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Extension > Agriculture > Livestock > Horse > Horse health > Gastrointestinal parasites

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Gastrointestinal parasites

Jeremy D. Frederick, DVM, University of Minnesota

Parascaris equorum (Round worms) usually appear in horses 3-9 months of age. Foals will have a decreased appetite, slow growth rate, and sometimes a dull dry hair coat and potbelly appearance. A veterinarian or diagnostic laboratory can find eggs in the horse's feces. Treatment with Pyrantel pamoate (Strongid T) or Fenbendazole (Panacur) will kill the adult worms. If a heavy burden is suspected, Fenbendazole is the more appropriate choice. Ivermectin and Piperazine can be used to kill the larval stages of the worm.

Strongylus vulgaris infects the cecum and ventral colon of the horse. When large numbers of larvae invade the intestine, the horse may become clinically sick showing signs of fever, diarrhea, inappetence, weight loss, colic and eventually death. With chronic infections, intermittent recurrent colic is a major indicator of disease. This parasite can be diagnosed on fecal flotation tests and is best treated with ivermectin (Eqvalan) and moxidectin (Quest) for the larval stages, and oxibendazole (Anthelcide), fenbendazole (Panacur), or pyrantel pamoate (Strongid T) for adults.

Cyathostome forms cysts in the wall of the cecum and colon. As they emerge, they damage the lining and cause inflammation which can interfere with digestion and absorption of nutrients. The horse may show clinical signs including: anorexia, weight loss, diarrhea and colic. Severe emaciation can result with long term infections. This parasite can be diagnosed on fecal flotation tests and can be treated with ivermectin (Eqvalan), oxibendazole, pyrantel pamoate (Strongid T), and piperazine for the adults and ivermectin (Eqvalan), moxidectin (Quest) or pyrantel tartrate (Strongid C) for larvae.

Anoplocephala Perfoliata (Tapeworm) live at the end of the small intestine and in the large colon. Can be present in any age horse over 6 weeks of age and is usually asymptomatic (no visible signs). The horse may have a slower growth rate or seem to lose condition, and in some cases diarrhea and colic can occur. Portions of the worms may be visible by the naked eye in the horse's feces. This parasite is diagnosed with a fecal examination. Treatment consists of twice the normal dose of pyrantel pamoate (Strongid T).

Gasterophilus (Stomach Bots) is often asymptomatic, however can cause lesions in the mouth, esophagus and stomach which may make the horse reluctant to eat. Treatment with ivermectin (Eqvalan) or moxidectin (Quest) is recommended.

Oxyuris equi (pinworms) can affect all ages of horse. The most noticeable clinical sign will be anal itching. The animals will be restless and not eat properly, and a yellow/gray discharge may be noticed coming from the anus. This worm can be diagnosed by examining the discharge for eggs or "trapping" the worm with scotch tape. Fenbendazole (Panacur), oxibendazole (Anthelcide), pyrantel pamoate (Strongid T), piperazine and ivermectin (Eqvalan) against adults. In foals fenbendazole should be the drug of choice.

Strongyloides westeri (Threadworm) usually seen in young foals between 10 days and 6 months. The worms cause inflammation, edema and erosion of the small intestinal lining which can result in impairment of digesting and absorption of nutrients. Poor growth, depression and mild diarrhea can be present. The infection can be diagnosed by a standard fecal flotation test. The foals ingest this through the mare's milk, so treating the mare with ivermectin (Eqvalan) within one week of foaling will reduce infection. The foal should be treated with oxibendazole (Anthelcide) or ivermectin (Eqvalan) if this worm is suspected.

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