THERAPEUTIC AND PERSISTENT EFFICACY OF FIPRONIL AGAINST BOOPHILUS-MICROPLUS (ACARI, IXODIDAE) ON CATTLE

Cattle infested with all parasitic life-stages of Boophilus microplus (Canestrini) were treated once with a pour-on formulation of fipronil at 0.25, 0.5, or 1.0% active ingredient (AI) to determine the therapeu tic efficacy. Additionally, cattle were infested at weekly intervals f or 13 consecutive weeks after treatment to determine the persistent ef fectiveness: of the three concentrations. Results of therapeutic effic acy of fipronil showed that with each increase in concentration there was a corresponding and often significant (P < 0.05) decrease in the n umbers, fecundity and fertility of engorged females. The overall contr ol of the Index of Reproduction (IR) achieved at 0.25 and 0.5% AI was 86.2 and 93.3%, respectively, while the control of the IR at the 1.0% AI concentration was 99.7%. The persistent efficacy of fipronil, like the therapeutic efficacy, was also dose related. The 0.25% AI treatmen t failed to provide complete protection (100%) against larval reinfest ation, although control of the IR remained high (> 95%) for the first 6 weeks after treatment. In contrast, at 0.5% AI the control of the IR remained at 100% for the first 4 weeks after treatment, and was > 90% for an additional 2 weeks (6 weeks posttreatment). The 1.0% AI concen tration afforded the longest residual activity, providing virtually 10 0% protection against larval reinfestation for 8 weeks after treatment was applied. Based on the results, neither the 0.25 or 0.5% AI concen tration completely eliminated the immature stages of the tick (nymphs and larvae) on the host at the time of treatment (therapeutic efficacy ), and the residual activity (persistent efficacy) did not prevent rei nfestation by lan ae for long periods after treatment. While the 1.0% Al concentration provided both therapeutic and residual efficacy thar would be suitable for use in the eradication program, caution is still warranted because a few engorged females detached for up to 25 days a fter treatment, although none reproduced after the 6th day. Published by Elsevier Science Ltd.