Parasite Groups

• Nematodes
• Acanthocephalans
• Cestodes

The major swine endoparasites may be divided into three categories: nematodes, or roundworms; acanthocephalans, or thorny-headed worms; and cestodes (tapeworms). Parasites are assigned to these categories according to their morphology, or structure.

Nematodes

Nematodes, or roundworms, are elongated, cylindrical, and tapered at both ends. Adults of this class range from less than a millimetre in length to more than forty centimetres. They have a complete digestive tract and tough, elastic, skin-like cuticle. The mouth area may be specialised for attaching to or feeding on the host. For example, the kidney worm, Stephanurus dentatus, has six teeth in its mouth 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.

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.

Acanthocephalans

The thorny-headed worm, Macracanthorhynchus hirudinaceus, is only member of this phylum that is a parasite of swine. This phylum differs from the nematodes in several respects. Each thorny-headed worm has a spiny proboscis for hanging onto the gut wall, but no digestive tract; food is obtained simply by absorption through the body wall. The thorny-headed worm looks something like a tapeworm; the body is long and flattened, with transverse wrinkles. However, it is not segmented as tapeworms are. Sexes are separate.

Cestodes

Tapeworms, or cestodes, are flat, ribbon-like organisms that usually live 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.