Internal parasites can be a critical threat to your horse's health. In recent years, reports of parasite resistance to common deworming medications have caused experts to rethink standard strategies for parasite control. We asked our consulting veterinarian, Dr. Barb Crabbe (author of The Comprehensive Guide to Equine Veterinary Medicine, Sterling Publishing Co., Inc.), to answer common questions and share her own practice's approach to the problem. Here's what she told us.
What has caused changes to deworming strategies? Over the last two decades, a number of different types of parasite populations have changed in ways that alter how those parasites respond to medications. Species of worms such as the small strongyle and tapeworm, for example, have become more problematic, while others, such as the large strongyle, have become less so.
In addition, widespread use of deworming drugs has led to resistance within some of these parasite populations. It is true that the deworming ingredients ivermectin and moxidectin remain effective against most classes of parasites, with very little reported resistance to date.
Still, if we overuse these medications in our environment, we risk encouraging the development of resistance in these drugs, too. The same is true for praziquantel, the medication most effective for treating tapeworms.
So what is the solution to avoiding resistance? Monitor your horse's parasite load using fecal egg counts on manure samples, so you can target dewormers more effectively and not overuse them.
My practice's approach is designed to avoid overusing the still-effective dewormers. It also identifies resistant horses (which harbor larger numbers of parasites than others and may be more resistant to deworming medications), plus monitors whether overall resistance is developing on a farm. Fecal egg counts are the key to achieving all these goals.
How does that approach work in practice? Our schedule calls for regular dewormings twice yearly, in the spring and fall. In, say, March or April, we deworm all horses on a farm with a product that contains moxidectin/praziquantel. In September or October, we use ivermectin.
We check fecal egg counts on all horses before the dewormings, then do a follow-up check?and additional deworming if needed?on any horses with an EPG (eggs per gram) higher than 200 prior to the initial deworming.
Then, in summer and winter, we check fecal egg counts again, administering a dewormer only to any horses with EPGs over 200, with the choice of product determined by the fecal exam. Once a farm is stable for a couple of years (no horses moving in or out) and we know from testing that there are no "high shedders" (horses with high parasite loads), then we drop the summer/winter fecal counts.
Why routinely deworm in spring and fall, instead of deworming only according to the EPGs? A for-sure deworming twice a year does three important things: It helps protect against possible false-negative fecal tests; it takes care of larvae that may not be shedding eggs at the time of the test; and it treats for bots.
Isn't it expensive to do all those fecal egg counts? Managing your deworming program with fecal egg counts costs about the same each year as interval deworming programs?the cost of lab testing replaces the cost of administering unnecessary chemicals to your horses. Moreover, getting together with other horse owners for group-testing "parties" can further reduce the expense.
What about the daily deworming strategy? In many situations, daily deworming with a pyrantel product is still a great strategy. Although some resistance to pyrantel has been reported, it's not necessarily more likely to develop in horses receiving daily doses. In fact, there's more resistance to pyrantel reported in European countries where daily dewormers aren't in use, than there has been here in the U.S. The advantage to daily deworming is that it reduces the overall numbers of eggs shed by individual horses, and can thereby help minimize pasture contamination.