First documentation of ivermectin resistance in Rhipicephalus sanguineus sensu lato (Acari: Ixodidae)

The brown dog tick, Rhipicephalus sanguineus sensu lato (Latreille, 1806), is an ectoparasite and disease vector of significant veterinary and public health importance that is distributed widely around the world. The intensive use of synthetic acaricides for tick control exerts a strong selective pressure for brown dog ticks to become resistant to them. Here, we investigated claims from the field regarding treatment failure associated with the use of veterinary products containing ivermectin (IVM) to control brown dog ticks infesting dogs in Yucatan state, Mexico. Dogs in six state municipalities were inspected to sample 15 R. sanguineus s.l. populations. Interviews were conducted with dog owners to gather information on the history of dog treatment with conventional acaricides and IVM. The larval immersion test was used on the progeny of adult female ticks infesting dogs to test for susceptibility to IVM. Dose-mortality regressions, lethal concentrations (LC), their confidence intervals, and slope were estimated by probit analysis. Ten tick populations (66.7%) were classified as resistant compared with the most susceptible population. A high inter-population variation in the phenotypic level of IVM resistance was evident (resistance ratio at LC50% and LC99% ranged from 1.0 to 30.5, and from 1.0 to 458.8, respectively). Tick populations classified as resistant were collected from dogs known to have been treated with IVM. To our knowledge, this is the first report of IVM resistance in R. sanguineus s.l. worldwide. Veterinary and pet owner education on integrated tick management practices is required to avoid widespread resistance and increased treatment failure with products containing IVM and other macrocyclic lactones used to control endo- and ectoparasites affecting dogs. Integrated tick management will also help mitigate the burden of brown dog tick-borne diseases on human and animal populations.