The next generation of dewormers is nearly here. Fort Dodge Animal Health anticipates final FDA approval for Moxidectin this year, while Pfizer Animal Health Group might see Doramectin reach the market in mid-1998. Like Ivermectin, Doramectin and Moxidectin are part of the Avermectin family. One might think of them as siblings, as all three are close, structurally related compounds that kill internal parasites in the same manner. However, the efficacies and effects of each have subtle differences in how they affect parasites.


Undoubtedly, Moxidectin’s claim to fame will be its significant improvement over Ivermectin in controlling small strongyles (the stage when the parasites become encysted cyathostomes in the horse’s body and are the most difficult to kill). Joseph A. DiPietro, DVM, MS, Professor of Veterinary Pathology and Dean of the College of Veterinary Medicine at the University of Florida and a researcher in clinical and critical trials of various deworming compounds, says, “The major difference between Moxidectin and Ivermectin is its apparent efficacy against mucosal stages of cyathostomes and its ability to prolong strongyle egg suppression post-treatment. This means that with interval deworming programs, fewer treatments with Moxidectin are likely to be necessary.”

In a clinical field trial conducted on 150 Standardbreds with naturally acquired parasite infections, DiPietro found that after the initial deworming, the average number of strongyle eggs per gram for the Ivermectin-treated group on Day 56 was 27.3; on Day 70 was 209.3; and on Day 84 was 466.4. The Moxidectin treated group averaged less than 1 strongyle egg per gram 70 days after deworming, and averaged only 3.7 strongyle eggs per gram on the 84th day.

Moxidectin’s success in suppressing encysted cyathostomes is an important breakthrough. “Moxidectin is not 100% effective, but it definitely does de