Efficacy of combination anthelmintics against multiple resistant strains of sheep nematodes

Abstract

Broad-spectrum anthelmintics were tested for their efficacy in either single active or combination formulations against multiple anthelmintic-resistant strains of Haemonchus contortus, Teladorsagia circumcincta and Trichostrongylus colubriformis. Three efficacy trials based on post mortem worm counts showed that the triple and quadruple combinations of benzimidazoles, levamisole, macrocyclic lactones and closantel were effective against most strains that were resistant to each of the components when drenched individually. Consequently, use of any one of these classes of anthelmintics by itself against multiple-resistant strains would lead to a rapid increase in the level of resistance. When used in combinations, the anthelmintics reduced the survival rate of most strains to <5%. However, because some strains of T. circumcincta and T. colubriformis were found to be resistant to all combinations currently marketed in Australia, it is clear that combinations should be used before resistance levels become too high.