MOXIDECTIN - PHARMACOKINETICS AND ACTIVITY AGAINST HORN FLIES (DIPTERA, MUSCIDAE) AND TRICHOSTRONGYLE NEMATODE EGG-PRODUCTION

The concentration of moxidectin, a macrocyclic lactone endectocide, in the blood serum of cattle resulting from single and daily subcutaneou s injections and oral dosing was determined as a function of time. Whe n given as a single subcutaneous (SC) injection, the drug peaked betwe en 4 and 6 h post-treatment. As a single oral dose, the peak serum lev el occurred at 1 day post-treatment. Daily SC injections and oral dose s 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 catt le drawn on Day 21 of daily treatment showed a decline in survival and egg production, but a negligible effect on egg hatching. Dose-mortali ty 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 ha ve larvicidal activity against the immature stages of the horn fly in the manure of treated cattle. Moxidectin administered at 100, 50 and 2 5 mug kg-1 as a daily oral medication was 100% effective in eliminatin g trichostrongyle egg counts by Day 3 of the treatment. Counts remaine d negative to the end of the trial.