THE MERCK VETERINARY MANUAL
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Parascaris sp in Horses

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Adult Parascaris equorum are stout, whitish worms, up to 30 cm long, with three prominent lips. The life cycle is similar to that of Ascaris suum (the roundworm of pigs, see Ascaris suumin Pigs), with a prepatent period of 10–12 wk. Large numbers of infective eggs can remain viable for years in contaminated soil. Adult animals usually harbor very few if any worms. The principal sources of infection for young foals are pastures, paddocks, or stalls contaminated with eggs from foals of the previous year.

In heavy infections, the migrating larvae may produce respiratory signs (“summer colds”). In heavy intestinal infections, foals show unthriftiness, loss of energy, and occasionally colic. Intestinal obstruction and perforation have been reported. Intestinal stages compete for absorption of essential amino acids. Diagnosis is based on demonstration of eggs in the feces. If disease due to prepatent infection is suspected, diagnosis may be confirmed by administration of an anthelmintic, after which large numbers of immature worms may be seen in the feces.

On farms where the infection is common, most foals become infected soon after birth. As a result, most of the worms are maturing when the foals are ~4–5 mo old. Treatment should be started when foals are ~8 wk old and repeated at 6- to 8-wk intervals until they are yearlings. All broad-spectrum equine anthelmintics are effective against the adult and immature worms in the small intestine and, therefore, ascarids are readily controlled by routine anthelmintic administration. However, there have been reports of resistance of P equorum to ivermectin and moxidectin in North America and Europe. Efficacy on any given farm should be monitored using a fecal egg count reduction test. In cases in which verminous pneumonia due to Parascaris migration has developed, therapeutic benefit may be achieved by treatment with ivermectin or fenbendazole (the latter at 10 mg/kg/day for 5 consecutive days) concurrent with appropriate antimicrobial therapy. Parascaris infection can be effectively prevented by daily administration of pyrantel tartrate once foals are eating grain regularly.

Last full review/revision October 2014 by Thomas R. Klei, PhD

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