Effects of alternation of drug classes on the development of oxibendazole resistance in a herd of horses.

Anthelmintic schedules that alternate between drug classes are widely used in horses. However, the results of investigations in which ovine nematode parasites were used have established that alternation of drug classes does not delay the development of drug resistance. This field study was designed to assess the effect of alternation of drug classes on the development of oxibendazole (OBZ) resistance in benzimidazole (BZD)-resistant equine small strongyles. A privately owned herd of horses was used for this study. These animals grazed the same pasture and had been treated with the same anthelmintic schedule for several years. Prior studies had determined that the animals were parasitized with BZD-resistant small strongyles. The herd was divided into 2 groups of 20 and 21 horses. Group-1 horses were given OBZ 6 times a year and had a total of 25 treatments with the drug. Group-2 horses were given OBZ 3 times a year, in alternation with non-BZD drugs, and had a total of 16 treatments. Statistical analysis revealed no difference between groups for the change in pretreatment fecal egg count following treatments 1, 4, 7, 10, 13, and 16. Alternation of drug classes did not effect the development of drug resistance in this trial. The efficacy of OBZ decreased with repeated treatments with the drug. By treatment 13, the efficacy, as evaluated by the fecal egg count reduction test, had decreased to less than 70% in both groups.