Characterisation of an avermectin resistant strain of Australian Haemonchus contortus

Le Jambre L.F., Gill J.H., Lelane I.J. and Lacey E.

International Journal for Parasitology, 1995, 25, 691-698.

Publication Date: October 12, 1994


A strain of Haemonchus contortus (CAVR) isolated in Australia was found to be resistant to ivermectin (IVM) with 0.4 mg kg−1 of the anthelmintic failing to significantly reduce worm burdens. Resistance to IVM was sex-influenced in the CAVR strain with adult males showing a greater sensitivity to IVM. Cross resistance to moxidectin was evident with approximately 15% of the population surviving a dose of 0.1 mg kg−1. The free-living stages of the CAVR isolate had reduced sensitivity to avermectin (AVM) inhibition of development and motility. Similar structure-activity patterns and resistance factors were obtained for a series of related AVMs as inhibitors of larval development and L3 motility in CAVR and White River II, an IVM-resistant H. contortus isolate from South Africa. Further, both isolates were found to be 3 times more sensitive to paraherquamide than a susceptible H. contortus isolate. This suggests that the same resistance mechanism is operating in both isolates. The CAVR strain is susceptible to the benzimidazoles, levamisole and closantel.

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