Investigation of intestinal nematode responses to naphthalophos and pyrantel using a larval development assay

Abstract

Responses of several nematode species to naphthalophos and pyrantel/levamisole were examined using a larval development assay in order to determine the potential of this assay for detection of resistance. Haemonchus contortus and Ostertagia circumcincta showed concentration-dependent responses to naphthalophos, however, the assay was unsuitable for Trichostrongylus colubriformis due to the low toxicity of the drug to the larval stages of this nematode. Measurement of concentration-dependent response to pyrantel in susceptible T. colubriformis was limited by a reduced toxicity against larvae at high drug concentrations, resulting in a parabolic response with a development–inhibition maxima of less than 100%. This limits the usefulness of the assay to detect pyrantel resistance in this species as the presence of a small resistant fraction in a field isolate may be indistinguishable from the parabolic susceptible response. On the other hand, responses of susceptible T. colubriformis to levamisole, and susceptible H. contortus to pyrantel and levamisole showed 100% development–inhibition over a range of drug concentrations, indicating that the appearance of a resistant fraction in a field population would be readily discernible from the susceptible response, allowing resistance detection for these drug/parasite combinations. This study has highlighted the varied suitability of the larval development assay technique for resistance detection with different combinations of drugs and parasite species.