Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 8/25/2003
Publication Date: 10/1/2003
Citation: MILLER, R., RICARDO-MARTINS, J., DUCORNEZ, S., BARRE, N., ANGELICA-SOLARI, M., CUORE, U., GEORGE, J.E. USE OF A MODIFIED-LARVAL PACKET TEST (LPT) TO MEASURE AMITRAZ SUSCEPTIBILITY IN BOOPHILUS MICROPLUS IN BRAZIL, NEW CALEDONIA, & URUGUAY, AND A COMPARISON OF THE MODIFIED-LPT TO A MODIFIED-SHAW TECHNIQUE FOR AMITRAZ TESTING IN BOOPHILUS MICROPLUS. PROCEEDINGS OF V INTERNATIONAL SEMINAR OF ANIMAL PARASITOLOGY, 1-3 OCTOBER 2003, MERIDA, YUCATAN, MEXICO. 2003. p. 118-123. Interpretive Summary: Quantification of amitraz resistance in Boophilus microplus has not been accomplished on a large scale. Two new bio-assay techniques capable of quantifying amitraz resistance have recently been developed. One technique, developed at the UDSA, ARS, Knipling-Bushland U. S. Livestock Research Laboratory, is a modification of the Food and Agriculture Organization Larval Packet Test (amitraz-LPT). The second is a modification of the Shaw (amitraz-Shaw) technique. Neither method has been evaluated in many laboratories. This study was conducted to determine if the amitraz-LPT could be used in laboratories around the world. The second objective was to evaluate the amitraz-LPT and -Shaw procedures and compare the results obtained from the techniques. The amitraz-LPT technique was able to consistently measure amitraz susceptibility at laboratories located in Brazil, New Caledonia, and Uruguay. The amitraz-LPT and -Shaw techniques were both able to quantify amitraz resistance. The amitraz-Shaw technique was easier to score, but took longer to run then the amitraz-LPT. Resistance ratios were higher when the amitraz-Shaw technique was used. This study verified that the amitraz-LPT can be useful in laboratories world-wide as a quick technique to measure amitraz susceptibility in B. microplus.
Technical Abstract: A modified Food and Agriculture Organization-Larval Packet Test (amitraz-LPT) designed to measure amitraz susceptibility in Boophilus microplus (Canestrini) was determined to be usable in Brazil, New Caledonia, and Uruguay. Amitraz resistance was measured in the Brazilian field-collected São Borja strain, the resistance ratio (RR) at the LC90 estimate (95% CI) = 24.2 (19.9-29.5). Additionally, the data obtained from a modified Shaw test (amitraz-Shaw) was compared to those obtained from the amitraz-LPT using susceptible and resistant strains of B. microplus. Both techniques were able to detect amitraz resistance. However, the amitraz-Shaw was easier to score and produced a larger RR for the highly resistant Santa Luiza strain than the amitraz-LPT, RR (95% CI) = 9,610 (7,531-12,264) and 29.5 (22.4-38.7), respectively. The amitraz-Shaw technique is suitable for discriminating dose studies requiring 2x LC99. However, the amitraz-LPT can be used at 1.5x LC99. The amitraz-LPT produced results in 24 hours whereas the amitraz-Shaw technique produced results in 72 hours. Both techniques represent a significant advancement in the ability to study the epidemiology and mechanisms of amitraz resistance and will lead to better management of cattle fever ticks.