Female ascarids can lay up to 200,000 eggs per day. Pinworms Though less dangerous than other internal parasites, pinworms are annoying to the horse because they cause severe anal itching. Adult worms crawl part way out of the anus to deposit their eggs on the adjacent surface. The eggs hatch outside of the horse's body and become infective in a few days, although they can survive unhatched for several months. The parasite is taken into the animal through contaminated water, grain, hay, or grass.
Young worms mature in the large intestine in three to four months, then begin the cycle anew.
A characteristic of pinworm infection is rubbing of the tail and anal region - causing broken tail hairs and bare patches around the tail. Pinworms can be treated successfully with the same drugs that are effective against strongyles and ascarids. An Ounce of Prevention Breaking the life cycle of parasites is as important as administering dewormers. Manure should be removed daily in stalls and weekly in pastures. Pastures and paddocks should be well drained and not overpopulated.