Surveillance deworming made simple

Now is the time to switch from calendar-based deworming to a targeted program—for the sake of your horse’s health, your pocketbook and the equestrian community at large.

If you were to provide horse owners with scientifically sound advice on how to protect current and future generations of horses from parasites, while saving themselves money in the process, you’d think most would enthusiastically adopt those recommendations. But, for some reason, that hasn’t been the case when it comes to parasite control.

Using fecal egg counts to devise a deworming program will protect future generations of horses and likely save you money right now.

For at least a decade, we’ve known that targeted deworming programs—which use regular fecal egg counts (FEC) to identify horses that need treatment—are far superior to the traditional calendar-based methods, in which each horse in a herd is treated every eight weeks. Not only is the old system financially wasteful, but it increases the risk of that parasites on the property will become resistant to a particular anthelmintic, rendering any product containing that chemical ineffective on that farm for years and possibly forever. 

FEC-based targeted deworming, sometimes called surveillance deworming, identifies the horses in a herd that are shedding significant amounts of parasite eggs and treats only those horses with the appropriate products on a schedule dictated by their response to the treatment. It’s an approach that saves money—because many horses will need deworming much less frequently—and reduces the risk of resistance within the parasite population. It’s the right thing to do for yourself, your horse and the equestrian community at large.

Yet, according to the United State Department of Agriculture’s National Animal Health Monitoring System (USDA-NAHMS) 2015 study, only 5.5 percent of farms with horses age 4 and older tested manure for parasite eggs and then based their deworming practices on the test results. This means that nearly 95 percent of people caring for mature horses chose to stick with a more expensive parasite control method becomes less and less effective over time. The numbers aren’t much better on farms with more varied equine populations: The same study found the largest group of FEC adopters were operations housing foals, but only 8.2 percent of those reported using surveillance deworming. Why?

My best theory is that switching to a targeted deworming program can seem a little daunting. After all, it’s a dramatic departure from protocols established three decades ago and followed religiously ever since. But, in this case, the change is worth it. Surveillance deworming is not the “set it and forget it” type system we’re accustomed to, but it’s not especially difficult, either. As an equine veterinarian, I’ve helped dozens of owners, on small farms as well as huge boarding stables, make the transition. And I hope that by providing you with some information, I will persuade you to consider making the switch, too. 

Switching to surveillance deworming

The first step in the transition to a targeted deworming program is to determine your horse’s parasite status. You’ll do this four to 12 weeks after you delivered the last calendar-based deworming dose, depending on the efficacy period for the product you last used. Waiting this long provides an accurate picture of how many and what types of parasites the horse carries around when there are no dewormers in his digestive tract. Go online to look up the efficacy period, often called an egg-reappearance period (ERP), of the last product you used. ERPs of all deworming products are slowly declining due to resistance, and they vary a bit with geography, but research continues to be done to keep this information up to date.

When you know your product’s ERP, the next step is one you’re already probably familiar with—picking up manure. You need only one or two fecal balls from your horse, not an entire shovelful (which I’ve been handed in a bucket more than once). You’ll want relatively fresh manure that hasn’t dried out or frozen solid. For a stall-kept horses this is easy to collect; just walk through the barn before the morning or afternoon mucking. If your horse is turned out with a herd, you’ll need to devise a collection method that ensures you get the right manure. One way is to tie your horse to a hitching post until he defecates. Feeding him can speed up that process. If you have several horses, you may spend the better part of the day waiting, but it’s time well spent. If the weather is mild and you’ll be passing the samples to your veterinarian the same day, the baggies can sit in the tack room. If it’ll be more than 12 hours before they are delivered, or if it’s a particularly hot day, store the baggies in a fridge or cooler so the parasite eggs do not die off, which will lead to false results.

Your veterinarian will send the samples to a laboratory for a FEC. You could, in theory, send them off to a laboratory yourself and receive the results yourself, but trying to interpret them that would be like trying to use your own bloodwork to make adjustments in your own medications: You run the risk of making some serious mistakes. Your veterinarian has the training and experience to fully understand FEC reports and to devise a deworming program based on them. Plus, veterinarians work to stay up to date on the latest research in this area.

The FEC results are reported in numbers of eggs per gram (EPG) of manure. These eggs are from small strongyles, which are the primary parasite of concern in horses. A few laboratory reports will also include a count of ascarid eggs, which affect mainly younger horses. If you’re interested in that information, tell your veterinarian. FECs cannot determine levels of tapeworms because they produce eggs only sporadically. Once a year you’ll want to deworm every horse on the property for those using a double dose of pyrantel pamoate or praziquantel. Also, bot eggs won’t show up in a FEC because they are laid outside of the horse. But deworming with ivermectin or moxidectin, which you’ll be giving for strongyles, will also control bots.

FECs provide an objective representation of how many mature, egg-producing strongyle worms your horse carried at the time the manure was produced. If a horse has less than 200 EPG, he has a low worm burden and is considered a “low shedder.” On the other end of the spectrum, a horse with greater than 500 EPG is considered a “high shedder” who carries a relatively heavy parasite burden, even if he looks slick and healthy. Studies have shown that 80 percent of worm eggs on a pasture are shed by 20 percent of the horses, so high shedders have a huge impact on the parasite status of other horses on a farm or in a field.

Don’t be surprised if the numbers from individual horses on the same farm with the same management vary greatly—each horse has a different genetic susceptibility to parasite burdens. Some horses will have high number of parasite eggs while others kept in the exact same environment will have next to none. Barring any significant change in health status, a horse will stay in the same shedding category for his lifetime, so you can’t “convert” a high shedder to a low-shedder status though aggressive deworming.

Building a surveillance program

If every horse on your farm turns out to be a low shedder, congratulations. This means you might need to deworm all your horses only once or twice a year. The exact timing will depend on your geographic location and even microclimate—worms are less active during cold winter months and dry, parched summers. On the other hand, worms are never dormant in some regions with moderate climates. Your veterinarian will be familiar with the parasite cycles in your area and make suggestions. Sometimes it makes sense to do FEC tests quarterly for the first year to see when numbers begin to rise past 500, which indicates that it’s time to deworm again. Once you’ve learned the pattern, you can scale back on the frequency of FECs. You’ll always do at least one a year, though, to ensure your program is still working.

Don’t be surprised if your veterinarian recommends products from the same chemical class for two consecutive dewormings. With so long between treatments, rotation isn’t necessary to maintain the efficacy of the products. You may, for instance, give a macrocylic lactone product in the spring and another macrocylic lactone product with pyrimidine added for tapeworms in the fall.

Moderate shedders, with FEC between 200 and 500, may need three treatments a year. Again, the timing will depend upon the climate, and quarterly FECs can help determine the pattern. With three treatments a year, you’re likely to be using products from two or more classes of drugs.

Keep in mind that high-shedding horses aren’t sickly or weak; they simply need more frequent deworming to stay healthy. These horses may need four treatments a year, rotated through all classes of dewormers. That’s about every 13 weeks. Even with regular treatments, high shedders also need quarterly FECs the first year and twice a year once you’re confident of the results of your program.

By now you can see that surveillance deworming means that there is no standard prescription for each type of horse or a scheduled course of treatment that you can put neatly on your calendar. There are simply too many variables to consider.

For instance, in my area of the country—northern Virginia—we are seeing eggs reappearing after pyrantel treatments in four weeks or less, and four to eight weeks after fenbendazole treatment, so some of our horses are dewormed more frequently. But then, when it’s super cold outside, we don’t have to deworm as much because the eggs aren’t hatching. Nor do we deworm during long, dusty periods in the summer. That said, warm winter weather that allowed the grass to keep growing meant we slid into another into deworming period before we got a good freeze. We emailed our boarders and said we’d be doing another round of treatments. In this system, you are constantly evaluating and thinking and adjusting.

Spotting resistance to deworming products

If your initial FEC reveals a large percentage of high shedders on your farm (more than a quarter of the tested horses), you’ll want to take a second step in laboratory testing to determine if the parasites on your farm have become resistant to the chemical last used.

To do that, administer the same dewormer as soon as possible after you get the results of the FEC—the same day would be ideal. Then take another sample, two weeks later, from the high-shedding horses. The timing is critical because you want to see how the dewormer affects the FEC.

Waiting a few days to deworm after the first test or doing the repeat testing too early or too late may allow other factors to influence the results. If the anthelmintic you used is still effective, you will see a dramatic drop in the FEC numbers—at least a 70 percent reduction across all horses on the farm (there is a mathematical formula your veterinarian can use to determine this). If that drop does not occur, you likely have resistance to that product on your farm.

It’s important to remember that it’s not the horse who is resistant to the chemical, it’s the parasites that populate your farm. That chemical will not be effective in any horse on your property, possibly for many years. If you have a closed herd, it could take eight to 10 years for a parasite population to diversify enough for the product to be effective again, or it may never happen. If you have a busier barn, with new horses coming in regularly, that process may occur within three to five years if you’re lucky. In the meantime, you’ll need to find an alternative product. It’s also important to know that dewormers are not intended to kill every parasite a horse carries. Any chemical lethal enough to parasites to achieve “0” on a FEC would be harmful to horses.

Surveillance deworming options for boarders

If you keep your horse at a boarding barn that’s still on a calendar-based deworming program, you might think you can’t switch to a targeted system. After all, you have just one horse in that environment, and boarding stable owners aren’t always open to suggestions about management changes. I’d still encourage you to try.

First, touch base with the barn owners. Don’t discuss this with fellow boarders before you’ve approached the management. If your horse is a low shedder, ask if you can submit his FEC results in lieu of regular dewormings. Loop your veterinarian into the conversation, if it’s helpful, and see everyone can agree to a target FEC number.

You might spend as much money on FECs as dewormings that first year, but if you can regularly demonstrate that your horse isn’t contributing to the parasite population on the farm, the owner may accept fewer FECs the following year. It’s important that you not portray this as a financial issue or an “anti-medicine” position, because it’s neither. You simply want to be a good steward of these important medications to protect their efficacy for other horses on that property.

If your FEC shows that your horse has a high worm burden, you’ll want to deworm him and then retest in two weeks to determine whether the product is still working on that property as I described early. If it is not effective, go directly to the barn owner, ideally with a veterinarian for backup. Don’t discuss the FEC results with other boarders. You’ll only start up a rumor mill that will leave everyone upset with everyone else.

The barn owner might want to retest to confirm the findings. That’s a smart thing to do. He or she may change the farm’s deworming protocol as a result or choose to do nothing. In that case, you could then give your horse a different product you know works, or you may decide that this isn’t the best boarding situation and look to move elsewhere.

In my experience, all but the most hard-line barn owners are willing to accept low FECs instead of deworming and, if you approach it all in a rational way, they usually come on board with targeted deworming after they understand how it works.

Surveillance deworming works best when an entire barn does it. That can happen even in the largest boarding barn. The first operation I helped switch to this system was a large facility with 60 horses where calendar-based deworming had been done since the 1980s. The barn owner designated a day to collect fecal samples from all horses and had the boarders help, pulling field-kept horses into stalls and waiting for them to produce manure.

With the test results in hand, I created a spreadsheet of horses to keep track of who needed treatments, who didn’t, and who would have follow-up testing. Horses were put into categories, with appropriate deworming protocols devised for each group. We even divided the heavy shedders into three treatment groups to test the efficacy of different types of dewormers that first year.

Fortunately, they all still worked. It was a complex process for sure, but once it was done, it was easy to sustain. New horses coming in have manure collected for an FEC the day they arrive, and they are put into the appropriate category based on the results. I do the same at my own farm where I have boarders.

Old routines can be hard to change, particularly when it comes to keeping horses, but the traditional method of deworming is a habit that every horse owner is obligated to break. If we are going to protect the efficacy of the available dewormers, thereby protecting our own horses as well as generations to come, we need to switch to the more modern, results-based system. It may seem like a hassle at first, but there is simply too much at risk to not make the effort.  

Don’t miss out! With the free weekly EQUUS newsletter, you’ll get the latest horse health information delivered right to your in basket! If you’re not already receiving the EQUUS newsletter, click here to sign up. It’s *free*!




Related Posts

Gray horse head in profile on EQ Extra 89 cover
What we’ve learned about PPID
Do right by your retired horse
Tame your horse’s anxiety
COVER EQ_EXTRA-VOL86 Winter Care_fnl_Page_1
Get ready for winter!


"*" indicates required fields


Additional Offers

Additional Offers
This field is for validation purposes and should be left unchanged.