Large Strongyles

What you need to know

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 dung.

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.

Large strongyle larvae are swallowed as your horse eats infected grass. 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, diarrhoea, fever and anaemia from the bleeding bite wounds. S. equinus and S. edentatus can cause liver damage.

Effective wormer chemicals: Ivermectin, Moxidectin, Oxfendazole, Oxibendazole, Fenbendazole (may require more than 1 syringe for a heavy horse).
 

More information

The large strongyles of horses are also known as blood worms, palisade worms, sclerostomes, or red worms. The three major species and their respective lengths are Strongylus vulgaris (up to 25 mm), S edentatus (up to 40 mm), and S equinus (up to 50 mm). (See also Small Strongyles.) Under favourable conditions, the larvae develop to the infective stage within 1-2 wk after the eggs are passed. Infection is by ingestion of infective larvae, which exsheath in the intestine and migrate extensively before developing to maturity in the large intestine. The prepatent period is 6-11 mo. The larvae of S vulgaris migrate extensively in the cranial mesenteric artery and its branches, where they may cause parasitic thrombosis and arthritis. Larvae of the other two species may be found in various parts of the body, including the liver, perineal tissues, flanks, and pancreas. These species do not produce lesions in the mesenteric arteries. Mixed infections of large and small strongyles are the rule.

Adult large strongyles have large buccal capsules and are active feeders, ingesting plugs of mucosa as they move about in the intestine. The associated blood loss may lead to anaemia. Weakness, emaciation, and diarrhoea
are also common. Strongylus vulgaris is especially important because of the damage it does to the cranial mesenteric artery and its branches. As a result of the interference with the flow of blood to the intestine and
thromboembolism, any of several conditions may follow, including colic; gangrenous enteritis; or intestinal stasis, torsion or intussusception, and possibly rupture. Cerebrospinal nematodiasis ( Cns Diseases Caused By Helminths And Arthropods: Introduction) can cause a variety of lesions and signs depending on the part of the CNS affected.

Diagnosis of mixed strongyle infection is based on demonstration of eggs in the faeces. Differential diagnosis can be made by identifying the infective larvae after faecal culture. When colic due to verminous arteritis is suspected, a painful enlargement at the root of the mesentery may be palpable per rectum. Serologic diagnosis based on a rise in _-globulins has been recommended but is not specific for S vulgaris . Parasitic arterial lesions have been demonstrated using arteriography.

Colic due to arterial lesions has been successfully controlled by anthelmintic treatments effective against arterial larvae. Ivermectin at its standard dose is effective against the larval stages of S vulgaris ; enbendazole and oxfendazole, at doses higher than that for the adult parasite, are also effective against larval infections. Daily administration of pyrantel tartrate is effective in preventing the establishment of arterial stages of S vulgaris . A number of anthelmintics, including the benzimidazoles, pyrantel, and ivermectin, are active against adult large and
small strongyles.

Parasite control programs assume that grazing horses are infected; hence, treatments are administered to minimize the level of pasture contamination and to reduce the risks associated with migrating larvae.

Removal of faeces from paddocks and pastures manually or by mechanical means also aids in
control. Whatever worming program is used, faecal samples should be examined periodically to monitor of the effectiveness of the program.