Submitted to: American Journal of Veterinary Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 23, 2003
Publication Date: June 23, 2003
Citation: DAVEY, R.B., GEORGE, J.E., MILLER, R. J. EFFICACY OF VARIOUS CONCENTRATIONS OF COUMAPHOS TO CONTROL ADULT, NYMPHAL, AND LARVAL STAGES OF AN ORGANOPHOSPHATE-RESISTANT STRAIN OF BOOPHILUS MICROPLUS ON INFESTED CATTLE. AMERICAN JOURNAL OF VETERINARY RESEARCH. 2003. v. 64(6), p. 684-689. Interpretive Summary: The success of the Boophilus tick eradication program is primarily a result of the systematic dipping of all livestock in the organophosphorus (OP) acaricide, coumaphos. But, because of the high incidence of OP resistant ticks throughout Mexico, the continued success of the program is jeopardized because the concentrations of coumaphos used in the program may not be high enough to eliminate strains of ticks that have a high level of OP resistance. This study was conducted to determine the level of control that could be expected to occur at concentrations that were up to 2-times higher than the required concentrations presently used in the program applied against an OP-resistant strain of ticks that was 13-times more resistant to coumaphos than a susceptible strain. The overall control achieved at 0.165% active ingredient (AI) coumaphos was only 52.9%, whereas at 0.299% AI the control was 75.8%, and at 0.566% AI control was 89.7%. Thus, none of the concentrations tested resulted in an acceptable level of control (99%) for use in the eradication program. The overall lack of control was principally due to the low level of control obtained against ticks that were adults at the time of treatment (range of control: 4.3-73.5%). Against nymphs the control ranged from 60.6% at 0.165% AI to 97.3% at 0.566% AI. Control levels against larval ticks was >98% at all coumaphos concentrations. Thus, results indicated the presence of highly OP-resistant ticks would place the program at risk of failure.
Technical Abstract: Calves infested with a highly organophosphorus (OP)-resistant strain of Boophilus microplus (Canestrini) were dipped in coumaphos, an OP, at concentrations of 0.165, 0.299, and 0.566% active ingredient (AI). The overall level of control was 52.9, 75.8, and 89.7%, respectively. None of the concentrations provided the minimum standard level of control of 99% considered to be acceptable for use in an eradication program. The lack of control was primarily due to the low level of control obtained when treatments were applied to ticks that were in the adult stage when treatment was applied. Against adult ticks the numbers, engorgement weight, egg mass weight, and fecundity of surviving females was similar for both untreated and treated females, regardless of coumaphos concentration. Control ranged from 4.3% at 0.165% AI to 73.5% at 0.566% AI. Against nymphs the control at 0.165% AI was 60.6%, whereas at 0.299% and 0.566% AI control was 89.5 and 97.3%, respectively. In contrast to other parasitic stages, the greatest impact for each concentration occurred against larval ticks. All measured parameters were significantly lower in treated ticks than in untreated ticks when ticks were larvae at the time of treatment. Regardless of coumaphos concentration, the level of control against larval ticks remained at >98%. Thus, the presence of OP-resistant ticks poses great risk to the eradication program.