Toxicological and biochemical characterization of coumaphos resistance in the San Roman strain of Boophilus microplus (Acari: Ixodidae)

TheSanRomanstrainof the southern cattle tick,Boophilusmicroplus, collected fromMexicowaspreviously reported to have a high level of resistance to the organophosphate acaricide coumaphos. An oxidative detoxification mechanism was suspected to contribute to coumaphos resistance in this tick strain, as coumaphos bioassay with piperonyl butoxide (PBO) on larvae of this resistant strain resulted in enhanced coumaphos toxicity, while coumaphos assays with PBO resulted in reduced toxicity of coumaphos in a susceptible reference strain. In this study, we further analyzed the mechanism of oxidative metabolic detoxification with synergist bioassays of coroxon, the toxic metabolite of coumaphos, and the mechanism of target-site insensitivity with acetylcholinesterase (AChE) inhibition kinetics assays. Bioassays of coroxon with PBO resulted in synergism of coroxon toxicity in both the SanRoman and the susceptible reference strains. The synergism ratio of PBO on coroxon in the resistant strain was 4.5 times that of the susceptible strain. The results suggested that the cytP450-based metabolic detoxification existed in both resistant and susceptible strains, but its activity was significantly enhanced in the resistant strain. Comparisons of AChE activity and inhibition kinetics by coroxon in both susceptible and resistant strains revealed that the resistant SanRoman strainhadan insensitiveAChE,with a reducedphosphorylation rate, resulting in a reduced bimolecular reaction constant. These data indicate amechanismof coumaphos resistance in the San Roman strain that involves both insensitive AChE and enhanced cytP450-based metabolic detoxification. Published by Elsevier Inc.