Survey of Rhipicephalus microplus resistance to ivermectin at cattle farms with history of macrocyclic lactones use in Yucatan, Mexico

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Abstract

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 (RR50 = 10.23, RR99 = 179.6), SDGO (RR50 = 7.37, RR99 = 115.3) and FND (RR50 = 7.09, RR99 = 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.

Introduction

Rhipicephalus (Boophilus) microplus (Canestrini) is an endemic pest of cattle in tropical and subtropical regions of the world, causing major economic losses to cattle producers through direct physical effects on the parasitized animal and indirectly through transmission of infectious disease agents such as Babesia bovis, B. bigemina and Anaplasma marginale (Solorio et al., 1999, Rodriguez-Vivas et al., 2004, Rodriguez-Vivas et al., 2005). In addition to the costs of chemicals, labor, equipment and production losses associated with treatment, the cost of maintaining tick boundaries is highly expensive (Nari et al., 2000).

Chemical acaricides have played an important role in controlling this damaging ectoparasite (Kunz and Kemp, 1994); however, as a consequence of its extensive use, R. microplus has developed resistance to all major classes of acaricides in several countries (George et al., 2004). Tick resistance is defined as a significant increase in the number of individuals within a tick population that can tolerate doses of drug(s) that have proved to be lethal for most individuals of the same species (FAO, 2004).

In Mexico, R. microplus resistance to chemical acaricides has rapidly developed in recent decades (Rodriguez-Vivas et al., 2007). Resistance to organophosphates (OP) acaricides first developed in the 1980s and resistance to synthetic pyrethroids (SP) emerged in the 1990s (Fragoso et al., 1995). Amitraz (Am) resistance in R. microplus from Mexico was confirmed in 2001 (Soberanes et al., 2002) and recently, Miller et al. (2008) reported the first case of R. microplus resistant to fipronil in Northern States of Mexico. Additionally, many tick populations have developed resistance to multiple classes of acaricides in Mexico (Miller et al., 2008, Rodriguez-Vivas et al., 2007). In the last 8–10 years, injectable and pour-on macrocyclic lactones (MLs, ivermectin, doramectin and moxidectin) have been used to control both gastrointestinal nematodes and cattle ticks in Mexico (Aguilar-Tipacamu and Rodriguez-Vivas, 2003, Rodriguez-Vivas et al., 2006a). Resistance to MLs has already been reported for gastrointestinal nematodes in bovines of several countries of the world (Borges et al., 2008); however, tick resistance to MLs has been rarely documented. The first reports of tick resistance to MLs were made in Brazil. Martins and Furlong (2001) reported cross-resistant R. microplus ticks to doramectin, ivermectin and moxidectin by in vivo test and recently, Klafke et al. (2006) also reported R. microplus resistant to ivermectin in the State of Sao Paulo. Due to the increasing MLs treatments to control parasites, resistance of R. microplus to ivermectin has been recently reported in Yucatan, Mexico (Perez-Cogollo et al., 2010).

Section snippets

Study background

The study was carried out from August 2008 to March 2009 in a tropical region of Mexico (Yucatan State). The state is located between 19°30′ and 21°35′ north latitude and 90°24′ west longitude of the Greenwich meridian. The climate of the state is sub-humid tropical with a summer rainy season. The monthly maximum temperature varies from 35 °C to 40 °C (mean 26.6 °C). The relative humidity (RH) varies from 65% to 100% (mean 80%) and the annual rainfall varies from 415 mm to 1290 mm depending on the

Results

The slopes, LC estimates of technical grade ivermectin to kill 50%, RR50 from 30 R. microplus field populations together with their respective CL95% are shown in Table 1. The LC estimates of technical grade ivermectin to kill 99%, RR99 with their respective CL95% are shown in Table 2. All 30 populations tested had significantly higher LC50 and LC99 estimates than the Deutch susceptible strain. However, with regard to RR50 and RR99 values, CHPAT (RR50 = 10.23, RR99 = 179.6), SDGO (RR50 = 7.37, RR99 = 

Discussion

Almost three decades have passed since the first MLs were introduced into the market; still they continue to be one of the major alternatives for bovine parasite control, especially in the cases of parasites resistant to other chemical groups (Borges et al., 2008). R. microplus resistant to MLs has been rarely documented worldwide; however, recently cases of ivermectin resistance were reported in southern Mexico (Perez-Cogollo et al., 2010). In this study various levels of ivermectin resistance

Acknowledgments

This project was funded by FOMIX-CONACYT (108773). We are very grateful to CONACYT-Mexico for supporting Luis Carlos Perez Cogollo in his Masters Degree at FMVZ-UADY. Thanks are also due to the farmers who generously gave their time and hospitality.

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