Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/30/2005
Publication Date: 9/15/2005
Citation: Miller, R., Davey, R.B., George, J.E. 2005. First report of organophosphate-resistant Boophilus microplus (Acari: Ixodidae) within the United States of America. Journal of Medical Entomology. 45(2):912-917. Interpretive Summary: The Cattle Fever Tick Eradication Program (CFTEP) was established to keep the United States free of cattle ticks and their associated diseases which would be devastating to the U.S. Cattle industry. The CFTEP prevents the re introduction of cattle ticks through an intensive system of inspection and treatment of infested cattle bound for importation into the U.S. and along a 700 Km long quarantine zone between Texas and Mexico, from Del Rio southeast to Brownsville. In order to eradicate tick infestations, the CFTEP relies heavily on one chemical, the organophosphate coumaphos. Resistance to chemical pesticides is common in Mexico. Therefore, tick infestations found within the United States have been routinely screened for resistance for the past 10 years. Until now, all tick populations tested have been susceptible to all pesticides evaluated. This manuscript presents a description of the first case of pesticide resistant cattle ticks found within the U.S. The information gained from this research was significant because it showed that ticks with the level of resistance detected were still controllable under the current practices of the CFTEP. This information will save time and taxpayer=s money because a change to a different pesticide or method of eradication will not be necessary when the inspectors of the USDA, APHIS, Veterinary Services encounter this level of resistance in ticks.
Technical Abstract: Boophilus microplus collected from Starr County, Texas were determined to be resistant to the organophosphorous acaricides, coumaphos and diazinon, but susceptible to permethrin and amitraz. Bioassay results obtained from the initial collection (Arce1) made prior to the treatment of all cattle with coumaphos, showed that the slope (SE) of the probit regression was 3.96 (0.22) whereas the slope (SE) of the susceptible reference population (Muñoz) was significantly steeper at 6.97 (0.38). This in combination with resistance ratios (RR) of 3.6 (3.4 3.8), 5.0 (4.5 5.5), and 6.5 (5.4 7.7) at the LC 50, 90 and 99, respectively, indicated that the Arce1 collection was resistant to coumaphos. A second collection (Arce2) was made 12 d after all cattle in the infested pasture were treated with coumaphos. Bioassays on the Arce2 ticks produced a slope that was not significantly different from the susceptible population, but the RR (95% CI) at the LC50 was 4.1 (3.9 4.3). The Arce2 population was also determined to be resistant to diazinon, RR (95% C.I.) = 7.1 (6.5 7.7, 11.7 (10.3 13.3), 17.7 (14.5 21.5) at the LC50, 90, and 99, respectively. Interestingly, the slope (SE) of the Arce2 population exposed to diazinon was different to that of the reference strain, 2.98 (0.12) and 6.09 (0.35) for the Arce2 and the Muñoz, respectively. The fact that coumaphos treatment changed the slope for coumaphos resistance, but not for diazinon resistance indicated that the mechanism for coumaphos resistance was likely to be different than that of diazinon resistance. The high dose strategy used by the Cattle Fever Tick Eradication Program was able to eradicate a coumpahos resistant B. microplus population with a measured RR of 4 6, after just two treatments of coumaphos 12 d apart.