Horse specific endoparasites

Nematodes (roundworms)

Nematodes are roundworms of varying sizes, which live in the intestine of their host. They attach themselves via their lip-disks or hooks to the wall of the intestine and feed on mucous, blood or intestinal contents.

Large and small strongyles (blood worms)

The large strongyle is the most common and most destructive of all internal equine endoparasites. It grows 1.5 to 5 cm long. Horses of all ages have strongyles, except very young foals. Many instances of colic may be related to damage to blood vessels caused by the migrating strongyle larvae.

Ascarids

Parascaris equorum is the largest internal parasite affecting the horse. It grows to approximately 12 to 40 cm. These parasites are most common in young horses. Immunity develops during adolescence upon exposure to P. equorum.

Pinworms

Oxyuris equi is found in the cecum, colon, large intestine and rectum of horses. Eggs may be passed out in the feces or deposited by female worms on the skin surrounding the anal opening, causing irritation and itching. The male adult grows 9 to 12 millimeters long, the female grows to approximately 150 millimeters and has a characteristically long, pin-like tail.

Insect larvae

Bots

Bots are larvae of horse bot flies, which lay their eggs on the legs or around the muzzle of the horse. After hatching, the larvae get into the horse's mouth and eventually reach the stomach where they develop into the full grown bots. There are three types of bots: Gastrophilus intestinalis, Gastrophilus haemorrhoidalis, and Gastrophilus nasalis. The bee-like horse bot flies are brown, hairy and have one pair of wings. The reddish mature larva (the bot) grows to 2 cm. It has a broad, round body tapering to a narrow hooked anterior end.