Submitted to: Journal of Medical Entomology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/1/2003
Publication Date: 7/1/2003
Citation: LI, A.Y., DAVEY, R.B., MILLER, R.J., GEORGE, J.E. RESISTANCE TO COUMAPHOS AND DIAZINON IN BOOPHILUS MICROPLUS (ACARI: IXODIDAE) AND EVIDENCE FOR THE INVOLVEMENT OF AN OXIDATIVE DETOXIFICATION MECHANISM. JOURNAL OF MEDICAL ENTOMOLOGY. 2003. v. 40(4). p. 482-489. Interpretive Summary: Coumaphos is the only acaricide approved for use in dipping vats at the U.S. ports of entry for Cattle Fever Tick Eradication Program. This acaricide plays a pivotal role in preventing the reintroduction of the cattle fever tick from Mexico through cattle importation. Resistance to this and other organophosphate acaricides posses a major threat to the continued success of the Cattle Fever Tick Eradication Program. A study was conducted to determine the levels of resistance to coumaphos and diazinon in Mexican ticks and to investigate the mechanisms of resistance. Ticks were collected from various areas of Mexico where failures of control have been reported. The levels of resistance were measured using the Larval Packet Test. Tick larvae from several Mexican locations demonstrated up to 10-fold resistance to coumaphos, and up to 34-fold resistance to diazinon. Bioassays using various synergists revealed a specific metabolic mechanism that may have contributed to the observed resistance to coumaphos in resistant ticks. The results of this study underscore the potential risk of tick resistance to coumaphos in Mexico to the U. S. Cattle Fever Tick Eradication Program. The findings from this study on the metabolic mechanism of resistance to coumaphos may lead to development of a new biochemical assay for detecting coumaphos resistance.
Technical Abstract: The levels of resistance to coumaphos and diazinon in several Mexican strains of Boophilus microplus (Canestrini) were evaluated using the FAO larval packet test. Metabolic mechanisms of resistance were investigated with synergist bioassays. Piperonyl butoxide (PBO) reduced coumaphos toxicity in susceptible strains, but synergized coumaphos toxicity in resistant strains. An enhanced cytochrome P450 monooxygenase (cytP450)-mediated detoxification mechanism may exist in the resistant strains, in addition to the cytP450-mediated metabolic pathway that activates coumaphos. PBO failed to synergize diazinon toxicity in resistant strains, suggesting the cytP450 involved in detoxification were specific. Bioassays with triphenylphosphate (TPP) failed to show a correlation between TPP synergism ratios and the LC50 estimates for either acaricide. Esterases may not play a major role in resistance to those acaricides in those strains. Bioassays with diethyl maleate (DEM) revealed a significant correlation between DEM synergism ratios and LC50 estimates for diazinon, suggesting a possible minor role for glutathione S-transferases in diazinon detoxification. Resistance to coumaphos in the Mexican strains of B. microplus was likely to be conferred by both a cytP450-mediated detoxification mechanism described here and the mechanism of insensitive acetylcholinesterases reported elsewhere.