INHERITANCE OF LEVAMISOLE AND BENZIMIDAZOLE RESISTANCE IN AN ISOLATE OF HAEMONCHUS-CONTORTUS

Reciprocal crosses between an isolate of Haemonchus contortus resistan t to both benzimidazole and levamisole anthelmintics and a susceptible isolate were performed in order to determine the mode of inheritance of these resistances. F-1 and F-2 generations and parent isolates were assayed for susceptibility to thiabendazole and levamisole in vitro. For each drug all of the filial generations were intermediate in susce ptibility between the parent isolates, and analysis indicated that res istance was inherited as an incompletely recessive character determine d by more than one gene in each case. There was no evidence of materna l inheritance. Results of both the in vitro assays and in vitro select ion, followed by determination of sex ratio in the survivors, as well as studies on adult worms, provided no evidence for sex-linkage. This work illustrates that in vitro assays coupled with minimal studies in sheep are useful for determining inheritance of resistance, yet use fe wer experimental animals than traditional studies. (C) 1998 Australian Society for Parasitology. Published by Elsevier Science Ltd.