High resistance of Rhipicephalus microplus to acaricides in the Recôncavo baiano, state of Bahia, Brazil

This research aimed to quantify resistance levels to cypermethrin, chlorpyrifos, amitraz, ivermectin, and fipronil in Rhipicephalus microplus larvae from cattle farms in the Recôncavo baiano region of the State of Bahia, Brazil. We also identified associations between resistance level and operational factors. Engorged adult female ticks were collected from 14 farms to perform bioassays for the five acaricide classes and determine the intensity of resistance. A nonparametric Mann-Whitney U test was employed to identify categorical factors associated with resistance intensities. A high frequency of resistance was observed across all five compounds evaluated, even on farms that reported never using these products. On all farms, we found tick populations resistant to at least three acaricides. While cypermethrin, chlorpyrifos, and ivermectin showed the highest intensity of resistance (RR50: 38.06, 126, and 27.35, respectively), amitraz and fipronil showed the lowest (RR50: 11.28 and 1.06, respectively). In the Recôncavo baiano region, the incorporation of amitraz and fipronil in the formulation of strategic control protocols is recommended.