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Parasite Prevention — Fall 2010 | Out Here Magazine

Take control of your horse's parasites with fecal testing, individualized deworming

By Carol Davis
Photography Courtesy of Pfizer Animal Health

Aggressive individualized deworming — which includes a return to fecal testing — is the best means to keep your horse safe from the damage of internal parasites, an animal health expert says.

"The average horse in a pasture or in a stall can be heavily parasitized, but you'd never know it," says Dr. Tom Lenz, a longtime veterinarian and senior director of Equine Veterinary Services for Pfizer Animal Health. "That's why fecal egg counts are so important."

When Lenz began his veterinary career 35 years ago, fecal tests were routine. But new over-the-counter deworming products made the process more efficient for horse owners, and in recent decades they have shifted away from fecal testing and simply dewormed the recommended four to six times per year. The problem is, that method is based on now-outdated data.

"Most horse owners and many veterinarians are following a 40-year-old plan for internal parasite control which recommended worming every horse in the herd or on the premises every two to three months," Lenz says.

The old plan also recommended rotating dewormer classes with every treatment to prevent resistance development by the parasites, he says.

"That plan was based on 1966 data that showed large blood worm (large strongyles) egg counts remained low for eight weeks after deworming. The information prompted increased deworming at shorter intervals, but eliminated regular fecal monitoring (to determine) effectiveness," he says.

Since then, large strongyles have all but been eliminated from well-managed horse herds, but small strongyles have become the most significant internal parasite in adult horses and have developed resistance to many of the commonly used dewormers, Lenz says.

Horse owners have stopped doing fecal exams, so they really have no idea whether the dewormer they're using is truly doing its job, Lenz says.

"I can worm a horse, but parasites will mutate and become resistant. If you're not running fecals, you'll never know that happened," Lenz says. "That's why it's really important to get a veterinarian involved and run fecals on the horses and see exactly what's going on."

Cost of a fecal test — which averages from $12 to $20 — may be off-putting at first, but it's worth it in the long run.

"Horse owners think it's cheaper to deworm, rather than get a fecal, but that's not true if you look at the big picture," Lenz says.

By taking the steps to establish an individualized, and more effective, deworming program for your horse, you'll deworm fewer times.

"In reality, it's less expensive, and it's much better to manage the parasite load in your horses," Lenz says.

Pfizer and Lenz recommend a four-step plan to protect your horses from parasites:

Step 1: Establish baseline egg counts. Your veterinarian should perform a fecal egg reduction test to obtain a fecal egg count for your horse. This will establish whether your current program is working properly. Test before and after deworming to monitor for resistance to various dewormer classes.

Step 2: Measure shedding levels. Assessing the shedding level (high, medium, or low) of a particular horse determines whether eggs are being passed to other horses.

Step 3: Know your variables. Before deworming, work with your vet to analyze your horse's parasite exposure risks. Factors can vary greatly from farm to farm and horse to horse.

Step 4: Determine your horse's individualized deworming plan. Your vet will analyze the data collected from your horses and farm to customize a schedule that will produce the best results.

Once you have determined which dewormers are not effective for your horses, Lenz says, you should no longer use them on your premises because resistance does not reverse.

Carol Davis is editor of Out Here.