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Tapeworms in Horses

By Thomas R. Klei, PhD, Boyd Professor and Associate Dean for Research and Advanced Studies, School of Veterinary Medicine and Louisiana Agriculture Experiment Station, Louisiana State University

Three species of tapeworms are found in horses: Anoplocephala magna, A perfoliata, and Paranoplocephala mamillana. They are 8–25 cm long (the first usually being the longest, and the last the shortest). A magna and P mamillana usually are in the small intestine; A perfoliata is found mostly at the ileocecal junction, in the cecum, and in the ileum. The life cycle is similar to that of Moniezia spp in ruminants (see Tapeworms) and involves free-living oribatid mites as intermediate hosts. Diagnosis is by demonstration of the characteristic eggs in the feces, but because the discharge of proglottids is sporadic, a single fecal examination may not be diagnostic. In light infections, no signs of disease are present; in heavy infections, GI disturbances may be seen. Unthriftiness and anemia have been reported. Ulceration of the mucosa is quite common in the area of attachment of A perfoliata and has been suggested as one cause of intussusception. Intestinal perforation, peritonitis, and subsequent colic have been associated with Anoplocephala infections. Colic from disturbances of the ileocecal area is more likely in horses with tapeworm infections than in those not infected. Colic associated with tapeworm infections often recurs. The site of attachment of tapeworms frequently becomes secondarily infected or abscessed. Anoplocephala spp can be effectively treated with pyrantel salts; normal dosages (6.6 mg/kg) of pyrantel pamoate are 87% effective, while double the normal dosage is >93% effective. Daily administration of pyrantel tartrate (2.65 mg/kg) removes Anoplocephala spp. Praziquantel (0.75–1 mg/kg) is 89%–100% effective in the removal of A perfoliata. Praziquantel (at 1 mg/kg) appears to effectively remove P mamillana; pyrantel salts do not. Mixtures of the macrocyclic lactones ivermectin or moxidectin with praziquantel are available and are highly effective against A perfoliata.

On facilities where tapeworms are prevalent, clinical signs of tapeworm infections can be prevented by pyrantel salts administered daily during the grazing season, or by administration of effective oral anthelmintics within an interval deworming program. Treatment of horses according to the latter program immediately before turn out and at the end of the grazing season is likely to be most beneficial.