Page 112 - September 2022

EQUINE HEALTH
“While our recommendations are to deworm less frequently than historically, we still tend to treat young horses more often than adults, but more frequent deworming increases the risk for developing drug resistance in the parasites.” - Dr. Martin Nielsen
parasite control if we don’t have a reliable drug. We need to more carefully use what we currently have. Even if we do get new products, their usefulness will be short-lived if we abuse them like we abused all the equine dewormers of the past,” he says.
Parasite problems are a numbers game. The industry has tried for 60 years to eradicate them, but that’s an unrealistic goal. If you have a horse or a herd/farm where you’ve kept the parasite numbers so low that the horses don’t encounter enough to build immunity, they are defenseless if they go somewhere else and pick up worms. They could easily develop heavy worm burdens, which could be damaging.
“Some people have experienced this situation—sending a young horse to be trained. They get a call a few months later from the trainer saying this horse has crashed; he has diarrhea and is losing weight and a little colicky. These horses are suddenly dealing with parasites they’ve never encountered before. It’s like when you send kids to day care or first grade. They come home every week with a different kind
of cold because they haven’t encountered these viruses before,” he explains.
“The same thing is true of parasites in horses. It’s better to have some exposure, let them develop some immunity but keep the worm numbers low enough to not be damaging, and then when they really need that immunity it will be there. The immune system ultimately will
be the thing that can bail us out of some huge problems in the future. Horses probably do a much better job of controlling parasites with their immunity than almost any other domestic animal, and this may be another area of research for the future,” says Reinemeyer.
Dr. Martin Nielsen, Associate Professor at the MH Gluck Equine Research Center, University of Kentucky, says foals and yearlings are the most important group in terms of parasite control. “While our recommendations are to deworm less frequently than historically, we still tend to treat young horses more often than adults, but more frequent deworming increases the risk for developing drug resistance in the parasites.
“The first major threat to foals is the large roundworm, Parascaris. The best time to treat foals for this parasite is at 2 to 3 months of age—as recommended in the AAEP parasite control guidelines. The reason for waiting this long is because efficacy of any anthelmintic is better in the later stage of the parasite. If you treat too early, the only ascarids within the foal are migrating larvae, and none of the current dewormers have any efficacy against this stage. In earlier years, ivermectin had efficacy against migrating larvae as well as the adult worms in the gut, but today ivermectin no longer works against ascarids. We no longer recommend using ivermectin for ascarid control,” he explains.
A second deworming treatment is appropriate at weaning time but becomes a bit more complicated. “The foal may still have ascarids and there might also be some strongyles,” says Nielsen. Depending on which parasite we want to target, we’ll choose a different drug, partly because of the levels of resistance we know are now present.
“A drug like fenbendazole is generally good for ascarids, but usually has no efficacy against small strongyles any longer There may be a few populations of horses in which it would work, but the general rule of thumb is that you should assume it no longer works for strongyles unless proven otherwise,” Nielsen says. If you are treating mainly for strongyles, you might use ivermectin but this drug is facing huge issues of resistance in roundworms worldwide.
“Weaning age is the transitional period where we start seeing the foal’s immunity kicking in against roundworms and see small strongyles coming in and taking over— becoming the dominant worm category,” Nielsen says. This may happen later in some foals than in others. You can’t tell which drug to use by looking at the foals.
The optimal deworming program for young horses will vary somewhat from farm to farm, depending on circumstances. “For the average foal, the first two treatments in the foal’s life should target ascarids at 2 to 3 months, and
The best time to treat foals for Parascaris is at 2 to 3 months of
age because efficacy of any anthelmintic is better in the later
stage of the parasite. The first major threat to foals is the large roundworm, Parascaris.
110 SPEEDHORSE September 2022
Courtesy Heather Smith Thomas
Courtesy Heather Smith Thomas