Efficacy of Daily Oral Treatments of Ivermectin Administered to Cattle Infested with Boophilus microplus (Acari: Ixodidae)1,2

The efficacy of ivermectin administered orally to cattle infested with all parasitic stages of Boophilus microplus (Canestrini) ticks was evaluated. Ivermectin capsules were administered to two separate groups of cattle at a dose rate of either 25 or 50 mg/kg for a period of 21 consecutive days. A third group of calves received a placebo capsule each day and served as a control. Although the overall control achieved at both doses of ivermectin was >99% against all parasitic stages, the 50 mg/kg/d dose was significantly more effective than the 25 mg/kg/d dose against each developmental stage of the tick. Each ivermectin treatment dose produced a significantly higher percentage reduction in female tick numbers against ticks that were adults at the time of treatment onset than was observed against immature ticks (nymphs and larvae), however the 50 mg/kg/d treatment was significantly more effective in reducing tick numbers, regardless of the developmental stage of the ticks. Both engorgement weight and egg mass weight of females were significantly lower in the ivermectin treated groups than were observed in the untreated group. The potential applicability for treating tick-infested cattle with different delivery systems, such as daily oral treatments, boluses, and medicated feed that contain ivermectin or other macrocyclic lactone compounds is also discussed.