Coming Attractions: The Newest Dewormers

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 demonstrate efficacy where Ivermectin more or less does not have much effect against that particular lifecycle stage," says Craig Reinemeyer, DVM, PhD (in parasitology), Associate Professor at the University of Tennessee and a participant in drug trials for pharmaceutical companies. "The cyathostome is far and away the most common parasite that people are going after in their regular worm control programs. Encysted cyathostomes of horses are different from other parasites in that they are relatively unaffected by any of the
standard dewormers. That basically ensured repeated transmission of the parasites."

Although Moxidectin provides similar efficacy as Ivermectin against common adult and larval gastrointestinal nematodes, earlier reports indicated Moxidectin might be slightly less effective than Ivermectin against bots. Says Dennis French, DVM, Professor of Veterinary Science at Louisiana State University and a veterinary clinician involved with parasite research, "I think that's one of the things that's delayed Moxidectin's entrance into the market: There was concern that at the suggested dosage level that it lacked some efficacy (against bots). We have done some re-evaluation of that, and found it to be at about a 99% efficacy level, so it's not a significant lack of efficacy."

Predictions are that Moxidectin might be available this fall as a gel. It has already been approved for use in Australia, New Zealand, and Brazil.


Data is a little sketchy on Doramectin, which is still undergoing clinical trials. As with Moxidectin, Doramectin also appears to kill encysted cyathostomes better than Ivermectin. "From the limited clinical trials that have been done with it," says French, "the egg counts seem to stay lower for longer periods of time than with any of the other dewormers."

Doramectin's efficacy against bots as compared to Ivermectin is still undetermined. "We've done some of these trials and, while this is an assumption, as far as I can tell, I think it will be effective against bots," says French.

Although the equine compound is still undergoing testing, a formulation for cattle (Dectomax) for gastrointestinal nematodes has been available in the United States for about a year. According to Pfizer Animal Health's Web site, stocker calves treated by Doramectin and Ivermectin at four different study sites found that "stocker calves treated with Doramectin had consistently higher weight gains than either Ivermectin injectable or Ivermectin pour-on-treated stockers (2.2-13.2 lb.), although none of the differences reached statistical significance."

Results from three of the study sites on calves found that fecal egg counts from animals treated with Doramectin were "significantly lower" than those treated with Ivermectin pour-on on Days 28, 35, 42, and 49 post-treatment (Arkansas study); than Ivermectin-treated animals on Days 42, 49, 56, and 84 post-treatment (Georgia); and Ivermectin pour-on on Days 7, 14, 21, 28, 35, 42, 49, and 56 post-treatment (Mississippi). The fourth site (Wisconsin) showed fewer eggs in Doramectin-treated animals than Ivermectin injectable solution-treated animals at Day 28, and "an indication of an efficacy advantage with doramectin...compared to both the Ivermectin injectable and pour-on" on Days 49 and 56 post-treatment.

Doramectin will be marketed as an in-jectable formula. No site reactions or side effects have been noted.

Biological Controls

On a more distant horizon, biological controls might prove to be a useful adjunct for dewormers. Preliminary research is already underway on a naturally occurring fungus that consumes parasite larvae that live in the environment. Says French, "There appears to be remarkable differences in horses that are fed this fungus and the parasite level counts in the feces versus those that don't get the fungus. The questions that remain are what does it do for a whole herd, do all horses have to get it, and how expensive is it going to be?"

French predicts horses would first be dewormed, then fed a grain ration with fungus on a daily basis. "Any larvae shed by the horse would be captured by these fungus and not allowed to develop on the pasture, so the re-infective potential as the horses graze on the pasture would be reduced."

Past And Future Perfect

Although Moxidectin and Doramectin might eclipse Ivermectin in controlling encysted cyathostomes, don't write Ivermectin off. Since its market debut in 1983, Ivermectin has not produced any documented parasite resistance in the horse; it remains to be seen whether Moxidectin or Doramectin will enjoy the same long-term results.

Experts are concerned, however, about what the future holds for all the Avermectins. "There's considerable concern about the eventuality of worms becoming resistant to virtually all the drugs that are currently available," says Reinemeyer. "That has already happened with the sheep industry. In some places in South America, Australia, and South Africa, very large sheep ranches have literally gone out of business because the worms on those ranches became resistant to every drug available. To date there is no documented Ivermectin resistance in horse worms, but that eventuality is rather frightening, because theoretically, the worms eventually should develop resistance to just about anything you throw at them."

Because of that risk, the future might see compounds and biological adjuncts that require fewer deworming treatments. "We will continue to explore alternative methods to chemotherapy, but routine treatment is likely to be the lynch pin of parasite control in horses," predicts DiPietro. "Unique delivery devices for horses are likely to be worked out which would allow an owner to administer a dewormer once or twice a year, and the drug would be pulse released over that timeframe to kill parasites."

Adds French, "There will be a need for treatment with dewormers, hopefully at much reduced levels, because when you continually expose parasites to a drug--say at six-week intervals--you're asking for resistance to develop. With an adjunctive type of therapy where you're reducing the intake of infected larvae off the pasture, you're helping to maintain the integrity of the drugs."

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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