Inheritance of cambendazole resistance in Haemonchus contortus

Fourth-molt larvae (M4) of cambendazole-resistant (CBZ-R) and cambendazole-susceptible (BPL) strains of Haemonchus contortus were recovered from donor lambs and inoculated orally into source lambs to effect mating in 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 parenteral strains (CBZ-R and PBL). 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% to 42%, indicating that there had been no reversion to susceptibility.