in order to ensure the breaking up of the mucosa and even digestion of the tissue. When pieces of mucosa are no longer visible, or after 3 hours, the bottles are removed from the water bath, and, if possible, sieved over a sieve with 0.038 mm apertures. Otherwise, leave for at least 30 minutes and decant carefully. Then add 40% formalin to a final concentration of 10% to preserve the worms in the sediment.
Liver, heart and lung
Examine each organ for the presence of parasites or parasitic lesions. Collect the parasites in 70% alcohol and note any lesions. Take 5 strips of liver – each approximately 5 cm thick – from the entire width of the liver. This is approximately a fifth of the liver’s mass and thus represents a fifth aliquot. Place these strips in a marked glass jar containing normal saline and incubate in a water bath at 38-40°C for about 2 hours, shaking occasionally.
Examine the right lung by opening the bronchi and trachea and thoroughly rinse in running water over a bucket. Collect all the visible parasites and note any lesions. Cut the lung into 1 or 2 cm cubes with a pair of scissors, place the cubes in a bottle with normal saline, and then incubate for 2 hours in a water bath at 38-40°C, shaking occasionally.
Slice the heart into 1 cm-thick slices and place these in a glass jar in normal saline. Incubate as for the liver and the lung. After incubation, pour the contents of the various glass jars into separate buckets. Rinse the organs thoroughly and add the respective washings to the saline in the respective buckets. Sieve over a sieve with 0.038 mm apertures, and preserve in marked bottles in 10% formalin.
Muscle and connective tissue
After the carcass has been skinned, examine the inside of the skin and the surface of the carcass for parasites. Collect in marked bottles in 10% formalin or 70% alcohol. Note the numbers and the sites where they occurred.
Make sections in the muscles: 3 in the triceps brachii, a longitudinal incision in the longissimus dorsi on each side, 2 lengthwise incisions in the lower surface of the tongue, a longitudinal incision in the iliopsoas on each side, and 2 incisions parallel to the lower jaw in the masseter muscles. Examine each cut surface thoroughly for the presence of cysticerci. If present, note the numbers and the site, then collect and preserve in 70% alcohol.
|NOTE: DO NOT EAT OR DRINK OR SMOKE WHEN HANDLING CYSTICERCI: THEY MAY BE INFECTIVE TO HUMANS|
It is important to know the host-parasite lists that are provided as Addendum A.
Pay special attention to the rumen, the eyes and the first 10 metres of the small intestine. If Trichostrongylus is suspected, gently scrape the mucosa of the small intestines with a glass slide and cover with another glass slide. Hold against the light to see the worms which are very small.
The cranial mesenteric artery must be ligated at the intestines and collected together with the entire aorta. Ligate the thoracic aorta in front of the heart as well as the iliac arteries – caudal to the bifurcation of the aorta. Cut between the ligatures and remove the entire aorta together with the anterior mesenteric artery. Open separately and examine for parasites. Examine the peritoneum visually and collect the larvae of other Strongylus species that may be present. Pay special attention to the large blood vessels of the caecum and the colon, where large migrating larvae of the Strongylus species may also be found.
The aorta of dogs must be ligated in the same way as described for horses. However, it is necessary to only take the thoracic aorta, since Spirocerca lupi causes lesions in this location.
Nematode egg counts
Faecal egg counts are the only possible way to diagnose helminth infections in the live animal. It should be understood, however, that the faecal egg count merely gives an indication of the presence of repro ductively active female worms and there is no correlation between the egg count and the actual worm burden the animal carries.
- Wide-mouthed specimen jars, 50 - 100 ml capacity, with screw caps.
- Spatula or spoon.
- Electronic, mechanical or triple-beam scale for weighing in grams.
- needed. They consist of 3 cylindrical nylon-meshed sieves that fit into one another. The outer sieve has apertures of 0.05 mm, a funnel-shaped top and has a stop-cock at the bottom. The inner sieve has apertures of 0.096 mm and a funnelshaped top.
- Burette stand, to clamp the trematode egg sieves.
- A counting chamber or a glass petri dish with parallel lines scored in the bottom.
- Stereoscopic and standard microscopes.
- Mechanical or electronic counter, e.g. a white cell counter.
- A wide-mouthed , 1 litre, glass jar.
- An electric blender
- Ball bearings, approximately 5 mm in diameter.
- Amyl alcohol in a bottle with a dropper.
- Wide-mouthed pipettes, approximately 5 mm in diameter.
- Rubber bulbs.
- McMaster slides.
- Any of the following solutions which are used to float the eggs to the top of the faecal emulsion: 400 g magnesium sulphate (Epsom salts)/litre water OR a mixture of 200 g mercuric iodide and 400 g potassium iodide, dissolved in 1 litre of water, OR 1 kg sodium chloride/ litre water OR 667 g sugar/litre water OR 400 g zinc sulphate/litre of water.
- An electric centrifuge.
- Centrifuge tubes.
- Glass rods, 5-8 mm in diameter.
- Glass slides with coverslips.
- A “flotation” fluid, consisting of 3 parts glycerine and 1 part water.
- A “Faecalizer” (TM) kit, with the apparatus and chemicals.
Collection of faeces
Sheep and goats
Hold the animal in such a way that it is easily managed. Insert a gloved, lubricated index finger in the anus and rotate rapidly to stimulate the defecation reflex. Collect at least 20 g of pellets in a marked bottle (approximately 20).
Calves and cattle
Place the index and third finger into the rectum of small calves and spread with the tips of the fingers. Move the hand slowly back and forth to stimulate the defecation reflex. In larger calves and adult cattle, insert the entire hand into the rectum and collect the faeces. Place in a marked bottle.
Place the horse in a crush and remove faeces manually, as described for adult cattle.
Dogs and cats
Specimens may be collected by enema or from the floor of the kennel, but should not be covered with grass, bedding or dirt.
Pigs and poultry
As for cats and dogs.
Methods for nematode egg counts
This description involves the different flotation techniques, which only give an indication of the presence or absence of nematode eggs. This method is recommended when a quick diagnosis is needed and should not replace the quantitative McMaster method. Sodium nitrate appears to be more effective than sugar water for the flotation of nematode eggs in ruminant faeces, although the difference is only about 6%. For dogs and cats, magnesium sulphate and zinc sulphate appear to be more effective, with the mixture of mercuric and potassium iodide being the most satisfactory for eggs of Toxocara canis. Sodium chloride seems to be most effective for the nematode eggs of pigs.
The centrifugal flotation technique is often used for rapid diagnosis of helminthoses in a variety of animals, but particularly in dogs. A level teaspoonful of faeces is placed in a centrifuge tube and well mixed with water. It is then filled to the brim and centrifuged for 2 minutes at 1 500 rpm. The water is decanted and the sediment mixed with one of the flotation fluids listed earlier. Centrifuge again for 2 minutes at 1 500 rpm and touch a glass rod onto the meniscus of the centrifuged mixture. The tiny drop which adheres to the glass rod contains the helminth eggs and is transferred to a glass slide and viewed under the microscope.
The ‘Faecalizer’ method is a patented, commercially available product that is widely used in private practice and some laboratories. It comes as a complete kit and the buyer only supplies their own glass slides and coverslips. The kit consists of a plastic well, a lid with an attached spatula, an applicator, and a perforated plastic plunger. The well is filled with the faeces to be examined, and a plastic tube which will contain the flotation fluid, is firmly fixed over the well. The tube is then filled with the flotation fluid and with the aid firstly of the applicator, followed by the plastic plunger, the contents of the well is thoroughly mixed with the flotation fluid. The tube is then filled with flotation fluid until the meniscus is convex. A coverslip is placed over the meniscus and the entire apparatus is left for 20 to 30 minutes to allow the eggs to rise and adhere to the coverslip. The coverslip is carefully removed, placed on a glass slide, and examined microscopically.
This method is also known as the modified McMaster method or the McMaster method; it was first developed in Australia in 1939 and has been adapted over the years to suit various conditions and people. It gives a reasonably accurate indication of the number of eggs that present per gram of faeces, but it must be stressed that it does not reflect the actual burden that the animal carries. The reasons for this will be explained in due course.
A faecal sample is collected from the animal to be tested. Weigh the sample and use 1 g of dog and cat faeces, 2 g of sheep and goat faeces, and 5 g of calf, adult bovine or horse faeces taken. The flotation fluid used in this case is a saturated sugar solution. For dogs and cats 59 ml of this sugar solution is used, for sheep and goats 58 ml, and for the other species 55 ml (note that the combined weight of faeces and the volume of the sugar solution is always 60). The faeces and sugar mixture are placed in a blender and blended for 10 to 20 seconds. If no blender is available, use a strong glass jar and add 10 iron ball-bearings to the faeces-sugar-water mixture. Shake well for about 2 minutes.
Add 4 to 6 drops of amyl alcohol to the suspension. This helps to break the surface tension and prevents air bubbles from accumulating. The bottle is closed and swirled to mix the suspension and the amyl alcohol. A wide-mouthed pipette is used to transfer the required amount of suspension to the McMaster slide. Since the slide has 3 chambers and each has to be filled, the bottle containing the suspension must be shaken in between each sampling. After the chamber has been filled, allow to stand for about 2 minutes for the eggs to float to the top. Examine the McMaster slide under the 10 X magnification of the standard microscope. All the eggs in a rectangle measuring 12 x 8.5 x 15 mm are counted and 3 rectangles are usually examined, and an average used. Each rectangle represents 0.15 ml of the original volume of the suspension, and this must be used when calculating the number of eggs per gram of faeces. The formulae below are used to correct for the dilution factor for each species.
Dogs and cats:
Total number of eggs counted ........................................... x 400 = epg Number of chambers counted
Sheep, pigs and goats:
Total number of eggs counted ........................................... x 200 = epg Number of chambers counted
Cattle and horses:
Total number of eggs counted ........................................... x 100 = epg Number of chambers counted
The eggs of Nematodirus, Strongyloides and Trichuris are characteristic and can be counted separately from the rest.
Faecal trematode egg counts
Use cold water when sieving or sedimenting faeces for trematode eggs. If the presence of Schistosoma is suspected add 10% formalin immediately after collecting the faeces – to prevent the eggs from hatching.
Mix faeces thoroughly with a spatula or spoon, or rub dry faeces between fingers. Weigh 2 g each of sheep and goat faeces and 5 g each of cattle and horse faeces. Blend for 10 to 15 seconds in cold water in a blender. Close the screw plug on the inner sieve and the stop cock on the outer sieve of the trematode egg sieves. Place the sieves in a burette stand and clamp vertically. Put the faecal emulsion into the inner sieve, rinse the blender with cold water, and add this to the faecal emulsion in the inner sieve. Spray the contents of the inner sieve with a strong stream of cold water until the water that runs from the outer sieve is clear. Remove the inner sieve, open the screw plug, and discard the contents. Wash the inner filter thoroughly and make sure that it is clean.
Spray the contents of the outer sieve with cold water and allow to settle, leaving a 2 to 3 cm layer in the bottom. Open the stop cock and allow the contents to run into a glass bottle, previously marked. Fill the bottle with cold water, allow the eggs to settle, and decant the supernatant after 2 to 5 minutes. Examine microscopically (stereo microscope).
A simpler, cheaper method is to emulsify the faeces with cold water, and sieve through a vegetable sieve into a widemouthed 1 litre jar. Fill the jar to the brim with cold water, leave for 5 minutes, and decant carefully. Repeat until the supernatant is clear. After decanting for the last time, pour the remainder into a small bottle and allow eggs to settle. Examine under the stereo microscope.
Sometimes blood is present in the faeces and Schistosoma or the protozoan Eimeria is suspected. In such cases, a drop of blood is transferred to a microscope slide, covered with a coverslip, and examined microscopically. If no Schistosoma eggs are found, take another drop of blood and make a blood smear, which can then later be stained with Giemsa and examined for Eimeria.
When Schistosoma infestation is suspected, scrape the rectal mucosa either with a glass slide inserted into the rectum by hand, or by means of a curette. With a little saline, break up the mucosa, cover with a coverslip, and examine microscopically. The specimen may also be preserved in 10% formalin and at a later stage ground up with a pestle and mortar, and examined under a stereo microscope.
The counting procedure for trematode eggs
Eggs are counted in a counting chamber with a stereo microscope. First, swirl the suspension in the small bottle and then pour small quantities into a counting chamber with parallel line scores in the bottom. The areas between the parallel lines are examined one by one and the eggs differentiated and counted. Once all the eggs have been counted, the contents of the counting chamber is discarded and some more of the suspension in the small bottle is poured into the counting chamber. Repeat the process until the entire specimen in the small bottle has been counted.
The egg count per gram of faeces is calculated as follows:
Total number of eggs present .................................................... = epg Mass of faeces
Differential egg count is calculated as follows:
Total number of Fasciola eggs ...................................... = epg (Fasciola) Mass of faeces
Total number of Calicophoron eggs .............................. = epg (Calicophoron) Mass of faeces
The number of Fasciola eggs added to the number of Calicophoron eggs must be the same total as the total number of eggs counted. For example, 340 eggs were counted in a 5 g faecal specimen of a bovine. The epg will then be: 340/5 or 68 eggs per gram.
Another example is: in a 5 g faecal sample of a bovine, 340 Fasciola eggs and 1 200 Calicophoron eggs were found. The epg is: 340/5 = 68 Fasciola eggs per gram and 1 200/5 = 240 Calicophoron eggs per gram. The total epg, therefore, is 308 epg – which is the same as 340 + 1 200/5.
Method for diagnosing cestodes
Examination of the live animal for the presence of cestodes is to be attempted. Proglottides may be seen in the faeces or may be removed manually from the rectum. The standard McMaster technique, as applied to nematodes – but using 10 to 15 g of faeces – may be used. In each case, the results are unsatisfactory and autopsy remains the best way to make a diagnosis.
Worm burdens that can cause clinical signs or death
Table 19 indicates the numbers of adult worms that can cause clinical signs or even death in cattle and sheep. It is included here as an aid in making diagnoses of helminthoses. This is because one of the problems appears to be the inability to correlate the worm burden with the effect.
|Fasciola hepatica||700-1 000|
|Calicophoron spp.||100 000-150 000|
|Cooperia spp.||40 000-50 000|
|Haemonchus contortus||8 000-10 000|
|Oesophagostomum radiatum||800-1 000|
|Ostertagia ostertagi||20 000-30 000|
|Calicophoron spp. (imm)||>40 000|
|Haemonchus contortus||1 000-3 000|
|Nematodirus spathiger||1 000-3 000|
|Haemonchus contortus||1 000 (lambs)-10 000 (adults)|
|Nematodirus spathiger||1 000 (lambs)-10 000 (adults)|
|Teladorsagia circumcincta||10 000|
|Strongyloides papillosus||11 000 (lethal in goat kids)|
|Trichostrongylus axei||25 000-40 000|
|Trichostrongylus spp. in the small intestine||20 000-25 000|
Daily egg output of female nematodes
Table 20 indicates the daily egg output of a single female nematode. It could therefore be used to interpret the results of the faecal nematode egg counts.
|Helminth species||Number of eggs||Output category|
|Oesophagostomum, Haemonchus||5 000-10 000||Very high output|
|Chabertia (erratic)||3 000-5 000||High output|
|Bunostomum, Gaigeria||600-800||Moderate output|
|Cooperia, Ostertagia, Trichostrongylus||200||Low output|
|Nematodirus||50||Very low output|
Anthropozoonosis: (plural -es ) – infections acquired by humans from other vertebrates.
Curative treatment: treatment with anthelminthics when animal develops clinical signs of helminthosis.
Definitive host (final host): this is the host in which the parasite attains sexual maturity and is able to reproduce.
Direct life cycle: life cycle in which parasite utilises only one host (synonym: monoxenous).
Ecology: study of the interrelationships between organisms and their environment, which includes factors such as temperature, humidity, pH, the presence or absence of light and other things which are necessary for the survival of the worms. Ecological studies usually apply to the free-living stages of helminths.
Histotropic phase: larval (usually 4th) stage which remains in the host tissues without any further development. This phase is induced by the immune status of the host – rather than the season.
Hypobiosis/hypobiotic: seasonally-induced resting stages that the 4th larval stages of some of the parasites undergo in the final host, before they develop into adult worms (also referred to as arrested, retarded, inhibited or suspended).
Indirect life cycle: life cycle in which the parasite uses more than one host.
Infection rate: indicates the size of the infection that the specific animal carries.
Infective stage: a stage in the life cycle, usually the 3rd larval stage that can enter the intermediate or final host.
Intensity: the number of individuals of a particular parasite species in each infected host. This is expressed as a number, for instance, an intensity of 2 500 means that the host is infected with 2 500 worms. The mean intensity is the total number of parasites counted in all the animals, divided by the number of infected hosts.
Intermediate host: the host in which the immature stage of a worm (L1, L2, L3) develops into infective form which is picked up by the final host; mainly applicable to trematodes and cestodes.
Life cycle: the development of a parasite through its various stages, viz. fertilization, laying of eggs, hatching and development of the larvae, infection of the final host and further development into adults.
Metaphylactic treatment: administration of an anthelminthic when animal is infected with worms
Paratenic host (synonym – transport host): a non-essential, passive transporter of the helminth; unlike in the intermediate host no development of the larva takes place.
Prepatent period (synonym – developmental period): the period after the infective stage has entered the final host and before the parasite demonstrates its presence – such as eggs in the faeces, blood or mucus in the faeces or the urine, loss of condition and various other clinical signs. This period refers to the period between infection and the presence of adult worms.
Prevalence: the number of helminths of a host species infected with a parasite divided by the number of hosts examined – and is expressed as a percentage. For example, during a survey, 200 sheep were examined and 134 were found to be positive for Haemonchus contortus. The prevalence is therefore 67 percent.
Prophylactic: preventative treatment.
Reservoir host: this is an animal that harbours the parasite, but is not adversely affected by it; for example wild animals which do not show clinical signs but are a potential source of clinical infections for domestic livestock.
Resilience: the ability to cope with the effects of infection.
Resistance (anthelminthic): the ability of helminths to develop resistance to anthelminthic substances on exposure.
Resistance (helminth): refers to the ability of animals to resist helminths.
Tolerance: innate unresponsiveness of a parasite to a drug, independent of prior exposure to that drug or to others belonging to the same chemical group.
Zooanthroponosis: infections of human origin that are acquired by other vertebrates.
Zoonosis: (pl-es) infections transmitted between man and other vertebrates.
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