BENZIMIDAZOLE RESISTANCE IN A FIELD POPULATION OF HAEMONCHUS-CONTORTUS FROM SHEEP IN KENYA

A controlled anthelminitic efficacy test was used to control results o f faecal egg count reduction test obtained earlier from a field survey where field strains of Haemonchus contortus were suspected to be resi stant to thiabendazole (TBZ), fenbendazole (FBZ) and levamisole (LEV). Worm-free sheep (32) were experimentally infected with 2 strains of H . contortus (M1-3) and 3 anthelmintics were tested. There was 85 and 4 3% reduction, as compared with controls, in mean worm burdens (MWB) of sheep infected with M1 strain and killed 5 days after treatment with TBZ and FBZ, confirming their resistance to these anthelmintics. TBZ h ad an efficacy of 98% in sheep infected with M2 strain but FBZ reduced MWB of the same strain in 70% indicating a moderate level of resistan ce to this anthelmintic. There was a 96% reduction in MWB of sheep inf ected with the suspected LEV-resistant M3 strain and then treated with LEV, indicating that this strain was not resistant to LEV. There is a need to determine the anthelmintic efficacies of a gastrointestinal n ematode population in any investigation on farms where chemotherapeuti c control measures against helminthiasis appear to have failed.