Strongyles, Bloodworms, Small strongyles

Strongyles are species of most damaging internal parasites. Strongyles, are divided into two groups, called the large strongyles and the small strongyles. The harmful effects of large strongyles are much greater than of the small ones. The larvae of large strongyles damage blood vessels, while the larvae of small strongyles remains in the wall of the gut.

The life cycle of this parasite has some unique, important features. The eggs are laid by adult worms in the intestine. They pass out in the feces and hatch into larvae in 1 to 2 days. Larvae feed on manure and continue to develop. After about a week, larvae become infective, move up on the blades of grass, and the horse eats them. Once inside the horse, they migrate to various organs and tissues where they do extensive damage. After the young adult worms return to the small intestine, they attach themselves to the gut wall, feed themselves on the gut lining, reach maturity and lay eggs. The life cycle of the small strongyle is different from that of the large strongyle in that the larvae penetrate the large intestine and become encysted. after 1 to 2 months, the larvae emerge and return to the intestine where they mature 1. A horse gets infected by ingesting larvae present in grass and forage.

Black horse

One of the three species, Strongyle vulgaris, is the most harmful because its larvae enter the arteries, creates blood clots which block small vessels. This destroys part of the blood supply to bowels and other digestive organs.

The larvae live in the arteries for about 5 months, then return to the intestine and develop into adult worms, where they attach to the wall of the bowel. A heavy infection can cause severe anemia. Small strongyles are the most common internal parasites in adult horses and occur frequently in foals. Larvae produce colic, bleeding and anemia, protein loss, and intestinal malabsorption.

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Control of strongyles is most important in all deworming programs. Since the arterial worms are not killed by most dewormers(except for ivermectin), it is extremely important to eliminate the larvae at early stages. An occasional treatment does not offer protection and will not prevent the potentially devastating consequences of these parasites. Most veterinary experts recommend the inclusion of ivermectin in all deworming programs.


  1. Horses: A Guide to Selection, Care and Enjoyment. J. Warren Evans

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