Deworming schedules are generally aimed at when the drugs will do the most good, eradicating worms before they pass eggs to re-contaminate pastures. In the north, horses are turned out to pasture in spring. “Horses should be treated at least 4 or 5 days before turnout,” says Dr. Craig Reinemeyer (President, East Tennessee Clinical Research, Inc.) because parasites must return to the environment between successive generations.
The minimum parasite control recommendation for adult horses is to maintain eradication of large strongyles (S. vulgaris). The simplest way is to treat with a larvicidal drug every 6 months, and keep up that schedule. “Three products work for this: any of the ivermectins, moxidectin (Quest or Quest Plus), and Panacur Power-Pak. Using any of these at 6-month intervals will work, and no matter what your climate is, spring and fall are probably the best times to do this. In the north especially, it should be done before horses go to pasture in the spring. Egg counts will spike in the spring, and worms will contaminate pastures. In the south it becomes a self limiting thing as the weather heats up,” says Reinemeyer.
By summer, if you have foals, you need to think about ascarid control. “We’ve been treating too much for these, however, and need to back off on onset and frequency,” says Reinemeyer. “Foals should not be treated routinely for any worms until they are at least 60 days old. Any earlier, treatments are not likely to be effective. The efficacy of these dewormers gets better as the worms get older. The younger the worm is when you expose it to the drug, the less likely the drug will work,” he explains.
So it’s best to give foals as much time as you can before deworming the first time. “After that it gets tricky, regarding intervals of deworming for foals. We are starting to see resistance of roundworms to the macrocyclic lactone class of drugs, which includes ivermectin and moxidectin. Dr. Gene Lyons at the University of Kentucky has found a local ascarid strain that is resistant to a second class of drugs as well. This is mostly due to overusing these drugs,” says Reinemeyer.
In the southern U.S, he doesn’t recommend deworming during hot dry summer months, at least in mature horses, since worm transmission is dramatically decreased. “In foals you could maintain minimum control recommendations for ascarids. In the north, summer is a more important season of transmission than winter and more important than summer in the south, so you may want to maintain strongyle control through summer. You don’t need to deworm the whole herd, however. We can test and find out which horses don’t need help and which ones do,” he says.
A problem in certain southern regions is summer sores (habronemiasis) caused by tiny stomach worms deposited in breaks in the skin by biting flies that serve as the other host. These worms were a serious problem until the advent of ivermectin, which kills them in the body tissues. “Now the only place I see them is in large stables where horses are on different regimens of worming or lack thereof,” says Craig.
“Some of the horses with lesions were dewormed with drugs that would be effective against the adult stages of the worms (in the stomach), but not the immature larvae in the body tissues. The fly population was large and probably constantly transmitting larvae. This problem always seems to be related to large numbers of horses in a stable. Even if your horse was treated with ivermectin, what about the horse 2 stalls away, where the fly is coming from?” Flies may feed on horses that were not dewormed; thus fly control should be part of the approach to eliminate summer sores.