Many aspects of horse care are based largely on customs and a sense that “It’s always been done this way.” And while it’s never a good idea to let tradition guide your management decisions, nowhere is this attitude more harmful than when it comes to parasite control. In fact, the longtime practice of routinely deworming horses every eight to 12 weeks is putting the health of horses around the world in jeopardy.

Here’s why those old practices are so wrong: Each time an anthelmintic (worm-killing) chemical is used, most of the targeted pests in a horse’s gut die, but a few—those least susceptible to the agent—survive. Those remaining parasites go on to reproduce. And when they do, they can pass on their ability to  withstand the dewormer to the next  generation. Over time, with each successive dose, the population of parasites that no longer responds to the deworming agent grows. Making the resistance even more of a concern is the fact that new, effective deworming products aren’t easy to develop. No new classes of dewormers are on the horizon right now.

Three-pronged approach

All of this means that it’s more important than ever to use dewormers judiciously. That requires adopting a three-pronged approach to deworming: targeting the horses most affected by parasites, timing treatments appropriately, and taking measures to limit the exposure of all horses to internal parasites.

The details of an updated deworming protocol will be specific to your situation. Your veterinarian is in the best position to help you develop a modern parasite control program for your horses. These, however, will be the guiding principles of that new strategy:

• Drop the idea of eliminating parasites.

Few worms cause your horse serious problems in small numbers, and trying to eliminate them all will do more harm than good.  Remember that your overall goal is to keep all your horses healthy—but that doesn’t mean parasite free.

• Identify the high shedders  in your herd.

 Research has proven that adult horses shed strongyle parasite eggs at different rates. In fact, in some herds, a small number of horses is responsible for most of strongyle eggs shed onto the pasture.

The only way to identify high-shedding horses is to conduct  fecal egg counts. These tests detect parasite eggs in manure samples. Targeting your deworming treatments to high shedders will greatly reduce the number of eggs on your pasture.  Low-shedding horses in your herd may require no more than twice-annual treatments to control  large strongyles.

• Choose the right chemical agent.

Another benefit of fecal egg counts is they identify the types of parasites present on your farm.  With this information, you can choose chemicals proven to work against those worms.

Fecal egg counts can also help you determine whether the parasites on your farm are developing resistance. Here’s how: An initial count  is done before a horse is dewormed and then the test is repeated 10 to  14 days later. Ideally, the  number of eggs will be reduced by 90 percent or more. If the count is reduced by less than 80 percent, then you need to investigate.

First, make sure that you are using the anthelmintic according to instructions. Check the expiration date, make sure you have stored it properly and administer the correct dosage. If all of that checks out, you’ll know the parasites on your farm are becoming resistant to that chemical. This means you’ll need to switch to a different chemical class to treat your horse effectively and slow the progression of resistance.

Keep in mind that because tapeworms shed eggs sporadically, they may not appear in a routine fecal egg count. Also, a blood antibody test can identify tapeworm infection. A test that looks for tapeworm-specific antibodies in the horse’s saliva is also available.

• Deliver the right dosage.

Giving a dose of a dewormer that is too small contributes to the development of resistance among the targeted parasites. To ensure you are giving the correct dosage, first calculate your horse’s weight in pounds with the help of a tailor’s tape. (Measure his girth circumference in inches, square it, then multiply by his body length—measured from the point of his shoulder to the point of his buttocks—then divide that number by 330.) Then, consult the product information to determine the proper dose for that weight.

• Time treatments for best effect.

You might think the goal of deworming is to kill as many parasites as possible with each treatment. But ironically, that will cause resistance to develop quicker. Instead, you’ll want to time your treatments for when a higher percentage of parasites are in refugia (“out of reach”). That means that while the dewormer is working inside one horse, the chemicals would have no effect on eggs already on the pasture or in other low-shedding horses.

Those eggs, for parasites that might be susceptible to the chemicals, would then “dilute” the number of resistance genes found in the next generation of worms. Administering dewormers when the refugia population is low reduces this effect. For  example, eggs do not survive long on the ground during very cold winters or in hot, dry summer weather. Frequent treatment of the whole herd at once also reduces refugia.