Equine Deworming – It’s All in the Poop
by Shanna Nelson DVM
At Fox Creek Veterinary Hospital, we take your horse’s manure seriously, and we perform all of our equine fecal egg counts in-house. Why is this important? We are able to accurately quantify the parasite burden your horse is experiencing, and designate him or her into the categories of low, moderate, or high shedder. This allows for targeted deworming practices catered to your horse’s specific needs. Often, this means you can deworm less frequently (gone are the days of dutifully rotating dewormers every 2 months). Even more importantly, it helps us slow down the increasing threat of parasite resistance to our drugs.
It is commonly stated that 20% of horses harbor 80% of parasites, and this seems to hold true. This means that most horses (low shedders) only have to be treated with dewormer twice per year. On the other hand, high shedders should be dewormed more frequently (often four times per year). Exactly why some horses are prone to higher parasite burdens is unknown, but genetic factors certainly play a role, as do environment and upbringing. Interestingly, high shedders are often not “sick” from their worms, and externally, they look like normal, healthy horses. However, they are shedding parasite eggs into the environment that can in turn infect other horses, some of whom (especially foals and geriatrics) are not as well equipped to handle the parasites, which means clinical disease can result. Significant parasite infections can result in weight loss, anemia, lethargy, colic, and, in severe cases, death. However, the goal is not to remove ALL worms (that is an impossible task), but rather to keep the infection down to manageable levels so that the host (your horse) is able to live a normal, happy, healthy life.
That said, why not just deworm every horse four times a year, to make sure parasite burdens stay low? Unfortunately, it doesn’t work this way. Deworming this frequently is unnecessary at best and harmful at worst, because excessive treatment with anti-parasitic drugs contributes to parasite resistance. We are already seeing that common drugs and old mainstays like fenbendazole (Panacur) and pyrantel (Strongid) are much less effective than they used to be, and are insufficient for treating significant parasite infections. Some worms are also developing resistance to ivermectin, and moxidectin (Quest) may be next in line if we are not careful. If our deworming drugs stop working, then we may see a rash of severe, treatment-resistant parasite infections, leading to skinny, sickly horses, impaction colic, and more—something that we have largely been able to avoid for the last several decades due to the ease of treatment with over-the-counter deworming drugs. At the moment, there is no “backup plan” in the works for treatment of resistant parasites, although scientists are actively searching for novel drugs. Therefore, performing targeted deworming not only ensures the most appropriate care for your horse, but also helps protect the equine population at large by minimizing the development of drug resistance.
The basic equine deworming schedule should consist of a treatment with either ivermectin or moxidectin in the spring, and another treatment (preferably with the drug not used previously) in the fall, after the first frost. The fall treatment should also include praziquantel to target tapeworms; combination products include Quest Plus and Equimax. High shedders will require an additional treatment or two between these, and products may include pyrantel, fenbendazole, or ivermectin again, depending on the parasites affecting the individual and the horse’s environment. We do recommend using name brand products, as previous studies of generic ivermectin have found it to contain less than the advertised quantity of active ingredient—the attractive price is not worth the reduced efficacy. Young horses may require more frequent deworming, but their specific parasite issues are another topic entirely.
Please contact us at 636-458-6569 to let us know if we can help you with your equine deworming needs. We can easily collect a stool sample from your horse when we are performing other treatments on the farm, or you are welcome to bring it by the clinic for analysis. After performing a fecal egg count and asking a few questions about your horse’s environment and deworming history, we will be able to advise you on a customized deworming plan for your horse.