Intestinal Threadworms (Strongyloides westeri)

Lifecycles: Infection occurs by eating larvae or through the skin. Larvae that enter through the skin migrate to the lungs, then up the wind pipe where they are coughed up and swallowed. Larvae mature into adults in the small intestine. Adults lay eggs that are passed in the manure.

How It Gets Into Your Horse: Larvae are swallowed as the horse eats infected grass or larvae go through the horse's skin. Infected mares pass the worm in their milk onto their young foals.

Dangers If Left Untreated: Threadworm larvae in the lungs can cause bleeding and respiratory problems. The worst damage often occurs in untreated foals who can suffer diarrhea, weakness, weight loss and poor growth.