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 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.

According to the researchers, a 5-day regimen of fenbendazole (10 mg/kg) has proven to be "highly effective" against ivermectin-resistant ascarids.

The team noted that the 5-day course of fenbendazole "is not recommended as the sole approach to routine ascarid management, but is an effective tool in the face of ivermectin-resistant ascarids."

Reinemeyer added, "Another important application of this regimen is that it is larvicidal, meaning that it kills ascarids while they're still migrating through the liver and lungs. Larval infections cannot be detected by any known tests, and this is probably the stage at which foals transmit infections from one farm to another."

This poses a particular problem for breeding farms where the mares are transported to another premise for breeding: "Their foals-at-side can pick up a resistant strain of ascarids at the breeding farm, and then take it back home. Even if they are treated with ivermectin upon their return, it will have no effect on the developing infection because that strain is ivermectin-resistant. Once those worms mature and begin to lay eggs--which is around 75 days post-infection--the new farm will be contaminated with a resistant strain."

The fenbendazole treatment provides a way to prevent introduction of resistant strains to previously naive farms, Reinemeyer added.


1 Moxidectin is not approved in the U.S. for use in foals less than 6 months of age. But, because ascarids can be found in young horses up to about 18 months, it could be used against resistant worms in weanlings and yearlings, Craig R. Reinemeyer, DVM, PhD, said.

About the Author

Erica Larson, News Editor

Erica Larson, news editor, 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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