Phenotypic, biochemical, and genomic analysis of organophosphate resistance in Rhipicephalus microplus from the state...

The cattle tick Rhipicephalus (Boophilus) microplus is a major ectoparasite of livestock, causing significant economic losses globally, with an estimated annual impact exceeding USD 3 billion in Brazil. Understanding the mechanisms of acaricide resistance is crucial to improve tick control strategies. This study investigated organophosphate (OP) resistance in ten R. microplus field strains from the Vale do Paraíba Region, São Paulo State, Brazil, through a combined approach integrating larval packet tests (LPT), biochemical enzyme inhibition assays, and sequencing of AChE1 and AChE2. Bioassays revealed high resistance levels to chlorpyrifos in most field strains. Biochemical tests using propoxur demonstrated significantly reduced acetylcholinesterase (AChE) inhibition in resistant strains, while sequencing identified 33 amino acid substitutions, including nine novel variants, distributed across AChE isoforms. The integration of phenotypic, biochemical, and genomic data shows that OP resistance in R. microplus from this region is primarily linked to target-site insensitivity, though the complexity of AChE polymorphism suggests multiple interacting mechanisms. These findings provide a comprehensive framework for resistance monitoring, reinforce the need for integrated control strategies, and highlight the importance of continued molecular surveillance to support sustainable tick management in Brazil.