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

Home » Pet and Animal News » * Pet Care & Information OVERVIEW » The Practical Guide to Equine Parasites and Dewormers > Large Strongyles (Strongylus vulgaris, S. equinus, S. edentatus)
Scientific name: Strongylus vulgaris, S. equinus, S. edentatus Common name: Large strongyles, Bloodworms Physical description of parasite: Large Strongyles are long, fat worms with biting mouths. Since they are full of their equine victims' blood, they usually are reddish-brown in color. Stages/lifecycles: Eggs of the three species of large strongyle can develop into infective larvae on pasture in as little as three days if warm, moist conditions exist. Once the larvae have been swallowed, the larvae drop their protective coating, or "sheath", on their bodies and migrate to different organs for further development. The larvae of Strongylus vulgaris, the most harmful of the three, move to the horse's arteries where they "go with the flow" of the blood for approximately two weeks. When they reach the mesenteric artery, the main artery that feeds the gut area, they stay there for approximately four months while they continue growing. Then they return to the large intestine through the arteries. Once in the large intestine, the larvae burrow into the intestinal cavity. After six to eight months, the worms are mature and eggs begin to pass into the manure. Strongylus equinus larvae move to the liver after they have shed their protective coating. They stay there for approximately six weeks, then move through various abdominal organs to the large intestine. After about nine months, adult worms lay their eggs. Strongylus edentatus also move to the liver where they remain for approximately nine weeks, then move to the lining of the abdominal cavity where they form nodules. The larvae also form nodules in the gut wall, which they break open to serve as a doorway into the large intestine. How the parasite enters the horse's system: Large strongyle larvae live in the manure in the grass. When the horse swallows infected grass, the growing process of the worms begins. Effects of parasite if left untreated:B Of the three species, Strongylus vulgaris does the most damage. That's because this species uses - and damages - the blood system as it moves around the horse's body. During their wanderings, the larvae rough up the walls of the arteries, leaving "tracks" in the lining. These tracks are perfect spots for blood clots. The clots break away from the walls and lodge in other blood vessels, blocking the blood flow to the intestine below the clot. The rough walls of blood vessels not only promote blood-clot development, but also weaken the walls. If the blood-vessel wall is weakened enough, it can burst, causing immediate death. If the blood clots block blood flow to the hind quarters, lameness and/or weakness can result. When they set up housekeeping in the large intestine, the worms literally bite off pieces of it. This may lead to severe colic, diarrhea, fever and anemia from the bleeding "bite wounds" in the stomach and intestine. Large strongyles were once the most common and most dangerous equine parasite. This is no longer true, thanks to education of horse owners and diligent deworming practices using highly effective dewormers. As long as we continue these good practices, there's every reason to believe that large strongyles will not resume their role as a major cause of colic. But if we let up, they'll be back. The other two species, S. equinus and S. edentatus, can cause liver damage.