Role of metabolic enzymes in conferring resistance to synthetic pyrethroids, organophosphates, and phenylpyrazole compounds in Rhipicephalus microplus

The study was conducted to work out possible correlation of resistance with functional enzymes in tick isolates of Rhipicephalus microplus collected from two agro-climatic zones of Uttar Pradesh, India. Adult immersion test was adopted using previously established discriminating concentrations of synthetic pyrethroids (SP) (deltamethrin, cypermethrin), organophosphates (OP) (diazinon, coumaphos), and phenylpyrazole (fipronil) compounds against these isolates. Quantitative enzyme profiling of esterases, cytochrome P450, and glutathione S-transferase (GST) enzymes was done to correlate resistance and enzyme activity. All the four isolates were found to be resistant to SP having resistant factor (RF) of 2.9-18.05 and to one of the OP compounds, diazinon showing RF of 3.5-15.07 but susceptible to coumaphos (RF < 1.4). The RF values of the four isolates were also found between 0.8 and 1.36 showing susceptibility to phenylpyrazole compound (fipronil). The enzymatic profiling revealed elevation in beta-esterase, CYP450, and GST level in comparison to reference susceptible Indian veterinary research institute-I (IVRI-I) tick line. Significant elevated activity (p < 0.05) of beta-esterase was observed in Lucknow (LKO), Kanpur (KNP), and Moradabad (MBD) isolates while LKO, AMR, and MBD isolates showed significant increase in GST activity (p < 0.05 to p < 0.001). Although higher enzyme activity was recorded in some isolates having higher RF, positive correlation between RF and enzyme activity was not recorded, thus confirming the role of multiple mechanisms in conferring resistance in field isolates.