Inheritance of avermectin resistance in Haemonchus contortus

A larval development assay was used to compare the responses of the Chiswic k Avermectin Resistant (CAVRS) isolate of Haemonchus contortus, an avermect in-susceptible isolate (VRSG) and their crosses to avermectins, The F-1 and F-2 generations of reciprocal crosses between CAVRS and VRSG were denoted as CAVRS males x VRSG females = CXV, and VRSG males x CAVRS females = VXC. The levels of avermectin resistance in the developing larvae of the F-1 of both CXV and VXC were indistinguishable from that in the avermectin-resista nt parent, indicating that the resistance trait is completely dominant. Ave rmectin dose-response curves for the CXV F-1 did not show a 50% mortality r ate at low concentrations, indicating that avermectin resistance is not sex -linked. This conclusion was confirmed when adult male worms of the F-1 of the CXV mating were found to have survived treatment of the host with 200 m u g kg(-1) ivermectin. This dose rate (200 mu g kg(-1) ivermectin) caused a 50% reduction in the number of adult males in the F-1 from both CXV and VX C crosses, but only a non-significant reduction in the number of adult fema les in the F-1. Dose-response curves obtained for the F-2 generations in th e larval development assay indicated the presence of 25% of avermectin-susc eptible individuals, suggesting that a single major gene largely controls t he avermectin-resistance trait. This genetic analysis of avermectin resista nce in an Australian H. contortus isolate indicates that the expression of the gene for avermectin resistance is an autosomal complete dominant in the larvae; however, in adults its expression is sex-influenced, with males ha ving a lower resistance to avermectin than females. (C) 2000 Australian Soc iety for Parasitology Inc, Published by Elsevier Science Ltd. All rights re served.