First Description of Acaricide Resistance in Populations of Rhipicephalus microplus Tick from the Lower Amazon, Brazil

SIMPLE SUMMARY: Ticks represent a major health and economical issue for cattle farmers since bovine are infested by the cattle tick Rhipicephalus microplui; This tick is responsible for direct economic loss and can be the vector of several diseases. Many acaricidal drugs are routinely used to control tick infestation, but tick populations are becoming resistant to different drugs. Our work evaluated for the first time the resistance of a tick population from the Lower Amazon region to three different and commonly used acaricidal drugs. We found that the tested tick population had a marked resistance to Cypermethrin and Deltamethrin and mild resistance to Amitraz. Farmers in the region should avoid the use of such acaricides to control ticks, especially Cypermethrin and Deltamethrin. ABSTRACT: There is limited information on the resistance to acaricidal drugs of the cattle tick Rhipicephalus microplus in the Lower Amazon region. Thus, we aimed to determine the efficiency of three widely used acaricide products (Amitraz, Cypermethrin, and Deltamethrin) in the control of this tick species. The adult immersion test was used on engorged female ticks sampled on farms in the Lower Amazon region, Brazil. For the test, homogeneous batches of 10 engorged females were placed in Petri dishes and immersed in the tested acaricidal drugs, using four replicates of each acaricide and three replicates as a negative control, immersed in distilled water. The acaricides were diluted as recommended by the manufacturers, and the ticks were submerged for five minutes. Mortality of engorged females, production of eggs, and percentage of larval hatching were evaluated. Tick mortality was 2.5%, 7.5%, and 0% for Amitraz, Cypermethrin, and Deltamethrin, respectively. The percentage of larval hatching was 53.7% for Amitraz, 88.7% for Cypermethrin, and 80.0% for Deltamethrin. As recommended by the FAO, for the acaricide to be considered effective, it must have a control rate ≥95%. Among the tested acaricides, Amitraz showed an efficacy of 90.5%, Cypermethrin 10.4%, and Deltamethrin 26.6%. Ticks from the lower Amazon showed marked resistance to Cypermethrin and Deltamethrin and mild resistance to Amitraz. This is the first report of acaricidal resistance in the region.