Benzimidazole resistance in small strongyles of horses.

Treatments with mebendazole, cambendazole or febantel were ineffective in reducing the faecal egg count of a group of 40 horses infected with small strongyle species. Evidence was also obtained that this apparent resistance extended to fenbendazole. In horses dosed with morantel tartrate egg counts were reduced to zero by 7 days after treatment. The development of resistance is discussed in relation to previous treatment of the horses and the mode of action of febantel.