Colorimetric assay for the detection of benzimidazole resistance in trichostrongyles.

A modified version of the aphid tile-test, which is used to detect insecticide resistance in single adult aphids that are resistant or susceptible to organophosphate or carbamate insecticides, was used to compare the levels of non-specific esterases in strains of the trichostrongyle nematodes Haemonchus contortus, Ostertagia circumcincta and Trichostrongylus colubriformis which were known to be resistant or susceptible to benzimidazole (BZ) anthelmintics. This colorimetric assay has shown that there is significantly more non-specific esterase in the infective-stage larvae of BZ-resistant strains than in susceptible strains and this may prove to be of use in the detection of resistance to benzimidazole anthelmintics.