Plasma pharmacokinetics and faecal excretion of ivermectin, doramectin and moxidectin following oral administration in horses.
Authors: Gokbulut C, Nolan A M, McKellar Q A
Journal: Equine veterinary journal
Summary
# Editorial Summary Gokbulut, Nolan and McKellar compared the plasma pharmacokinetics and faecal excretion profiles of three oral antiparasitic compounds—ivermectin and doramectin (avermectins) and moxidectin (milbemycin)—administered at 200 µg/kg to horses, tracking blood and faecal concentrations over 80–197 days to establish whether their clinical efficacy could be explained by their absorption and elimination patterns. Moxidectin demonstrated a substantially longer mean residence time (17.5 days) compared to doramectin (3 days) and ivermectin (2.3 days), whilst achieving a larger area under the plasma concentration curve (92.8 ng·day/ml versus 46.1 and 53.3 ng·day/ml respectively), though all three compounds reached similar peak plasma concentrations within approximately 8 hours and showed detectable faecal excretion for up to 8 days post-administration. The extended plasma persistence of moxidectin and, to a lesser extent, doramectin suggests these compounds may provide prolonged prophylactic cover against gastrointestinal parasites, potentially catching emerging and maturing cyathostome populations during the treatment interval. For equine practitioners, these findings support the use of longer-acting formulations where parasite control intervals need to be extended, though the clinical significance ultimately depends on comparative efficacy data rather than pharmacokinetics alone—a gap the authors acknowledge requires confirmation through formal efficacy trials.
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Practical Takeaways
- •Moxidectin's longer residence time (17.5 days) suggests extended parasite control compared to ivermectin, potentially requiring less frequent dosing intervals
- •All three antiparasitics show rapid faecal excretion within 24-30 hours, which may impact pasture contamination and reinfection cycles when timing pasture management
- •Choice between these compounds should consider duration of activity needed for your parasite control programme, with moxidectin and doramectin offering longer prophylactic windows against developing cyathostome populations
Key Findings
- •Moxidectin demonstrated significantly larger area under curve (92.8 ng×day/ml) compared to ivermectin (46.1 ng×day/ml) and longer mean residence time of 17.5 days versus 2.3-3 days for avermectins
- •Maximum plasma concentrations were similar across all three compounds (21.3-30.1 ng/ml) achieved at approximately 7.9-8 hours post-administration
- •All three compounds were detected in faeces within 8-30 hours and remained detectable for 8 days, with highest faecal concentrations at 24 hours
- •Extended residence time of moxidectin and doramectin may provide prolonged prophylactic activity against emerging and maturing cyathostomes during treatment