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Symptoms
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The infected horse backs up against a post or wall and rubs her tail and hindquarters back and forth incessantly. In time, the skin on the hindquarters becomes excoriated and the hair is rubbed off, giving the characteristic ratty-looking tail.
Location
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The adult pinworm lives in the colon. The egg-bearing female migrates through the anus and deposits her eggs on the horse's perineum. The eggs are sticky and adhere to fences, bedding, stable walls, and other spots where the horse rubs her bottom. According to Dr. Robert Holland, DVM, from the Horse Owner's Veterinary Handbook, clear tape adhered to the perianal area allows the veterinarian to take a sample and determine if pinworm eggs are present.
Life Cycle
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Eggs hatch within one week. Infective larvae drop to the ground, where they contaminate feed. Once ingested, they pass into the colon and develop into adults. The entire life cycle is completed in three to four months.
While this sequence is typical, pinworms sometimes complete all stages of their life cycle in the horse's large intestines.
Effects
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When a large number of pinworms infest the large intestine, it can cause inflammation of the colon and episodes of colic--abdominal pain. According to Dr. Don Hudson, DVM, writing in the Eq Group website, pinworms are the least damaging of all the internal parasite groups. Unfortunately, the consistent irritation can cause the horse to appear quite unattractive.
Treatment
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To reduce the burden of infestation, clean the anal area with warm, soapy water daily. Use disposable paper towels to avoid transmitting eggs to other horses. Pinworms are sensitive to many of the deforming agents. The anthelmintics used in routine deforming programs will also control pinworms.
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The Effect of Pinworms
Pinworms, scientifically called Oxyuris, are common parasites, occurring primarily in stabled horses. The adult pinworm is a slender worm about 3 to 6 inches long. Horse pinworms cannot be transmitted to people, dogs or other domestic animals.