While equine dewormers and rotational deworming practices have been around since the 1960s, our understanding of parasite control has changed quite a bit. Historically, horses were dewormed on fixed schedules with treatments monthly, bimonthly, even daily in some cases (yikes!), while typically rotating a variety of products throughout the year. A large portion of the equine industry still follows these routines to this day, however current research has made it very clear that our previous approach to blind, rotational deworming has quietly created a much bigger issue: parasite resistance. Fecal egg counts (FECs) have become a key part of the conversation to combat this problem in recent years, and for good reason. In addition to basic management practices, they’re one of the most important tools we have for slowing parasite resistance and creating an effective, sustainable deworming program for your horse and herd.
First things first… What are we targeting, and why?
Decades ago, our rotational deworming programs worked well against the main parasite concern of the time, known as large strongyles. Over time, another group of parasites known as small strongyles (or cyathostomins) has become the most prevalent parasite we target in adult horses, and they have a much different (and very frustrating) lifecycle. After a horse ingests these parasites on pasture, the larvae have the ability to encyst within the intestinal walls and essentially “hide” for months to years before emerging, which is when they can cause the most damage. On top of that, most dewormers are only effective at killing parasites within the intestinal lumen (opening), not the encysted ones. This ability to hide during their lifecycle has played a major role in the development of resistance to many commonly used dewormers, particularly in regions densely populated with horses throughout the country. Furthermore, larvae encysted in the intestinal wall don’t actively shed eggs. Horses can look completely normal while carrying large numbers of encysted larvae, and it’s possible for a horse to have a significant encysted parasite burden and still show up as a “low shedder” on a fecal egg count. Frustrating, right?
So, how does this “parasite resistance” happen? How do we accidentally create superworms and why do we need to care about them?
Antiparasitic resistance is exactly what it sounds like: parasites developing the ability to survive drugs that used to kill them, then passing that ability to their offspring. When a few naturally resistant worms survive a deworming treatment, they reproduce and pass their inherited resistance onto the next generation. The more doses of the drug that are administered on a frequent, routine basis, the larger the population of parasites that are immune to the drug grows. From a technical standpoint, antiparasitic resistance is said to occur when a dewormer fails to reduce egg counts by at least 95% after treatment. But from a practical standpoint, it really means: the product isn’t working like it used to. This phenomenon is already widely researched and documented with our current deworming medications, and without responsible, targeted deworming strategies, we run the risk of eventually losing effective dewormers altogether.
The American Association of Equine Practitioners (AAEP) now recommends shifting away from routine, rotational deworming and instead using a more strategic, targeted approach to deworming based on FECs (fecal egg counts).
What do FECs tell us, when do we do them, and how do we incorporate them into real-life equine parasite control?
A fecal egg count is a test that measures how many parasite eggs your horse is shedding in their manure. While the amount of eggs shed can fluctuate daily, horses tend to stay fairly consistent over time- some are naturally low shedders, while others consistently report much higher egg counts. Research shows that in reality, most adult horses tend to be low-to-moderate egg shedders, and typically only a small minority of horses are shedding the majority of the eggs in a given herd. This finding is often described as “the 80/20 rule” by Dr. Martin Nielson, one of the nation’s leading experts in equine parasitology, and it’s one of the most practical contributions to improving our current deworming strategies. It’s important to note that in most cases, you cannot tell if a horse is a high shedder simply by looking at them; testing is the only way to know. FECs help us identify which horses are contributing to the most pasture contamination of parasite eggs, allowing us to alter our management plan to target them more frequently than the low-shedders. As a result, it can also prevent over-treating the low shedders, which would ultimately build resistance by repeatedly exposing the parasite population to drugs unnecessarily. Translation: your whole herd does not need the same deworming plan, and doing so will cause more harm than good in the long run.
Additionally, we can assess a horse’s resistance to certain dewormers by performing a Fecal Egg Count Reduction Test (FECRT), where FECs are checked before and after deworming at the appropriate time based on the type of dewormer used. As we discussed above, the egg count should decrease by over 90% when tested at the appropriate interval after treatment. Failure to see an appropriate reduction in egg count indicates resistance to a dewormer, and helps us choose a more effective product in the future to prevent continued pasture contamination of resistant parasites.
Current guidelines for performing FECs recommend annual testing (typically in the spring or fall) to identify the low, moderate, high shedders as well as performing FECRTs, especially on high shedding horses. Most adult horses only need 1-2 deworming treatments per year, and additional treatments should be based on individual fecal egg count results, targeting treatment only where needed rather than deworming every horse the same way. It’s important to note that we recommend treating each horse for tapeworms and encysted larvae once annually in the fall using a product that’s effective against these types of parasites, typically Moxidectin/ Praziquantel (Quest Plus).
Good pasture management is also an essential component of successful parasite control. Simple practices like rotating pastures, frequent manure removal (ideally multiple times per week), avoiding overcrowding, and properly composting manure can significantly reduce parasite exposure and reinfection pressure. These management strategies work hand-in-hand with targeted deworming to keep overall parasite burdens lower across the herd.
At the end of the day, preventing parasite resistance comes down to a few key things: using fecal egg counts to guide decisions, implementing good pasture management, following research-based deworming recommendations, and working with your veterinarian to develop a plan that fits your individual horse and herd. Remember, the goal of a responsible deworming program is not to eliminate every parasite from a single horse, but to maintain the health of the entire herd while preserving the effectiveness of the dewormers we still have. If you’re not already incorporating fecal egg counts into your horse’s routine preventative care, that’s the best place to start. From there, we can build a plan that’s tailored to your situation and horse’s needs. Smarter parasite control isn’t about doing more… It’s about doing the right things, at the right time, for the right horses.
References
American Association of Equine Practitioners. (2024). AAEP Internal Parasite Control
Guidelines. Retrieved from
https://aaep.org/wp-content/uploads/2024/05/Internal-Parasite-Guidelines_Updated.pdf
Colorado State University James L. Voss Veterinary Teaching Hospital Equine Field Service.
(2021). Equine Recommended Deworming Schedule. Retrieved from
Nielsen, M. K. (2012). Sustainable Equine Parasite Control: Perspectives and Research Needs.
Veterinary Parasitology, 185(1), 32–44. https://doi.org/10.1016/j.vetpar.2011.10.012. Retrieved from https://ker.com/wp-content/uploads/Sustainable-Equine-Parasite-Control.pdf
Nielsen, M. K., Slusarewicz, P., Kuzmina, T. A., & Denwood, M. J. (2024). US-wide equine
strongylid egg count data demonstrate seasonal and regional trends. Parasitology,
151(6), 579–586. https://doi.org/10.1017/s0031182024000489
