ANTHELMINTIC RESISTANCE ON PLEASURE HORSE FARMS IN NORTH CENTRAL FLORIDA

One hundred and ninety-two horses on 15 privately owned pleasure horse farms in north central Florida were tested for the presence of worms resistant to either fenbendazole or ivermectin. On each farm, horses were divided into three groups of not more than 10 horses each and given either ivermectin (200 mcg/kg) or fenbendazole (5 mg/kg) or left untreated as controls. Fecal egg counts were performed on the day of treatment and at 2, 4, and 8 weeks following treatment, using a modified McMaster technique with a minimum sensitivity of 25 eggs per gram (EPG) of feces. Mean fecal egg counts of ivermectin-treated horses on all farms combined were reduced from a pretreatment value of 428 EPG by an average of 99, 98, and 77% at 2, 4, and 8 weeks after treatment, respectively. Mean fecal egg counts of fenbendazole-treated horses on all farms combined were reduced from a pretreatment value of 456 EPG by an average of 52, 61, and 53% at 2, 4, and 8 weeks after treatment, respectively. Ivermectin was fully effective on all 15 farms whereas fenbendazole was judged to be fully effective on only one of the 15 farms.