Evaluation of mebendazole used concurrently with piperazine monohydrochloride in horses.

Forty horses from a herd known to have benzimidazole-resistant small strongyles were treated with mebendazole (8.8 mg/kg) or combinations of mebendazole and piperazine monohydrochloride (25, 40, or 55 mg of piperazine base/kg). Pretreatment and 7-day posttreatment fecal examinations were done. Fecal cultures and strongyle egg per gram (epg) counts, and in vitro testing for benzimidazole resistance were performed. Results of fecal examinations prior to treatment were similar in all horses, and results of testing were positive for benzimidazole resistance. Horses treated with mebendazole and piperazine at all dosages had much lower geometric mean strongyle epg counts (2 to 4) and greater percentage reduction in geometric mean strongyle epg counts (99.7 to 99.9) at 7 days after treatment, compared with those measurements for horses treated with mebendazole alone. Adverse reactions to treatment were not observed.