INHERITANCE OF CAMBENDAZOLE RESISTANCE IN HAEMONCHUS-CONTORTUS

Abstract

Fourth-molt larvae (M4) of cambendazole-resistant (CBZ-R) and cambendazole-susceptible (BPL) strains of H. contortus were recovered from donor lambs and inoculated orally into source lambs to effect mating in a 2nd lamb. Eggs produced by these parasitic matings were collected, cultured and inoculated into lambs to compare the anthelmintic activity of cambendazole (CBZ), given at a dose rate of 20 mg/kg, against the F1 adults with its activity against the 2 parental strains (CBZ-R and BPL). In 2 replications, CBZ was 94 and 99% efficacious against the progeny of the cross matings and 91 and 95% against the BPL strain; CBZ was only 39 and 42% efficacious against the CBZ-R strain. In 1 test of F2 progeny of the reciprocal matings, CBZ was 81.2% efficacious against the F2 adults, whereas CBZ was 97.5% efficacious against the BPL strain and 37% against the CBZ-R strain. The results indicated that heredity of resistance to CBZ in H. contortus is not sex-linked and is probably a result of a heterozygous recessive allele. The infective larvae of the CBZ-R groups in these 3 trials were from the 20th and 24th passages through lambs without exposure to any anthelmintics, yet the anthelmintic activity of CBZ ranged from 37-42%, indicating that there had been no reversion to susceptibility.