ARS | Publication request: First report of fipronil resistance in Rhipicephalus (Boophilus) microplus of Mexico

Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 9, 2010
Publication Date: N/A

Interpretive Summary: The southern cattle tick, Rhipicephalus (Boophilus) microplus, is a major impediment to the beef production in Mexico and while many Mexican ranchers use integrated pest management (IPM) methods to some extent, most must rely heavily on the chemical control of these ticks, especially in the southern and coastal regions of the country where the environmental conditions are ideal for the development of R. microplus. Because of the progressive development of resistance in Mexico, new classes of acaricides have been introduced. Fipronil and ivermectin are two relatively new chemicals used in Mexico for the control of R. microplus. Almost immediately after its introduction, reports of treatment failures to fipronil were reported. Additionally, resistance to ivermectin has been reported in Brazil. This study was initiated to determine the presence of resistance in R. microplus to 5 different classes of pesticide commonly used in Mexico. Fipronil was included to provide information supporting or refuting recent reports of treatment failures after fipronil application. Ivermectin was tested because that chemical has been used for the treatment of internal parasites of cattle for many years in Mexico and recently has become important for the control of R. microplus on cattle in Mexico and on wildlife in the border region of the United States. Here, we report on the detection of resistance found in 6 strains of the southern cattle tick collected in northern Mexico to include the first account of a strain of R. microplus with resistance to 4 classes of acaricide. Resistance to fipronil seems to be due in part to elevated esterase activity that was preselected for in Mexico by the widespread use of permethrin in the 1980s. However more work needs to be completed in order to know the true mechanism(s) of fipronil resistance. Control of southern cattle ticks in Mexico must rely on IPM, stressing resistance monitoring for correct pesticide use and rotation to delay the development of resistance to newer and older pesticides. This information benefits producers, scientists, and industry. The knowledge of what, where, and, how ticks develop resistance to important pesticides aids in the development of rational pest management programs, saves money, and increases the productivity of the industry.

Technical Abstract: Six strains of Rhipicephalus (Boophilus) microplus collected from northern Mexico were surveyed for resistance to 5 classes of acaricide. All were resistant to permethrin. Two strains were resistant to amitraz and 3 were found to be resistant to coumaphos. Two strains were resistant to fipronil including the El Zamora strain which was resistant to a total of 4 classes of acaricide. Selection with fipronil for 3 generations produced a resistance ratio of 8.3 and 9.4 at the LC50 and the LC99 estimates, respectively. Permethrin resistance in El Zamora was due to elevated esterase (CZEST9) and could be a contributing factor of resistance to fipronil since the Coazacoalcos strain, also containing elevated CZEST9, showed resistance to fipronil even though it was collected in Mexico 13 years earlier, before fipronil was on the market, and the fact that elevated CZEST9 is a rare mechanism for pyrethoid resistance in Mexico. The implications of resistance for the control of the southern cattle tick in the future are discussed.