Parasites – Pinworms (Oxyuris equi)
Pinworms have the most efficient life cycle of all the parasites that infect the horse. They don't migrate through any organ tissue, and they have developed a means of reproduction by which the eggs don't leave the herd of horses.
While the horse is relaxed or sleeping, female pinworms crawl out of the horse's rectum, deposit eggs and a sticky substance on the perianal region of the horse, and crawl back into the rectum. Infective pinworm eggs are ingested orally and, once in the colon, the larvae develop through various stages before becoming sexually mature in about five months. As horses migrate, they take the eggs and adults with them.
Anal Irritation From Pinworms
Fortunately, about the only damage that pinworms cause is itching of the tail head. This annoys the horse but doesn't threaten its life. Because pinworms spend their entire lives in the lumen of the intestine and don't migrate, they cause very little physical damage to the horse. Horses can have massive pinworm infections without exhibiting significant health problems.
Keep an eye out for skin irritation around the anus area, rubbing of the tail, as well as biting and licking of hindquarters.