The preventive efficacy of ivermectin and abamectin administered in different routes (subcutaneous, intramuscular and pour-on) and doses (200 and 500mcg kg-1) against Cochliomyia hominivorax in the scrotal sac of cattle after castration, was evaluated. Animals from six different farms, in the state of São Paulo and Minas Gerais, Brazil were used. For each study, 30-45 uncastrated bulls, depending on the number of groups, were selected. On day zero of the study, the procedure was carried out by the method of bloody castration and the animals were treated after this process. The efficacy of ivermectin and abamectin administered via the pour-on (500mcg kg-1), subcutaneous (200mcg kg-1) as well as the efficacy of abamectin intramuscularly (200mcg kg-1) were evaluated. In each experiment, one group of animals was kept as control. The animals were evaluated from the 3rd to the 14th day after castration process/treatment. The efficacy values for both active principles were ≤30% the next day 10 post-treatment (PT), and in five experiments, both ivermectin and abamectin, regardless of the route of administration, were ineffective (0.0%) on the day 10th PT. Based on the results found in this study, conducted in different properties from southeastern Brazil, both ivermectin and abamectin, when used the protocol presented, were considered ineffective in preventing scrotal myiasis in bulls, irrespective of the route of administration and dose used.
preventive efficacy; macrolactones; myiasis; resistance
