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Do you remember when your barn calendar said “deworm” every six to eight weeks? Like the good horse owner you are, you carefully rotated the deworming medication you used to avoid causing parasites to develop resistance. Then “strategic deworming” came along. Every six weeks turned into every six months—and your budget for deworming medications shrunk, replaced by costs of fecal egg counts used to monitor your horse’s actual parasite load. It seems to make sense, but is it really working? You bet it is, and in this article I’ll tell you how.

I’ll begin by explaining what strategic deworming really means, and why it’s better not only for your horse, but for the environment. Then, I’ll give you a run down of how this deworming strategy works—both for individual horse owners and for large well managed barns. Finally, if you haven’t implemented strategic deworming in your own horse life yet, I’ll give you a step-by-step plan to get it going

WHAT IS STRATEGIC DEWORMING?

Strategic deworming can be simply defined as, deworming your horse in response to his current needs vs. deworming on a regular schedule. The foundation of this protocol is performing routine fecal analyses. There are two common types of fecal exams; quantitative and qualitative. Quantitative fecal analysis determines the level of parasitism in each horse, while giving you an idea as to the types of parasite present. Qualitative fecal exam simply identifies the types of parasites found.

A strategic deworming program relies on quantitative fecal analysis. If a fecal exam reveals greater than 200 eggs per gram (EPG) of feces it is recommended to deworm that horse. If your horse frequently yields high egg counts, it is recommend to perform a fecal egg count reduction test. This includes performing two fecal exams, one before deworming and one 10-14 days after deworming to ensure that the treatment is effective. Deworming at least twice a year, even with a low or negative fecal egg counts, is still recommended for all horses due to the life cycles of certain parasites.

So you might be thinking that if less than 200 EPG is good, than zero must be better. That’s not entirely true. Most parasitologists believe that small numbers of parasites are not only acceptable, but beneficial. When your horse is exposed to parasites throughout its life, it will develop some natural immunity. I realize this seems contradictory, but the take home message is that a small population of parasites are good and stimulate immune response. However, a large population can be detrimental to your horses’ health.

In addition to regular fecal analysis, it is important to consider environmental management. This is where we encounter more challenges since environmental management is a multi-step process that takes a great deal of commitment to perform. This includes, but is not limited to, regularly cleaning stalls, paddocks and pastures in an effort to eliminate/reduce the amount of manure contaminating your horse’s environment. Avoid spreading manure onto pastures where horses graze and mow grazing pastures below four to five inches. Avoid high stocking densities—most sources recommend one horse per one to two acres. Last, but not least, try to group your horses by age in separate pastures. Younger horses tend to shed higher amounts of parasites, where older horses tend to shed fewer. By co-mingling age groups you are unnecessarily exposing your older horses to higher amounts of parasite eggs.

WHY IS STRATEGIC DEWORMING BETTER?

The primary advantages of strategic deworming include minimizing the pressure on effective dewormers, identifying resistant horses and monitoring resistance on farms.

Minimizing the pressure on effective dewormers: Resistance to dewormers is well documented and widespread. There are currently a few products in the equine world that have had little to no resistance documented in the United States, such as ivermectin and moxidectin. However, if we continue to rely heavily on some of these products we may see resistance in the near future. In fact, recent studies have already reported ivermectin resistance in horse populations in South America. This data is limited, but should serve as a warning that with increasing intercontinental shipments of horses resistance is likely to quickly become a problem in the U.S. You may ask “so why don’t we just develop newer, stronger drugs?” It takes years of research and development, not to mention millions of dollars to get a new product on the market. As a result there is not enough of a financial gain for pharmaceutical companies to pursue these endeavors.

Identifying resistant horses: It is important to detect the horses within a population that carry a heavier parasite burden than others. This may help identify a horse with poor immunity against internal parasites. Does that sound like someone else’s problem? After all, you have routine quantitative fecal analysis performed on your horses manure, and they regularly come back negative. Think again. Someone else’s high shedding horse can quickly become your problem if that horse with a heavy parasite load moves into the barn where you board. This new horse will begin contaminating its new environment with the first pile of manure that it passes. Consequently, your horse will be exposed to this parasite population—quickly making someone else’s problem yours.

Monitoring resistance on farms: Despite our best efforts, resistance will likely occur. That is why it is crucial to continue to perform quantitative fecal tests. These tests will be our first indicator that a farm maybe contaminated with a resistant parasite population or to confirm that our current treatment plans are effective.

WHAT CHALLENGES ARE WE FACING WITH STRATEGIC DEWORMING?

Now that you understand what strategic deworming really is and how it works, here are some of the challenges you’ll face if you want to get things going. Regular monitoring of fecal egg counts is necessary, and this is where we encounter our first challenge…compliance with the program. In order for strategic deworming to be effective you have to be committed year round or else it will be impossible to determine its’ success. The primary reason that horse owners are initially resistant to switch to this program is cost. You may think that this type of program is expensive, when in fact strategic deworming saves money. Quantitative fecal exams can be performed by most veterinarians, and those that don’t offer this service can send them out to local diagnostic labs. The approximate cost of a fecal exam is eighteen to twenty dollars. Thus the money spent on deworming products when deworming every six to eight weeks with an interval deworming program is instead spent on fecal testing, resulting in comparable costs.

Another challenge is the possibility that the fecal test may come back inaccurately low. This can be partially due to parasites present in their larval form and not currently passing eggs into your horse’s manure. However, you can have complete confidence in a high egg count, (greater than 200 eggs per gram) and should treat your horse based on the product that your veterinarian recommends. And remember that deworming twice a year is still recommended even with low or negative fecal egg counts in part to account for potentially false negative tests.

So even with these challenges you must be wondering if these programs work and if they are cost effective? In short, yes! In our practice, we have been advocating strategic deworming for a number of years, and have successfully implemented programs with both individual horse owners and in large barns. We have been able to reduce deworming frequency in a large segment of our patient population down to twice a year based on fecal egg counts. We have successfully identified consistently high shedding horses within barns and have been able to segregate these horses and reduce expose to low shedders in the barn. Our clients are saving money, we are exposing both our patients and the environment to fewer chemicals, and we have minimized overuse of the few remaining effective deworming products.

Despite our success, resistance is inevitable and is quickly becoming a problem for all horse owners. The recent report of resistance to ivermectin and moxidectin adds two more products to the ever growing list of anthelmintics with decreasing effectiveness. This statement is not meant to make you shrug your shoulders in defeat, but instead intended to spur you into action to do your part to help preserve our pharmaceutical resources and protect our horses. If your haven’t implemented a strategic deworming program for your horses, do it now. For specific plans and treatments contact your local veterinarian.

Dr. Amanda Johnson is an associate veterinarian with Pacific Crest Sporthorse in Oregon City, Oregon.