Oxyuris equi (Pinworms)

What you need to know

Pinworm larvae mature in the large intestine, feeding off the intestinal lining. Adult females move to the anal area where they lay eggs covered with a sticky fluid that causes severe itching. Horses become infected with pinworms when they ingest eggs that have dropped into feed or water. Heavily infected horses may be nervous and stop eating. The severe itching makes the horse rub its tail and rump so much the tail hairs break off. Rubbing, biting and scratching can open up the skin to infections.

Effective wormer chemicals: Ivermectin, Moxidectin, Oxfendazole, Oxibendazole, Pyrantel pamoate, Fenbendazole.

More information

Adult pinworms, Oxyuris equi , are more common in horses <18 mo old and are found primarily in the terminal portion of the large intestine. The females are 7.5-15 cm long; males are smaller and fewer in number. The gravid females pass toward the rectum to lay their eggs, cementing them to the perineum around the anus. Masses of eggs and cement around the anus appear as a white to yellow, crusty mass. The eggs, which are flattened on one side, become embryonated in a few hours and are infective in 4-5 days. Adult pinworms are of little significance in the intestine but cause perineal irritation after egg laying. Rubbing of the tail and anal regions, with resulting broken hairs and bare patches around the tail and buttocks, is characteristic and suggests the presence of pinworms. Faecal examination may or may not disclose a pinworm infection. Samples collected around the perineal region may contain dried female worms or eggs. Application of cellophane tape to the skin of the perineum or scraping the area with a tongue depressor may recover ova for microscopical examination but is likely to result in a large number of false negative tests.

Most of the broad-spectrum drugs recommended for the strongyles (see Large Strongyles Small Strongyles) are effective against pinworms.