A mapping of Rhipicephalus microplus resistance to acaricides in the littoral region of Argentina.

The aim of this work was to perform an analysis of the geographical dispersion of Rhipicephalus microplus resistance to acaricides in the littoral region of Argentina, which is the most favorable area for the development of this tick in the Argentinean territory. The data were obtained through the application of in vitro bioassays. Larval immersion test (LIT) were the bioassays used to detect resistance to ivermectin and fipronil, while adult immersion tests (AIT) were employed for the diagnosis of resistance to pyrethroids, amitraz and the combinations of cypermethrin with organophosphates. Cases of resistance to ivermectin were determined in localities from all the provinces of the littoral region (except for Formosa province). Samples from Corrientes, Entre Rios, and Santa Fe provinces were also determined as resistant to fipronil. Resistance to cypermethrin was a common result for all the tested samples. Samples from Corrientes and Santa Fe provinces were determined as resistant to amitraz and to the combinations of cypermethrin with organophosphates, and samples from Entre Rios province were diagnosed as resistant to amitraz. Muti-resistant population of R. microplus were detected in three provinces. The widespread dissemination of R. microplus resistance to acaricides in the littoral region of Argentina may be associated with a high frequency of annual chemical treatments, which is a consequence of different factors as ecological conditions very favorable to R. microplus, the free movement of cattle without mandatory tick control between farms within the tick infested area, and the presence of cattle breeds susceptible to tick infestation.