• Nematodes
• Cestodes
• Trematodes

Internal parasites of sheep may be divided into three categories: nematodes, or roundworms; cestodes, or tapeworms; and 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 two groups. The roundworms are by far the most economically important internal parasites of sheep. Flukes produce damage of economic importance in some geographic areas, while adult tapeworms are usually of minor importance.

Nematodes

Nematodes, or roundworms are elongated, cylindrical, and tapered at both ends. Adults of this class that affect sheep range from less than a millimetre to several 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 sheep hookworm, Bunostomum trigonocephalum, has cutting plates in its mouth to perform such functions. Males of the strongyloid type 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 which is 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.

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 and organs of both sexes. This phenomenon of both sexes in one body is known as hermaphroditism. Cestodes absorb nourishment directly through their integument from the gut contents of the host animal.

Posterior of male Haemonchus showing bursa and spicules		Vulvular region– female Haemonchus

Trematodes

Flukes, or trematodes, characteristically are flat, unsegmented worms which are shorter and broader than tapeworms. An exception to the usual trematode shape is the rumen fluke, Paramphistomum cervi, which is cone-shaped. Suckers characteristic of the fluke species are located at the front of the fluke and are used as organs of attachment to the host. Different fluke species live in different parts of the host and are often named for their sites of predilection. Two examples are the rumen flukes and the liver fluke (Fasciola). Trematodes are hermaphroditic, with each adult having both male and female reproductive organs.