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Less is More

Worms are “smarter” than we might think. Small strongyles in mature equines are wising up, developing resistance to certain common classes of dewormers. In fact, resistance has already been duly noted in goats and sheep. Trouble is, there’s nothing new at this point on the dewormer research front, so we’d best use products sparingly and with care.

We have been set in our current deworming ways for decades. In 1966, paracytologists Drudge and Lyons solidified a curriculum now diligently followed by concerned horse owners in cooperation with their veterinarians. Deworm each horse in your herd that’s older than two months approximately every two months on a rotational schedule. Goal? Zap threatening large and small strongyles primarily, along with roundworms, tapeworms and those troublesome youngsters, the migrating larvae.

Then rest (reasonably) assured that your equine eating machines are absorbing and utilizing nutrients from their food and that their intestines are motoring along nicely with the parasites that have naturally evolved with their species. Because your anthelmintic equine dewormer is doing its job—expelling or killing helminthes or parasites—your horses aren’t tired or weak, they’re not coughing, their appetite is good and they doesn’t resemble skinny, yet pot-bellied stoves with sunken eyeballs and coarse, dry hair. They’re certainly not manifesting diarrhea or in the worst, most dangerous scenario, they’re not colicking. Your dewormer is deworming: Hurray!

So if your deworming regimen’s not broke, should you fix it? Some scientists now suggest “yes” because you may be killing your horse’s resistance with kindness and over-treatment, which also wastes money.

Horses actually need a small population of parasites to maintain equilibrium, so you don’t want to entirely rid your horses of parasites. You simply seek to halt future contamination and infestation of the horse’s environment with eggs. Parasites prefer pasture to stalls, so if your horses are inside most of the time, they are less likely to have a major problem.

“The most effective means of controlling intestinal parasites in horses is frequent removal and correct disposal of manure,” according to “Current Therapy in Equine Medicine,” by N. Edward Robinson and Kim A. Sprayberry (Elsevier Health Sciences, 2008).

But let’s get real: Although most farm owners endeavor to manage manure—including rotating pastures regularly—it’s simply easier andfeels safer to also use a rotated or feed-through dewormer. The authors recommend an individualized control program, to determine which drugs are effective on a given farm, because we’re not just treating the horse, we’re treating the environment.

Changing Old Habits

It’s an approach already being recommended to receptive clients of the Veterinary Medical Center in Easton, Md., where Elizabeth Callahan, DVM, DAVBP, wrote a very user-friendly synopsis, a “textbook approach,” on how to deworm smarter in 2009 and beyond. See her exemplary short paper at­ed%20Deworming%20Complete.pdf.

First, you want to assess exactly how parasites are affecting each of your horses. Just like with people, some experts feel it doesn’t make sense to treat every horse in your herd the same way, i.e., “a horse of a different color” likely needs a different dewormer at a different dose at a different time. In addition, different areas of the country will necessitate their own programs. Cold weather reduces pasture parasites, for instance, as does a scorching, dry summer.

Additionally, says Dr. Callahan, it’s thought that about 20 to 30 percent of your horses—high shedders—carry the majority of parasites and require more frequent deworming. Half may do a very good job of worm “self-control.” Remember, too, that certain categories of parasites—we won’t exactly call them “good”—stimulate the horse’s immune system to fight off more “bad” parasites.

How do you create deworming programs tailored for your horses? Start with an initial fecal egg count done on all horses, including new arrivals, to prevent pasture contamination; you’ll then do this routinely. Check individual dewormer efficacy by doing egg counts on 10 percent of your horses, deworming them, then rechecking those same horses in 14 days. If parasite numbers are reduced less than 80 percent, your horses’ parasites are resistant to that dewormer and you won’t use it alone again—combinations may still be effective.

Callahan initially performs a simple fecal flotation in her hospital. If a slide has more than an egg or two, it’s off to a lab for further examination. Horses with low fecal egg counts (200 eggs per gram) may not need to be dewormed more than twice a year, while high shedders (500 eggs per gram), may need to be treated more often.

“It is time-consuming and costs money to do fecal egg counts. Those who own only a few horses seem more interested in the ‘as-needed,’ new approach,” says Dr. Callahan. “Old habits die hard.”

Can you cease deworming a horse with several consecutive low stool samples when he’s living alone in the pasture or is contained in a stall? Probably not a great idea. Better to deworm at least twice a year for most parasites, and once a year for tapeworms, which are difficult to pick up on a fecal slide sample. A blood test for tapeworms is in the research stage.

Dr. Callahan cautions that there’s “no ‘best’ dewormer” and encourages infrequent use of the stronger moxydectin for high shedders. Other suggestions:?Always use a weight tape to ensure accuracy of dosing. Age matters, too—horses under 18 months and foals need a different program, as they’re more susceptible to parasites, and parasites may kill foals. Research also continues into the optimum treatment methodology for these younger age groups.

Communication with your veterinarian is crucial, and at first, yes, this sounds more expensive than your usual regimen—not to mention time consuming. However, if you’re willing to give it a try, once you’ve identified “how much” and “when” for each horse, you can achieve effective deworming that really protects each horse.