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. Fecal examination may or may not reveal 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 microscopic examination but false-negative tests are common.
Most of the broad-spectrum drugs recommended for treatment of strongyles (see Gastrointestinal Parasites of Horses: Large Strongyles in Horses) are effective against pinworms.
Last full review/revision March 2012 by Thomas R. Klei, PhD