Hair Worms (Trichostrongylus axei)

Lifecycle: Eggs hatch when eaten by the horse. Larvae migrate to the stomach and mature. Adult worms in the stomach and in the small intestine irritate and erode the villi, or finger-like projections, of the gut, damaging the capillaries and lymph vessels. Eggs are laid and passed in the manure.

How It Gets Into Your Horse: Hair worm larvae are swallowed as your horse eats infected grass.

Dangers If Left Untreated: When damaged, villi are unable to digest and absorb nutrients properly. Dark, foul-smelling diarrhea may result. Severe damage can cause bleeding into the intestine, leading to anemia and loss of condition. Foals are particularly susceptible to hair worm infection.