Pathogenesis, diagnosis

Oxyuris equi

Pathogenesis

Fourth stage larvae have a relatively large buccal capsule and they feed by ingesting plugs of intestinal mucosa to which they are attached. This causes mucoal erosions which appear to produce little in the way of clinical signs. Adult worms live freely in the intestinal lumen feeding on gut contents.

However, the main pathogenic effects appear to be related to the egg laying habits of the female worms. Fertilized females travel down to the rectum and extrude their anterior end through the host's anal opening. They lay eggs on the peritoneum in clumps contained in a greyish-yellow gelatinous material. This causes an irritation resulting in anal and perineal pruritis.
	Perineum of horse showing O. equi eggs

Clinical signs

	Infected animals rub their hind ends frequently in attempts to relieve the pruritis and irritation caused by the egg-laying females. Rubbing causes broken hairs, bare patches and inflamed skin over the rump and tail head causing the tail to assume an ungroomed "rat-tailed" appearance as shown in the acompanying two images. Animals are also restless causing them to feed less producing a loss of condition and often a dull staring coat.
Horse with O. equi infection		Horse with O. equi infection

Diagnosis

Diagnosis of O. equi infections is straightforward. The clinical signs described above are clearly indicative and combined with observing egg masses on the peritoneum, make this an infection that is easy to recognise. Sometimes owners will notice the long-tailed female worms in the feces of infected animals. The egg laying habits of female worms means that Oxyuris eggs are rarely found in feces taken directly from the rectum. They must be looked for in the perineal clumps or in fecal samples taken from the ground. Oxyuris eggs are diagnostic. They are ovoid, yellow, slightly flattened on one side and with a mucoid plug at one pole.

Egg of Oxyuris equi

Treatment and control

O. equi is susceptible to a wide range of available anti-parasitic drugs. The hind quarters of clinically affected animals should be washed to remove egg masses before treatment to avoid further contamination of the host's environment and to reduce the likelihood of reinfections.