Many equine intestinal parasites are becoming resistant to current dewormers. Because of this resistance, most equine veterinarians are recommending fecal egg counts to determine the level of parasitism in each horse. By doing so, you can determine which horses require deworming and which are not as susceptible to parasites, and this will allow less deworming products to be used. The less dewormers that are used, the less resistance will develop. Unfortunately, we do not have any new classes of dewormers that are being developed so we have to control intestinal parasites as best we can with the products that are now available.
In many cases, horse owners are still deworming their horses every 8 weeks, which was recommended 40 years ago, and is the reason we now have resistance developing. We know in a herd of horses that only about 20 percent of horses shed 80 percent of the parasites; because of this, only the 20 percent shedding parasites need to be dewormed more than twice a year. So the key is finding the horses that represent that 20 percent on your farm by having your veterinarian perform a fecal egg count. A recent study out of England indicated that adult horses generally have a consistent egg per gram count over a grazing season whether they’re treated with deworming medications or not. In younger horses, fecal egg counts were likely to vary more but in adult horses, the ones that were shedding low numbers of parasites continued to shed low numbers, and those shedding high numbers continued to shed high numbers. This indicates that in most cases of adult horses, the results of a fecal egg per gram count are consistent over time and these tests need to only be repeated frequently in young horses less than 5 years of age.