Status of Rhipicephalus microplus resistance to ivermectin, fipronil and fluazuron in Mato Grosso do Sul, Brazil.

Southern cattle tick resistance to pour-on and injectable acaricides has yet to be evaluated on a broader scope, and the paucity of information on the subject may hinder efforts to control this parasite. The objective of this study was to evaluate the resistance profile of ten populations of Rhipicephalus microplus to the acaricides fluazuron, fipronil and ivermectin in cattle herds in Mato Grosso do Sul, Brazil. The larval immersion test (LIT) was used to evaluate susceptibility to ivermectin and fipronil and the adult immersion test (AIT) was performed to evaluate fluazuron. Samples were randomly obtained in ten farms, and in general, we found resistance in five samples to fluazuron and in four samples to ivermectin and fipronil. Six samples showed incipient resistance to ivermectin and fipronil. Five of the ten evaluated samples showed resistance and/or incipient resistance to all the active ingredients, and the other five to two active ingredients. Among the samples classified as resistant, the average resistance ratio for ivermectin was 2.75 and 3.26 for fipronil. These results demonstrate the advanced status of resistance to the most modern chemical groups for the control of R. microplus in the state of Mato Grosso do Sul.