In the past, deworming protocols have consisted of periodic deworming (every 2-3 months), rotating deworming products. Another common program was to maintain horses on a daily dewormer (pyrantel). It has become apparent within the last few years that these programs have produced a population of parasites that is resistant to one or more types of dewormers. This is problematic, since there haven't been any new types of dewormers released in the last 25 or so years, and as far as we know, there won't be any new ones for some time. So what do we do? Basically, we're going to have to minimize the production of resistant parasites through proper management of our horse herds.
So, how do we do that? Well, we know that there are some horses who have a natural immunity to intestinal parasites. Also, there are horses who are more susceptible to parasites and tend to shed more parasite eggs in their manure. Our goal is to identify this second group of horses and target our deworming therapy to them. We accomplish this by performing a test called a fecal egg count (FEC). This tells us if the horse has a high or a low parasite burden. Typically, within a herd of horses there will only be a few individuals who have a high enough egg count to warrant treating with a dewormer. These horses are dewormed and another FEC is performed afterward to determine if the egg count decreased appropriately.