Female worms are normally full of eggs which pass out in the feces. Additional female pinworms crawl out the anal opening and deposit their eggs on the surrounding skin, causing irritation and itching to the host. For relief, the horse usually rubs against a fence post or other solid object, resulting in hair loss near and on the tail and occasionally producing a secondary infection. The mature female has a slate-grey or brownish color and a narrow tail which may be more than three times as long as the rest of the body. The life cycle is completed in five months.
Pinworms are actually less damaging to the equine system than any other internal parasite group.
Yet the constant annoyance and irritation can spoil the looks and action of an otherwise attractive horse. Pinworms can sometimes be seen in the manure of heavily infected animals. Control and Treatment Horse owners need to understand that an internal parasite control program is a continual battle. Regular cleaning of stables and paddocks. Avoiding overcrowding of pastures.