Large Strongyles (Strongylus vulgaris, S. equinus, S. edentatus)

Lifecycles: Large strongyle eggs can develop into infective larvae on pasture in as little as three days. Once swallowed, the larvae drop their protective coating, or "sheath" and migrate to different organs for further development. Strongylus vulgaris larvae are very dangerous, moving through the horse's arteries to the mesenteric artery, the main artery that feeds the digestive system. S. vulgaris larvae continue to grow in the mesenteric artery for about 4 months, then return to the large intestine where they burrow into the intestinal cavity. After 6-8 months, the worms mature and eggs are passed in the manure.

S. equinus larvae move to the liver for about 6 weeks. Then they migrate through abdominal organs to the large intestine. After 9 months, adults mature and lay eggs.

S. edentatus larvae also move to the liver, where they remain for about 9 weeks. Then they move to the abdominal cavity where they form nodules in the lining and the gut wall.

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

Dangers If Left Untreated: S. vulgaris cause severe damage. Migrating larvae rough up artery walls, leaving tracks where blood clots can form. Clots break away from the wall and lodge into other blood vessels, blocking blood flow to the intestine. Artery walls weakened by larval damage are also prone to burst, leading to immediate death. In the large intestine, large strongyles literally bite off pieces of flesh, often leading to severe colic, diarrhea, fever and anemia from the bleeding bite wounds. S. equinus and S. edentatus can cause liver damage.