Critical tests evaluating efficacy of moxidectin against small strongyles in horses from a herd for which reduced activity had been found in field tests in Central Kentucky.

Critical tests were performed in 2009 and 2010 in four 2-year-old horses naturally infected with internal parasites. The horses were from a herd (Farm MC) where reduced activity of ivermectin and moxidectin on small strongyles was demonstrated previously from EPG (eggs/gram of feces) data in field tests. Also, in critical tests in horses from the same herd, ivermectin was less effective on immature small strongyles in the lumen of the large intestine than when the drug was first marketed. The main interest in the present critical tests was to determine the efficacy of moxidectin (400 μg/kg) on small strongyles. This was done to try and find indications of why there has been a return of strongyle EPG counts sooner after treatment in field tests than when moxidectin was first commercially available. Removal of adult small strongyles for the four treated horses was >99% to 100%. Efficacy on immature (L(4)) small strongyles was 82%, 96%, 98%, and >99% for the individual horses. Identification of small strongyles recovered from two of the horses revealed that three genera and 11 species were present. Specimens of Cylicocyclus ashworthi are reported for the first time in horses in Kentucky although eggs of this species have been identified. Moxidectin, in the present study, was excellent on removing adult small strongyles but was less effective on immatures (L(4)) in the intestinal contents. The question as to why moxidectin efficacy on small strongyles has declined in field tests may have been answered at least to a certain extent. It seems that a significant factor is "quick development" of a few remaining immatures in the gut lumen of horses. Also, possible activity may have decreased on encysted stages in the large intestinal lining. In any event, after treatment of some horses with moxidectin, the life cycle of small strongyles is shorter now than at the onset of usage of this compound.