Moxidectin: pharmacokinetics and activity against horn flies (Diptera: Muscidae) and trichostrongyle nematode egg...

The concentration of moxidectin, a macrocyclic lactone endectocide, in the blood serum of cattle resulting from single and daily subcutaneous injections and oral dosing was determined as a function of time. When given as a single subcutaneous (SC) injection, the drug peaked between 4 and 6 h post-treatment. As a single oral dose, the peak serum level occurred at 1 day post-treatment. Daily SC injections and oral doses resulted in a gradual increase in blood serum level over the 21 days of treatment but did not reach a plateau during this time. Horn flies, Haematobia irritans (Linnaeus), feeding on the blood of treated cattle drawn on Day 21 of daily treatment showed a decline in survival and egg production, but a negligible effect on egg hatching. Dose-mortality data on adult horn flies showed an LC-50 and LC-90 value of 10 ppb and 19 ppb in the blood, respectively. Moxidectin was also found to have larvicidal activity against the immature stages of the horn fly in the manure of treated cattle. Moxidectin administered at 100, 50 and 25 micrograms kg-1 as a daily oral medication was 100% effective in eliminating trichostrongyle egg counts by Day 3 of the treatment. Counts remained negative to the end of the trial.