Veterinary Parasitology 190 (2012) 210–215 Contents lists available at SciVerse ScienceDirect Veterinary Parasitology jou rn al h om epa ge: www.elsevier.com/locate/vetpar Ivermectin resistance status and factors associated in Rhipicephalus microplus (Acari: Ixodidae) populations from Veracruz, Mexico A. Fernández-Salas a,b , R.I. Rodríguez-Vivas b , M.A. Alonso-Díaz a,∗ , H. Basurto-Camberos a a Centro de Ense˜ nanza, Investigación y Extensión en Ganadería Tropical, Facultad de Medicina Veterinaria y Zootecnia, Universidad Nacional Autónoma de México, Km. 5.5 Carretera Federal Tlapacoyan-Martínez de la Torre, C.P. 93600, Martínez de la Torre, Veracruz, Mexico b Facultad de Medicina Veterinaria y Zootecnia, Universidad Autónoma de Yucatán, Km. 15.5 Carretera Mérida-Xmatkuil, Mérida, Yucatán, Mexico a r t i c l e i n f o Article history: Received 6 March 2012 Received in revised form 27 May 2012 Accepted 5 June 2012 Keywords: Rhipicephalus microplus Resistance Macrocyclic lactones Associated factors a b s t r a c t The objectives of the present study were to (1) determine the status of resistance or sus- ceptibility to ivermectin (IVM) in Rhipicephalus microplus tick populations in Veracruz, Mexico, (2) determine the level of resistance (resistance ratios [RR] and lethal concen- trations for 50% [LC 50 ] and 99% [LC 99 ]) mortality in each R. microplus population, and (3) identify factors associated with resistance. Populations of R. microplus were sampled from 53 cattle farms to evaluate their resistance using the larval immersion test. Mor- tality data were subjected to probit analysis to calculate LC 50 and LC 99 . Resistance ratios were calculated in relation to a susceptible reference strain. A logistic regression model was used to evaluate the relation between resistance and possible associated factors. Thir- teen tick populations were susceptible to ivermectin, eighteen had incipient resistance and twenty-two had significant resistance. RR 50 of the susceptible tick populations varied from 0.59 to 1.07. The populations that showed the highest level of resistance were: ANTE (RR 50 = 8.21; RR 99 = 46.0), PALO (RR 50 = 6.25; RR 99 = 35.47), P.VIE (RR 50 = 5.89; RR 99 = 180.3), AURO (RR 50 = 5.36; RR 99 = 13.82 and CEDR (RR 50 = 4.11; RR 99 = 26.47). Cattle farms that used macrocyclic lactones ≥4 times per year were more likely to develop R. microplus resistant to ivermectin (OR = 13.0; p = 0.0028). In conclusion, more than two-thirds of the farms sampled in Veracruz, Mexico, showed some level of ivermectin-resistant R. microplus populations and the number of ML applications per year is factor associated with the resistance of R. microplus to IVM. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Rhipicephalus microplus is the major threat to the cattle industry in tropical, subtropical and temperate areas of the world. Chemical acaricides have played an essential role in its control, but their intensive use has favored the devel- opment of resistant populations (Rodríguez-Vivas et al., 2006a,b). Macrocyclic lactones (MLs) have emerged as an ∗ Corresponding author. Tel.: +52 232 3243941; fax: +52 232 3243943. E-mail addresses: alonsodm@unam.mx, alonsodma@hotmail.com (M.A. Alonso-Díaz). alternative to mitigate the negative effects of ticks, includ- ing tick populations resistant to most acaricides (Lanusse et al., 1997). Macrocyclic lactones are endectocides derived from the actinomycetes Streptomyces avermitilis (aver- mectins) and S. cyaneogriseus (milbemicins) that are used for the control of gastrointestinal nematodes (GINs) and ectoparasites (Lifschitz et al., 2002; Sumano and Ocampo, 2006). In Mexico, the pharmaceutical industry reported that IVM is the preferred anthelmintic to control GINs in ruminants and it is also used to control cattle ticks (Soberanes, 2010). However, as an effect from its inten- sive use, reports of R. microplus resistant to IVM have been documented in Latin America. 0304-4017/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.vetpar.2012.06.003