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ARS Home » Plains Area » Kerrville, Texas » Knipling-Bushland U.S. Livestock Insects Research Laboratory » LAPRU » Research » Publications at this Location » Publication #358340
Title: Baculoviral expression of presumptive OP-resistance mutations in BmAChE1 of Rhipicephalus (Boophilus) microplus (Ixodida:Ixodidae) and biochemical resistance to OP inhibition

Author
item Temeyer, Kevin
item Schlechte, Kristie
item MCDONOUGH, WILLIAM - Non ARS Employee

Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/9/2019
Publication Date: 9/3/2019
Citation: Temeyer, K.B., Schlechte, K.G., McDonough, W.P. 2019. Baculoviral expression of presumptive OP-resistance mutations in BmAChE1 of Rhipicephalus (Boophilus) microplus (Ixodida:Ixodidae) and biochemical resistance to OP inhibition. Journal of Medical Entomology. 56(5):1318-1323. https://doi.org/10.1093/jme/tjz062.
DOI: https://doi.org/10.1093/jme/tjz062

Interpretive Summary: The southern cattle fever tick, Rhipicephalus (Boophilus) microplus, was eradicated from the United States under a program jointly administered between affected states and the federal government begun in 1906. These ticks transmit at least two highly fatal cattle diseases, bovine babesiosis and anaplasmosis. The ticks and cattle diseases that they transmit remain established throughout Mexico, Latin America, and South America, and incursions into the U.S. constitute a significant continuing threat to U.S. cattle producers. All cattle imported into the U.S. from Mexico are treated by dipping with an organophosphate (OP) acaricide and hand inspected for presence of ticks. Tick resistance is prevalent to most available acaricides, including coumaphos, the OP used in the cattle dip vats. Research established that OP-resistance in insects, spider mites, and other pests is produced by one or more genetic mutations in the neural enzyme, acetylcholinesterase (AChE), resulting in production of an altered AChE structure that is resistant to OP inhibition. Previous research found a large number of variations in tick AChE structure that were associated with the presence of OP resistance. We report baculovirus expression of recombinant tick AChEs, each containing a single resistance associated mutation, to determine their individual contribution to AChE insensitivity to OP inhibition. Surprisingly, none of the individual AChE mutations alone resulted in substantially reduced sensitivity to OP-inhibition, strongly suggesting that OPinsensitivity in the tick AChE enzyme, BmAChE1, is the result of multiple mutations, each contributing a small (approximately 2-fold or less) incremental decrease in sensitivity to OP inhibition. Further, an OP-insensitivity mutation (G119S) found in African mosquitoes transferred into recombinant tick BmAChE1, alone resulted in over 1000-fold decreased sensitivity to OP inhibition of AChE activity. This observation strongly suggests possible further increases in tick resistance to OP acaricide, which threatens the effectiveness of the U.S. importation barrier to prevent re-entry of cattle fever ticks from Mexico and other countries.

Technical Abstract: The southern cattle fever tick, Rhipicephalus (Boophilus) microplus, transmits bovine babesiosis and anaplasmosis, and is endemic to Mexico, Latin and South America. R. (B.) microplus infestations within the U.S. are a continuing threat to U.S. cattle producers. An importation barrier between Texas and Mexico keeps the ticks from re-entering the United States. All cattle imported into the U.S. from Mexico are dipped in an organophosphate (OP) acaricide and hand inspected for the presence of ticks. Tick resistance has developed to most available acaricides, including coumaphos, the OP used in the cattle dip vats. OP-resistance can result from one or more mutations in the gene encoding the enzyme acetylcholinesterase (AChE), which results in production of an altered AChE resistant to OP inhibition. Previous research reported a large number of BmAChE1 mutations associated with OP resistance. We report baculovirus expression of recombinant tick BmAChE1 (rBmAChE) enzymes containing a single resistance-associated mutation to assess their contribution to OP inhibition resistance. Surprisingly, none of the naturally occurring BmAChE1 resistance-associated mutations alone resulted in markedly reduced sensitivity to OP-inhibition, suggesting that OP-insensitivity in BmAChE1 is the result of multiple mutations, each contributing a small decrease in OP sensitivity. Further, an OP-insensitivity mutation (G119S) found in mosquitoes was expressed in rBmAChE1, which resulted in 500-2000-fold decreased sensitivity to OP inhibition. Recombinant BmAChE1 with the G119S mutation demonstrated the lack of any structural prohibition to high level OP-insensitivity; strongly suggesting possible increases in tick resistance to OP acaricide which would threaten the effectiveness of the U.S. importation barrier to ticks.