The larvae hatch in the intestine and grow into adults and the female lays her eggs around the rectum. The adult worm occurs in the stomach and in the small intestine, irritating and eroding the finger-like projections, or villi, of the gut, damanging the capillaries and lymph vessels within the villi. Signs of infection may be dark, foul-smelling diarrhea, because the damaged villi are unable to digest and absorb properly in the intestine. If there is severe damage to the villi, the underlying small vessels could be damaged as well, causing bleeding into the intestines. Bleeding may lead to anemia and loss of condition.
Infection occurs by ingestion of grasses contaminated with worm eggs.
Foals are very open to infection by hair worms, so broodmares should be dewormed and moved to clean pastures. Intestinal Threadworms Threadworms are long and hair-like, with adults growing to 8 to 9 mm in length. Infection can occur either by the horse swallowing larvae or the larvae can pass through the horse's skin. Young foals can also be exposed to the worm by nursing from infected mothers. Larvae mature to adult worms in the small intestine.