Resistance to the deworming drug fenbendazole can develop in small strongyles, according to a study reported in the October 2003 Parasitology Research. This news follows on the heels of a disturbing find that parasites may also develop resistance to the dewormer ivermectin, the most broad-spectrum deworming drug available for horses.
The first study was done in 2002 at the Gluck Equine Research Center, University of Kentucky, on yearling Thoroughbreds. It focused on the efficacy of the five-day regimen of fenbendazole (see April 2003). Researchers included 58 yearlings in the study, and dosages used ranged from 7 to 10 mg/kg.?? Only three of the yearlings had negative fecal strongyle egg counts after treatment.?? Reductions in egg counts at the high dosages ranged from 0 to 78%.????
The study was repeated in 2003 with 36 yearlings, all treated at the 10 mg/kg dose.?? Only one had a negative fecal-egg count after treatment, and that horse had been negative before as well.?? Mean reduction in egg count after treatment was only 22%.??Cultures of the fecals showed all eggs present were from small strongyles.??These results were felt to confirm fenbendazole resistance in the small strongyles on these farms. (See also ivermectin resistance, November 2003.)
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