Susceptibility of Rhipicephalus microplus tick populations from western Pará to different acaricides.

The continuous and inappropriate use of acaricides for the control of the cattle tick has led to the emergence of populations resistant to different acaricides. Considering the scarcity of studies in western Pará, the aim of this study was to evaluate the susceptibility of Rhipicephalus microplus populations to doramectin, ivermectin, and amitraz. Engorged females were collected from cattle originating from Novo Progresso, Uruará, Rurópolis, and Mojuí dos Campos and incubated in batches until oviposition. The larvae obtained were then used in the Larval Packet Test (LPT) and the Porto Alegre (POA) susceptible reference strain was tested concurrently using the same impregnated paper packets. Lethal concentrations (LC₅₀ and LC₉₅) were estimated using PROBIT analysis and resistance ratios (RR₅₀) were calculated. Resistance to ivermectin was detected in Novo Progresso (RR₅₀ = 3.1), while Uruará and Mojuí dos Campos were susceptible. For doramectin, incipient resistance was detected in Novo Progresso (RR₅₀ = 1.71) while Rurópolis and Uruará were susceptible. Regarding amitraz, Mojuí dos Campos tick population was resistant (RR₅₀ = 9.66), while Novo Progresso was susceptible to this acaricide. The present study reveals heterogeneous susceptibility profiles of R. microplus populations from western Pará to amitraz, ivermectin, and doramectin. To the best of our knowledge, this is the first study to evaluate the susceptibility of R. microplus populations to macrocyclic lactones from western Pará.