The cattle tick Boophilus microplus

B. microplus ticks are a constraint on America's cattle industry from Mexico to Argentina (Guglielmone et al., 2000). Annual losses due to this tick were calculated as $150 million in Argentina, $800 million in Brazil and $40 million in Uruguay (Horn and Arteche, 1985; Spath et al., 1994; Rimbaud Giambruno, 1999). The plunge vat has been the most effective mean of applying acaricides but, in many regions, the existence of small cattle populations reduces the cost/efficiency of this method. Furthermore, in some areas, it is difficult to find a readily accessible and sufficient water source to charge and replenish the vats (Nari, 1990). Resistance of B. microplus to conventional acaricides (organophosphates, pyrethroids and formamidines) is also a relevant problem in the region (Cardozo, 1995; Caracostantogolo et al., 1996; Martins, 1996). In those regions with widespread resistance to these compounds, their lower cost in relation to MLs is no longer an advantage for treatment. When tick susceptibility to conventional acaricides is lost, control measures become more expensive. MLs are excellent tools in these conditions. In order to optimize the use of MLs, it is necesary to do epidemiological studies in each region to determine the optimum time to apply treatments according to the status of the non-parasitic tick population, tick burdens in cattle and persistent efficacy of each ML (Martins et al., 1996).

In Table 6.4.2, the comparative efficacy of the differents MLs available in the South American's veterinary market for the treatment of B. microplus is presented. Since the present review, no new MLs have been developed for the control of B. microplus.

The results in Table 6.4.2 show that cattle can be treated successfully against B. microplus using ivermectin, moxidectin and doramectin. The percentage efficacy in the different study trials ranges from 84.7 to 99%, at least during 21 days. However, in some trials, excellent efficacy was observed for even more than 35 days post-treatment.

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