Helminthology

Author: J BOOMKER
Diagram credit: J BOOMKER

Classification and life cycles

Helminthologists use the Linnaean binomial or trinomial system, which is periodically updated to include new species or revisions of existing ones. The classification presented below is by no means complete, but should be sufficient for the purposes of the veterinarian. The 3 main phyla of worms dealt with in this textbook are the nemathelminthes (nematodes), cestodes and trematodes.

Nematodes (roundworms)

Classification

Life cycles of the Nematodes

Nematodes have 2 basic types of life cycles: the direct (monoxenous) and indirect (heteroxenous) types. The direct life cycle is, as the name implies, direct and is simple and uncomplicated; an intermediate host is not present (Figure 1). The indirect life cycle, however, is more complicated and there may be one or more intermediate hosts (Figure 2). These may be molluscs, arthropods, mammals and even humans. The general life cycles are discussed here, while specific life cycles are discussed in more detail when dealing with the worms themselves.

General principles

Life cycles show some variation within the different groups, but there are distinct similarities, i.e. four larval and one adult stage (Figure 3). Each stage is separated from the next by a moult, during which time the cuticle is shed. When laid, the eggs of both these groups of nematodes contain 4 to 8 blastomeres and are said to be segmented. Further development takes place and eventually the larva is fully formed and ready to hatch. The conditions necessary for survival and hatching of the eggs include sufficient moisture, the presence of oxygen, and a temperature of about 26°C. Under these conditions, the eggs will hatch in about 24 hours. Development is slowed down at temperatures below 10°C and is accelerated as the temperature rises. Above 40‒45°C the eggs will die. The eggs, L1, L2 and L3, are collectively referred to as the free-living stages, since they occur outside the host.

Figure 1 Schematic representation of a direct life cycle.

Figure 2 Schematic representation of an indirect life cycle.

Once the larva is fully developed, the eggs hatch. Moisture is essential for the survival of the 1st stage larva and temperature requirements are the same as for the eggs. The larva feeds on bacteria in the substrate and grows, and then enters the first moult (M1) to form the second larval instar or stage. Like the L1, the L2 will also feed on bacteria in the substrate, it grows and eventually undergoes the second moult (M2) to form the third larval stage, also known as the infective stage. The L3 usually retains the skin or cuticle of the M2 and is called a sheathed larva. The L3 cannot feed because of this sheath, and is therefore dependent on food accumulated during the first 2 stages. The sheath protects the larvae against desiccation. Under cool, moist conditions the larvae remain active for long periods – some surviving for months before they die. A rise in the temperature activates the larvae, with resultant depletion of their food reserves. They migrate from the faeces when a thin film of moisture is present and move horizontally to the nearest herbage – and then migrate vertically. Migratory patterns are random and influenced by moisture and temperature. The larvae are attracted to diffuse light and become more active as temperatures rise. They retreat into their sheaths to conserve moisture and cease moving under dry, sunny conditions. The L3 must now enter the definitive host to develop further – failing which they will die.

After having entered the final or definitive host, the larvae (L3) will cast off the sheath of the second moult, start feeding, and after a variable period they will moult (M3) to the fourth stage (L4). The fourth larval stage grows rapidly and during the later stages, the first indications of sexual differentiation become apparent. After a period of inactivity, the fourth stage larvae will moult (M4) to the fifth stage. The fifth stage worms are young adults. They grow, and will develop to be the mature, reproductively active adult worms (Figure 3).

The larvae of the Nematodirus spp. – the long-necked bankrupt worms of sheep and goats – develop inside the egg and only when they have reached the infective stage (L3), will the eggs hatch, should the environmental conditions be favourable. The infective larvae may enter their respective hosts: (1) per os (that is, they are eaten with the food when the animal feeds, as happens in the majority of the trichostrongylid and strongylid nematodes); (2) percutaneously (by entering through the skin by means of enzymes and physical action, as is seen with the hookworms or Ancylostomatidae); or (3) by both methods (Bunostomum spp.). Details are provided when the worms are discussed individually (below).

Figure 3 Representation of a nematode life cycle.

Subclass Adenophorea

Life cycle of the Trichinelloidea

This superfamily contains two families: the Trichinellidae and the Trichuridae. The Trichinellidae have an intricate life cycle that starts with the ingestion of meat containing L1 infective larvae, which develop into adults within 2 to 3 days. The females are viviparous, producing larvae as soon as 6 days after infection. The larvae penetrate the intestinal wall and are distributed throughout the body via the thoracic duct, the...

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