FARAD recommends a meat WDI of 14 days for up to 0.4 mg/kg (0.18mg/lb) per os. These calculations assume that the kinetics of ivermectin are linear. The milk WDI would be 6 days based on a study by Scott et al,8 that demonstrated that at 6 days, goats' milk was clear of the drug after an oral dose of 0.2 mg/kg. Based on this information, oral administration up to 0.4 mg/kg will require a milk WDI of at least 8 days in dairy goats.
Subcutaneous route in goats-Ivermectin was detected up to 25 days in milk from lactating goats given 0.2 mg/kg SC.2 There were no differences between plasma and milk pharmacokinetic variables, and the milklasma AUC ratio was 1.08, as stated earlier. The elimination half-life was 4 days for plasma and milk, and it would take 40 days (10 half-lives) to eliminate 99% of the drug via milk when administered by this route. Limited tissue residue data from an NRSP-7 study10 provided an ERH of 4.34 days (ke = 0.1594 days-1) in fat, which is the slowest depleting tissue. Application of our algorithm resulted in a WDI of 22 days. As this FARAD estimate is less than the cattle WDT, and there were limited available data, we recommend the cattle WDT of 35 days for an extra margin of safety.