Engorged females of Rhipicephalus microplus were collected from 30 cattle farms in Yucatan, Mexico to evaluate ivermectin resistance. The larval progeny of each tick sample were produced in laboratory and evaluated using the larval immersion test to obtain the larval mortality. Concentration-mortality data were subjected to probit analysis to generate lethal concentrations (LC). Resistance ratio (RR) of each tick sample was calculated by dividing its LC with that of an ivermectin-susceptible strain (Deutch). Field populations of R. microplus demonstrated various levels of resistance to ivermectin. The top three resistant populations were CHPAT (RR(50)=10.23, RR(99)=179.6), SDGO (RR(50)=7.37, RR(99)=115.3) and FND (RR(50)=7.09, RR(99)=50.22). The level of ivermectin resistance in most tick samples from cattle farms in Yucatan, Mexico were relatively low. However, the intensive use of macrocyclic lactones to control both endo- and ectoparasites in the region will likely lead to a more serious resistance problem that may cause control failure in the future.
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