Management of anthelmintic resistance: inheritance of resistance and selection with persistent drugs

Abstract

Resistance to the benzimidazole (BZ) anthelmintics is inherited as an incomplete dominant/ incomplete recessive trait and is now widespread in populations of gastrointestinal nematode parasites of sheep. Unlike benzimidazole resistance, which is common in Haemonchus contortus, Trichostrongylus colubriformis and Ostertagia circumcincta, resistance to levamisole is relatively rare in H. contortus, although common in the other 2 species. One explanation for the slow spread of resistance to levamisole in H. contortus is that it is inherited as an autosomal recessive trait, while in T. colubriformis levamisole resistance is inherited as a recessive sex-linked trait. With the introduction of the avermectin/milbemycin class resistance has developed to the relatively short-acting ivermectin, but this time it is inherited as a completely dominant trait. The potentially more serious situation of a persistent anthelmintic selecting a dominant resistance gene was investigated using a simulation model. Efficacy against incoming infective larvae (L3) was assumed to decline or remain high over the period of drug persistence (3 days to 4 weeks), thus allowing the estimation of the relative importance of selecting resistant L3s on the development of resistance in the worm population. These factors were also examined against a background of initial efficacy levels, against adults, and mode of inheritance. Persistence and initial efficacy were found to be far more important in determining the rate of selection for resistance than was selection of resistant L3 as drug efficacy declined.