Therapeutic and persistent efficacy of a long-acting (LA) formulation of ivermectin against Rhipicephalus (Boophilus) microplus (Acari: Ixodidae) and sera concentration through time in treated cattle

The concentration–time profile, therapeutic, and persistent efficacy of a single subcutaneous injection of cattle with a long-acting (LA) formulation of ivermectin at a concentration of 630 μg/kg of body weight were determined against Rhipicephalus (Boophilus) microplus. Ivermectin sera concentration in treated cattle increased to 13.0 ppb within 1 d after treatment, and peaked at 26.2 ppb at 11 d post-treatment. Ivermectin sera levels remained above the threshold level for control of feeding ticks (≥8 ppb) for 42.6 d after treatment. Therapeutic efficacy of ticks on treated animals was >99.9%, and tick number, index of fecundity, engorgement weight, and egg mass weight of ticks from treated animals remained dramatically less than ticks from untreated animals. Tick number and reproductive capacity of ticks infested on treated animals at 14 and 28 d post-treatment were less than for ticks on untreated animals, whereas engorgement weight and egg mass weight of treated ticks remained lower than that of untreated ticks 49 d post-treatment. However, the level of control against ticks infested at 14 d after treatment (99.9%) was the only post-treatment infestation interval that provided the required 99% control necessary for use in the U.S. tick eradication program. The 14 d post-treatment infestation was also the only interval at which infested ticks were exposed to ivermectin levels above the threshold level of 8 ppb for the entire parasitic development period. Cattle would have to be treated at intervals of no more than 31 d apart to ensure that no viable ticks could reach repletion and detach from the host. Although this treatment interval is >2-fold longer than the present treatment requirement (14 d), it is dramatically less than the label claim for the LA ivermectin formulation of 75 d of prevention against re-infestation.