Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 10, 2007
Publication Date: March 25, 2008
Citation: Davey, R.B., Miller, R., George, J.E. 2008. Efficacy of amitraz applied as a dip against an amitraz-resistant strain of Rhipicephalus (Boophilus) microplus (Acari:Ixodidae) infested on cattle. Veterinary Parasitology. 152(1-2):127-135. Interpretive Summary: The U.S. Cattle Fever Tick Eradication Program has relied on the exclusive use of the organophosphate (OP) acaricide, coumaphos for almost 40 years as the primary method for eliminating infestations of Rhipicephalus (Boophilus) microplus ticks on cattle. However, the widespread development of OP resistance in Mexico has created a critical need to find alternative classes of acaricides to prevent re-establishment in the U.S. For many years the U.S. program has considered the formamidine acaricide, amitraz as a viable alternative to coumaphos, but recent reports of amitraz resistance in Mexico has caused the program to reconsider the use of amitraz in the U.S. Thus, a study was conducted to evaluate the efficacy of amitraz applied to cattle that were infested with a highly amitraz-resistant strain of ticks to determine the level of control that could be expected. Laboratory bioassay studies showed that selection pressure with amitraz applied to ticks during 13 out of 18 generations resulted in a 28.9-fold increase in the level of resistance. Additional on-animal trials were conducted in which cattle infested with the resistant strain of ticks were dipped at concentrations of 0.02, 0.044, and 0.096% active ingredient (AI) amitraz. Results showed that the level of control achieved against resistant ticks ranged from 34.6% at 0.02% AI to 81.2% at 0.096% AI amitraz. However, none of the dosages of amitraz provided the required 99% level of control that would be needed for use in the U.S. program. Therefore, it would be extremely unwise to recommend the use of amitraz in the U.S. program given the fact that amitraz resistance appears to be developing in Mexico.
Technical Abstract: Selection pressure with amitraz applied to Rhipicephalus (Boophilus) microplus (Canestrini) in 13 out of 18 generations resulted in a 28.9-fold increase in resistance level, indicating a shift in the phenotypic composition of the ticks from a heterogenous mixture of both susceptible and moderately resistant ticks at the beginning to a homogenous composition of predominantly resistant ticks at the end. Larvae of the last selected generation of resistant ticks were infested on cattle 3 separate times then the cattle were dipped in a total immersion vat charged at 0.02, 0.044, and 0.096% active ingredient (AI) amitraz. There was a dose related, though not always significant, response to amitraz whereby each increase in concentration produced somewhat higher adverse effects. The number of ticks per calf and index of fecundity (IF) of females recovered from cattle treated at 0.02 and 0.044% AI were not different from that of untreated ticks. However, female weight and egg masse weight of females recovered from cattle treated at 0.044% were lower than untreated females or females treated at 0.02% AI. All measured parameters obtained from the 0.096% AI treated group were significantly lower than all other groups, with the exception that the number of ticks per calf and IF values were not different from the other amitraz treated groups. Treatment at 0.02% AI produced significantly lower overall control (34.6%) than treatment at 0.096% AI (81.2%), while the level of control at 0.044% AI (57.6%) was intermediate between the other treated groups. However, none of the evaluated amitraz treatments provided the required 99% control that would be necessary for use in the U.S. Cattle Fever Tick Eradication Program. Regression analysis indicated a concentration of ca. 0.2% AI amitraz would be needed to achieve 99% control of the resistant ticks.