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Endoparasites - Parasite Groups

Equine endoparasites may be divided into three categories: nematodes, or roundworms; cestodes, or tapeworms; trematodes, or flukes. Parasites are assigned to these categories according to their morphology, or structure. Growth and life cycles of parasites within each group are generally distinct from those of the other groups. The roundworms are by far the most economically important internal parasites of horses.

 

Nematodes

Nematodes, or roundworms, are elongated, cylindrical, and tapered at both ends. Adults of this class range from 5 millimetres to more than 50 centimetres in length. They have a complete digestive tract and a tough, elastic, skin-like cuticle. The mouth area may be specialised for attaching to or feeding on the host. For example, the large strongyles (Strongylus species) have mouth capsules with teeth to perform such functions. Males of certain species of nematodes attach to females for mating by using a structure called a bursa. This is a posterior expansion of the cuticle or skin, which is bell-shaped or funnel-shaped and supported by finger-like projections called rays.

 
Anterior end of typical nematode   Posterior end of male T.axei showing bursa, rays and spicules, (polarised light)

Mating is also assisted by structures called spicules, used by the male to hold open the genital orifice of the female. The shape and arrangement of the male bursa and spicules vary from species to species and are frequently used to identify different nematodes.


Cestodes

Tapeworms, or cestodes, are flat, ribbon-like organisms that live most often in the small intestine of their host. The head, or scolex, of the tapeworm has suckers, hooks, or a combination of suckers and hooks used to attach the worm to the wall of the intestine. Proglottids (tapeworm segments) are generated from the scolex. In some species, the strobila or body of the worm may become several meters long. Each mature proglottid is a complete functional unit, incorporating a digestive system, organs of both sexes, and other organs. This phenomenon of both sexes in one body is known as hermaphroditism. Cestodes absorb nourishment directly through their tegument from the gut contents of the host animal.


Trematodes

Flukes, or trematodes, are characteristically flat, unsegmented worms. The equine intestinal fluke, found in Africa and India, is saucer-shaped. Suckers are located at the front and back of the fluke and are used as organs of attachment to the host. Horses are sometimes infected by the common liver fluke, Fasciola hepatica.


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