Persistence of ivermectin in plasma and faeces following treatment of cows with ivermectin sustained-release, pour-on or injectable formulations.

Sixteen young dairy cows were randomly allocated to 4 groups of 4 animals each: Group 1 cows were each given a single Ivomec SR Bolus; Group 2 cows were treated with the Ivomec Pour-on formulation; Group 3 cows were injected with the Ivomec Subcutaneous Injection; Group 4 cows were untreated controls. Blood and faecal samples were collected from all cows on the day before treatment and on days 1, 2, 3, 7, 14, 21, 28, 35, 42 and 49 after treatment for HPLC determination of plasma and faecal ivermectin concentrations. Group 1 (SR Bolus) cows had mean (+/- S.D.) plasma ivermectin concentrations ranging from 5.6 +/- 1.8 micrograms l-1 (p.p.b.) at 14 days to 11.0 +/- 4.7 micrograms l-1 at 49 days. Faecal ivermectin concentrations were little changed from 4.0 +/- 2.0 micrograms g-1 (p.p.m.) dry wt (dry weight) [0.5 +/- 0.2 microgram g-1 wet wt (wet weight)] at 14 days to 3.0 +/- 2.0 micrograms g-1 dry wt (0.5 +/- 0.4 microgram g-1 wet wt) at 49 days. Group 2 (Pour-on) cows showed a rapid rise in plasma concentrations to 32.9 +/- 15.7 micrograms l-1 2 days after treatment, followed by a gradual decline to 1.3 +/- 0.07 micrograms l-1 at 28 days. Faecal ivermectin concentrations rose sharply to 18.5 +/- 7.4 micrograms g-1 dry wt (2.8 +/- 1.2 micrograms g-1 wet wt) 2 days after treatment, then fell to 0.04 +/- 0.004 microgram g-1 dry wt (0.006 +/- 0.0004 microgram g-1 wet wt) at 28 days. Group 3 (Injection) cows also showed a rapid rise to an early plasma peak of 46.1 +/- 22.7 micrograms l-1 3 days after treatment, followed by a gradual decline to 1.3 micrograms l-1 at 35 days. Faecal ivermectin concentrations rose to 1.2 +/- 0.34 micrograms g-1 dry wt (0.2 +/- 0.05 microgram g-1 wet wt) at 3 days, declining to 0.08 +/- 0.0001 microgram g-1 dry wt (0.01 +/- 0.0008 microgram g-1 wet wt) at 28 days. No ivermectin was detected in the plasma or faeces of Group 4 (Control) cows. Concentrations of ivermectin potentially toxic to dung-breeding or dung-feeding invertebrates were excreted for the duration of the study in dung of cows treated with the SR Bolus and for 28 days in the dung of cows treated with the Pour-on or injectable formulations.