Fenbendazole: Better Treatment for Fighting Resistant Ascarids (AAEP 2010)

Foal owners need to be especially mindful of Parascaris equorum, commonly known as ascarids, the most dangerous worms found in these youngsters, according to D. Craig Barnett, DVM, senior equine technical services veterinarian for I
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Foal owners need to be especially mindful of Parascaris equorum, commonly known as ascarids, the most dangerous worms found in these youngsters, according to D. Craig Barnett, DVM, senior equine technical services veterinarian for Intervet-Schering Plough Animal Health (ISPAH). He reported that ascarid populations at several breeding farms have developed resistance to ivermectin and moxidectin1, drugs which are commonly used to treat these worms, and researchers are trying to determine the best way to deal with the problem.

Fenbendazole, he said, might be the answer researchers have been looking for. At the 2010 American Association of Equine Practitioners Convention, held Dec. 4-8, 2010, in Baltimore, MD, he discussed the results of a recent study carried out by Craig R. Reinemeyer, DVM, PhD, president of East Tennessee Clinical Research; colleague Julio C. Prado, DVM; and Wendy E. Vaala, VMD, Dipl. ACVIM, equine technical services specialist for ISPAH. Reinemeyer and his team inoculated 16 foals with an ivermectin- and moxidectin-resistant strain of ascarids and divided the foals into two groups, then treated one group with fenbendazole paste (10 mg/kg) on days 11 through 15 post-infection, and the other with ivermectin on day 15.

After two and a half months, when the ascarids had matured and begun to lay eggs, the researchers performed fecal egg count reduction tests and total worm counts to compare the two regimens.

They found that the foals treated with a five-day fenbendazole regimen had significantly lower egg counts than ivermectin-treated foals. Ivermectin exhibited minimal efficacy against the resistant population (as expected), whereas fenbendazole reduced the egg counts by 99.5%. The number of adult ascarids in the foals treated with fenbendazole was also significantly lower; there was a 96.3% reduction of adult worms compared to the foals treated with ivermectin

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Erica Larson, former news editor for The Horse, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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