Efficacy of macrocyclic lactone endectocides against Boophilus microplus (Acari: Ixodidae) infested cattle using different pour-on application treatment regimes

The efficacy of pour-on formulations of three macrocyclic lactone endectocides (moxidectin, ivermectin, and eprinomectin) was evaluated on cattle against Boophilus microplus (Canestrini) using two different treatment regimes. A single application treatment regime with each endectocide showed that fewer ticks per calf were recovered from all treated calves than from untreated cattle, but the level of control among the three treatments was similar (range; 78.7-87.7%) against all stages of ticks on the calves at the time of treatment. The engorged female and egg mass weights of all treated ticks were less than that of untreated ticks. Among the treated groups, the ivermectin and eprinomectin-treated females weighed less and produced lower weight egg masses than those from moxidectin-treated cattle. In a double application treatment regime with a 4-d interval between treatments, there were fewer ticks per calf recovered from the treated cattle than from untreated cattle. In addition, all treated females weighed less and produced lower weight egg masses than those from untreated cattle. Control with moxidectin (90.3%) was lower than with either ivermectin (98.9%) or eprinomectin (99.7%). The mean female and egg mass weight of the ivermectin and eprinomectin-treated groups was also less than that of the moxidectin treatment. A single application treatment against either 18- or 20-d-old adult ticks indicated that both moxidectin and ivermectin were less effective against 20-d-old ticks that were nearer to completing their parasitic development on the animal. In contrast, eprinomectin was the only endectocide tested that was equally effective against both 18- and 20-d-old ticks.