Potential of xenobiotic metabolizing enzymes as markers for tick resistance to ectoparasitic drugs

Abstract

Ticks are hematophagous ectoparasites that significantly impact cattle production systems in tropical and subtropical regions. Their control primarily relies on chemical compounds, which can be applied alone or in combination. However, the sustained use of chemical acaricides has often led to the emergence of resistant tick populations. Research efforts are being conducted to understand the mechanisms underlying tick resistance to ectoparasitic drugs, which could also be used as resistance markers. Studies on the expression and catalytic activities of different cholinesterases have been conducted to understand the mechanisms of tick resistance to organophosphates like chlorpyrifos. This work evaluated acetyl-cholinesterase (AChE) catalytic activity in larvae from five (5) tick populations with varying in vitro susceptibility against a commonly used combination of the pyrethroid cypermethrin (CPM) and the organophosphate chlorpyrifos (CPF). The in vitro activity of CPM+CPF was evaluated using the Adult Immersion Test (AIT). AChE activity was measured in tick homogenates through a spectrophotometric method. Tick populations were classified as highly susceptible (n=3) and low susceptible (n=2) against the combination. AChE activity (19.9 ± 4.75 nmol/min.mg) in highly susceptible populations was approximately two times higher (p<0.001) than in low susceptible tick populations (11.4 ± 4.51 nmol/min.mg). Interestingly, AChE activity in low susceptible populations was less sensitive to inhibition by CPF or its active metabolite, CPF oxon. These preliminary results encourage further research into identifying biomarkers of tick resistance to acaricides, particularly to organophosphates and pyrethroid compounds.