Scientific name: Strongylus vulgaris, S. equinus, S. edentatus
Common name: Large Strongyles, Bloodworms

Description: Large Strongyles are the most destructive and damaging internal parasites. Adult worms are reddish-brown and range from 3/4 to 2 inches (2 to 5 cm) in length. Strongylus vulgaris move through the horse's bloodstream, damaging arteries and creating the potential for blood clots and aneurysms. Strongylus equinus produce cysts in the liver, pancreas and intestine. Strongylus edentatus localize in portal veins and cause swelling of the liver and the abdominal lining. Adult worms attach themselves to the cecum and large colon to suck blood. This often leads to bleeding ulcers in the intestinal lining. Infection can also lead to weakening of blood vessel walls. Severely damaged walls can burst, causing immediate death. Horses with Large Strongyles may become anemic.

Life cycle: The life cycle of the three species of Large Strongyles is fairly similar. Female worms produce millions of eggs that are covered with a protective sheath and passed with manure. In warm, moist conditions, eggs develop into infective larvae in less than one week. When a horse ingests larvae through a contaminated food source, the protective sheath is dropped and larvae migrate to the liver, intestinal wall and arteries. The worms grow in these areas for several weeks before eventually returning to the large intestine. After six to eight months, the worms are fully mature, and females begin to lay eggs.

Symptoms: Signs associated with large strongyle infection include fever, poor appetite, weight loss, colic, lethargy, a rough hair coat and intermittent bouts of diarrhea and constipation.

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