Submitted to: Journal of Medical Entomology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/20/2009
Publication Date: 11/5/2009
Citation: Temeyer, K.B., Olafson, P.U., Miller, R. 2009. Genotyping mutation in BmAChE3: A survey of laboratory and Mexican strains of Rhipicephalus (Boophilus) microplus that are resistant or susceptible to coumaphos. Journal of Medical Entomology. 46(6):1355-1360. Interpretive Summary: The southern cattle tick was eradicated from the United States after a cooperative program between the states and federal government that lasted for nearly half a century. These ticks are still widespread in Mexico and can infect cattle with two microorganisms that cause fatal diseases in naïve cattle. The ticks are kept out of the United States by a program that treats and inspects all cattle imported from Mexico, but the ticks are becoming increasingly resistant to the pesticides used to treat the cattle. Coumaphos, the main pesticide used to treat imported cattle from Mexico, targets an enzyme called acetylcholinesterase in the central nervous system of the ticks. Susceptible ticks are killed due to interference with normal nervous system functioning, but as the enzyme becomes insensitive to coumaphos, it takes higher concentrations to kill them. Tick resistance to pesticides, including coumaphos, is widespread in Mexico. There are not many alternative pesticides available to treat the imported cattle, so coumaphos resistance in the Mexican ticks is a threat to the U.S. cattle industry. Southern cattle ticks contain three genes that specify three different acetylcholinesterases, and the main target of coumaphos has not been previously determined. A recent study was completed to determine if six mutations in one of the three genes for tick acetylcholinesterase might be responsible for the resistance to coumaphos. Ticks from laboratory strains and from other strains collected in Mexico that were known to be either resistant or susceptible to coumaphos were genotyped to determine if they contained the mutations in their acetylcholinesterase, BmAChE3. Results of the study determined that distribution of the mutations in the resistant and susceptible tick strains did not appear to correspond to the resistance or susceptibility to coumaphos suggesting that one of the other acetylcholinesterases may be the primary target of coumaphos in susceptible tick strains.
Technical Abstract: BmAChE3 mutations I48L, I54V, R86Q, V137I, I492M, and T548A were previously identified in the organophosphate (OP) acaricide-resistant San Román strain of Rhipicephalus (Boophilus) microplus. Recombinant BmAChE3 acetylcholinesterase containing the R86Q mutation was shown to exhibit nearly 20-fold reduction in the rate of phosphorylation by paraoxon relative to the wild type sequence. In addition, the R86Q mutation was present in resistant laboratory strains at elevated frequency compared to OP-susceptible strains, but was insufficient to alone generate the OP-resistant phenotype (Temeyer et al. 2007. J. Med. Entomol. 44: 1013-1018). Here, we developed assays to genotype the remaining 5 mutations and evaluated frequency of all 6 BmAChE3 mutations in individual R. microplus ticks from laboratory and Mexican field-collected strains. We found a substantial number of individuals in known OP-susceptible strains that appeared to be homozygous for each of the mutations surveyed, the exception being I48L, which was infrequent in all strains, leading us to conclude that none of the mutations alone were responsible for generation of phenotypic resistance to OP acaricide.