ARS | Publication request: First report of the cattle tick Rhipicephalus microplus resistant to ivermectin in Mexico

Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 25, 2009
Publication Date: February 26, 2010
Citation: Miller, R. 2010. First report of the cattle tick Rhipicephalus microplus resistant to ivermectin in Mexico. Veterinary Parasitology. 168:165-169.

Interpretive Summary: Cattle fever ticks were removed from the United States through an eradication campaign that began in 1906 and continues today. The success of the eradication program is based heavily on the use of pesticides to quickly eradicate populations of ticks discovered within the United States close to the Mexican border. This study describes the discovery of ticks resistant to a commonly used pesticide, ivermectin. Ivermectin is an important chemical used as a treatment for tick-infested deer which have become a complicating factor in the eradication of ticks because of their ability to move long distances and difficulty to treat for tick infestation. By knowing the level of ivermectin resistance and the distribution of ivermectin-resistant ticks in Mexico, we can better assess the risk this resistance places on the USDA, APHIS, Cattle Fever Tick Eradication Program and, ideally, make adjustments in order to maintain the integrity of the program.

Technical Abstract: 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 LC50 and LC99 estimates. The LUADY (RR50: 2.04 and 2.29, RR99: 2.67 and 3.55), SPN (RR50: 3.55 and 3.68, RR99: 8.19–11.06) and SFDO (RR50: 6.84 and 8.59, RR99: 54.17 and 87.86) ticks had significantly higher LC50/LC99 than the reference susceptible Deutch strain, demonstrating resistance in the field-collected populations. Furthermore, there was significant difference between LC50/LC99 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 LC50 nor LC99 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.