First report of the cattle tick Rhipicephalus microplus resistant to ivermectin in Mexico

Three cattle farms with ticks, Rhipicephalus microplus, thought to be resistant to ivermectin in Yucatan, Mexico were studied (SFDO, SPN, LUADY). Each field-population was collected and tested twice several months apart. The larval immersion test was used on the progeny of collected adult females to test the susceptibility to ivermectin. Dose-mortality regressions, lethal concentrations (LC), their confidence intervals and slope were estimated by probit analysis. Resistance ratios (RR) were determined in the three investigated populations at the LC 50 and LC 99 estimates. The LUADY (RR 50: 2.04 and 2.29, RR 99: 2.67 and 3.55), SPN (RR 50: 3.55 and 3.68, RR 99: 8.19–11.06) and SFDO (RR 50: 6.84 and 8.59, RR 99: 54.17 and 87.86) ticks had significantly higher LC 50/LC 99 than the reference susceptible Deutch strain, demonstrating resistance in the field-collected populations. Furthermore, there was significant difference between LC 50/LC 99 of the SFDO, SPN and LUADY tick populations, which indicates not only the presence of resistant populations, but also different levels of resistance to ivermectin in the field populations studied. There was no difference observed at the LC 50 nor LC 99 estimates at two different times of collection from any of the three populations studied. In conclusion, we report for the first time field populations of R. microplus resistant to ivermectin in Mexico.