Helminthology

Helminths of horses

Author: E VOLKER SCHWAN

Equine helminth infections are probably less economically important now than they were several decades ago when horses were used extensively for transport and farming. In South Africa, helminth infections are now of importance mainly in horses owned by rural communities which are dependent on work horses, and also horses used for sport and leisure activities.

The use of macrocyclic lactones has resulted in the large strongyles becoming a less important cause of colic in horses –while small strongyles are now of greater importance. Apart from the strongyles and helminths that cause mortalities in foals, most of the other helminth infections either occur sporadically, or have a limited pathogenic impact.

Nematodes

Large strongyles (Strongylosis)

Strongylus species

General

Strongylosis of horses is caused by Strongylus vulgaris, Strongylus edentatus, and Strongylus equinus . These species are collectively also referred to as ‘large strongyles ’, which, in contrast to the ‘small strongyles ’of horses –are migratory (termed ‘migratory strongyles ’). The genus name derives from the Greek word ‘strongylos ’ (= rounded) –referring to the cylindrical shape of the worms. Adult worms are medium-sized and stoutbodied. Microscopically, the species have a large buccal capsule, which can be armed with teeth. Typically, the buccal capsule is surrounded by leaf crowns. The posterior end of males terminates in a typical, well-developed, strongylate bursa with spicules.

Strongylus vulgaris male worms are 10 to 19 mm long and female worms 13 to 25 mm long. Both are 1.0 to 1.4 mm wide. The oval buccal capsule is armed with two typically ear-shaped teeth at its base.

Strongylus edentatus male worms are 19 to 28 mm long and female worms 28 to 45 mm long. Both are 1.3 to 2.2 mm wide. As the species name indicates, Latin ‘e ‘(= without) and ‘dens ’(= tooth) –the cupshaped buccal capsule is ‘toothless ’.

Strongylus equinus male worms are 24 to 35 mm long and female worms 35 to 50 mm long. Both are 1.1 to 2.3 mm wide. The oval buccal capsule is armed with one dorsal tooth with a bifid tip and also two sub-ventral teeth at its base.

Fresh adult strongyles are dark red, and are sometimes almost black in colour. The predilection site in the definitive host is the caecum and colon ( Figure 52 ).

The eggs of the large strongyles are of the strongylid-type, which are oval and thin-shelled. The surface of the egg is smooth. They contain 4 to 8 blastomeres when laid, and these measure 64-99 x 36-58 μm. The eggs are indistinguishable morphologically and morphometrically from those of other equine GIT strongylids.

Hosts

Apart from the horse, the definitive host range includes donkeys, mules and zebra species.

Distribution

The large strongyles have a cosmopolitan distribution; Strongylus vulgaris is the most common species.

Transmission

The large strongyles have a direct life cycle. Sexually mature females are oviparous. Eggs are shed with the faeces into the environment. Depending on the prevailing climatic conditions, there is a typical pre-parasitic development from egg to free-living L1, L2, and then finally the sheathed infective L3 –this all in as little as 3 days.

Ingestion of sheathed free-living larvae is the only mode of infection for definitive hosts. Following uptake, the larvae of the large strongyles typically undergo extensive intra-abdominal migration –which differs markedly in the three species.

Strongylus vulgaris: Infective larvae exsheath in the stomach and undergo a histotropic phase in the mucosa of the caecum and colon –where they develop to L4. The L4 stage then enters arterioles of the gut and migrates on the endothelium to the cranial mesenteric artery and its main branches, where development to pre-adult stages takes place. Pre-adult stages are carried back via the arterial bloodstream to the wall of the caecum and colon –where they form nodules which eventually rupture and release the worms into the lumen of the caecum and colon. The pre-patent period is 6 to 7 months and the patent period is up to 18 months.

Strongylus edentatus: Infective larvae pass into veins of the intestinal mucosa and migrate via the portal system to the liver, where they migrate into the parenchyma –becoming encapsulated in granulomas in which they develop to L4. L4-stages emerge from granulomas and migrate to the liver capsule, which they cannot penetrate. Eventually, they enter the hepatic ligaments and migrate retroperitoneally of the abdominal wall –where they are found in large numbers in the flanks. Aberrant migration of larvae is, however, common. Under the peritoneum, L4 develop to pre-adult stages. Probably via the hepatorenal ligament, pre-adult stages migrate back to the colon, where they eventually penetrate the lumen. Final maturation takes place in the caecum and colon. The pre-patent period is 10 to 11 months and the patent period up to 18 months.

Strongylus equinus: Infective larvae exsheath in the stomach and undergo a histotropic phase in the mucosa of the caecum and colon, where they develop to L4. The L4-stage migrates via the peritoneal cavity into the liver parenchyma –where extensive migration continues. Eventually, L4 enter the pancreas, and partially also the peritoneal cavity. In the pancreas, larvae develop to pre-adult stages which enter the lumen of the gut via the connective-tissue adhesion of the pancreas and head of...

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