Veterinarians report that the best-known guideline among horse owners for deworming horses is to administer an anthelmintic treatment twice a year to every adult horse. This method, when paired with fecal egg counts to identify shedders that need more frequent treatment, is the responsible way to approach parasite control. Any other manner poses drawbacks when it comes to caring for not only our individual horses but also the collective equine population. And although consulting the guidelines published by the American Association of Equine Practitioners (AAEP) can be very helpful when you have specific questions, knowing which ones to ask can be one of the biggest challenges in beginning your research. To help simplify and increase the effectiveness of your deworming program, two of the leading experts in equine parasitology and population medicine have addressed some of the most commonly asked parasite control questions.

Why does my horse need to be dewormed?

Heavy parasite loads are particularly harmful to foals and other young horses that do not have fully developed immune systems to help keep the “worms” in check, causing clinical signs ranging from respiratory disease to sometimes deadly impaction colic. Such burdens can also harm adult horses, causing nutrient loss, diarrhea, and colic, but ascertaining whether your horse is affected and if those parasites are drug-resistant is not as simple as looking at him—most adult horses show no external signs. Therefore, working with your veterinarian to create a deworming plan based on the results of regularly scheduled fecal testing is critical to successful parasite control.

“You need guidance on when to use dewormers, because you can’t tell whether a horse is parasitized on your own,” says Linda Mittel, MSPH, DVM, a faculty member at the Cornell University College of Veterinary Medicine, in Ithaca, New York. “Some of the horses with the highest levels of parasites are actually round and fat, and they look fine.”

​Why do we typically deworm our horses in the spring and fall?

The recommendation varies based on geographical location, but most of us need to plan to deworm our horses when the grazing season starts in the spring and in early fall, says Nielsen. This is because horses consume the most pasture during this time and, so, ingest worms and shed eggs. By late summer and early fall, parasite burdens have peaked.

We can’t count on excessive heat or cold to take care of the problem of parasite stages on pasture. “They develop quicker in warmer temperatures but live longer in colder temperatures, and live the longest under freezing conditions,” he says. “The external stages have a strategy for all conditions, including desiccation.”

Whatever month the grazing season falls in your part of the world, “killing the adults will cause (fewer) infective eggs to contaminate your pasture,” says Mittel.

What is parasite resistance, and should I be concerned about it?

All horse owners should be concerned about drug-resistant parasites. Simply put, parasites are becoming resistant to the drugs designed to combat them; it is a genetic adaptation that worms pass to succeeding generations, and once they develop resistance to a specific drug, they are unlikely to revert back to susceptibility. Widespread parasite resistance to available anthelmintics puts us at risk of not having effective treatment options for horses in need of such treatment. Currently the only method of testing for resistant parasites is with the fecal egg count reduction test (FECRT).

“I can guarantee that your horse is harboring drug-resistant worms right now,” says Martin Nielsen, DVM, PhD, Dipl. ACVM, an associate professor at the University of Kentucky M.H. Gluck Equine Research Center, in Lexington. “If you haven’t tested for drug resistance, you are fumbling in the dark.”

What is the difference between the fecal egg count (FEC) test and the FECRT? Why are they important in the overall parasite control strategy?

The FEC test is a quantitative assessment of your horse’s parasite egg output; you can determine whether your horse is a low, moderate, or high shedder. You can use the results to determine how frequently you need to deworm your horse. Veterinarians strongly recommended performing FECs on adult horses at least twice per year and performing a FECRT as part of your annual deworming protocol. You can use the latter to see if your horse is infected with anthelmintic-resistant parasites and determine which product to use to combat them. Performing these tests involves collecting two samples: one prior to administering the dewormer and the second 14 days after deworming. The main goal of testing before and after deworming your horse is simple: “You need to know if the dewormer works,” says Nielsen. “By conducting an FECRT, you will be able to check this. If you don’t do this, you likely end up using products that aren’t effective.”

Also be sure to have routine FEC tests performed on all new arrivals to determine their shedding level and check for resistant parasites before they transmit them to others in their pasture (via the horses ingesting eggs and/or worms while grazing).

With all the focus on identifying shedders and whether the dewormers are working in those horses, it’s easy to get into the mindset of managing the horses, when really we need to remember that we’re managing the parasites therein.

“It is important to understand that we are testing and controlling the parasite population, not the horses,” says Nielsen. “Each horse represents a biological sample of that parasite population, so they all have to be included in the same strategy. A boarding situation where each horse owner takes care of their own horse without knowing what the next person is doing and when is not a strategy. All horses have the same types of parasites, and people should not worry about a given horse bringing in some particularly bad worms. All the worms are already there. What you do need to think about, however, is what sorts of resistance you are introducing every time a new horse arrives on a farm. Without testing, you will have no clue about which drugs work and which ones don’t.”

How do I know if my pastured horse needs deworming?

All pastured horses are exposed to parasites. “These horses have access to two parasites in particular, small strongyles and tapeworms,” says Mittel, and require monitoring. “That’s where we would come in to test fecal egg counts (for strongyles) and use the results to determine if they need to be dewormed.”

Horses with low numbers of eggs on their FEC test results might not need to be dewormed, while high shedders do. We’ll discuss tapeworm control strategies in a moment.

My horse lives in a drylot and does not share a pasture with other horses. Do I still need to test and treat him for parasites?

If your horse’s only turnout is in a drylot, he has a low likelihood of acquiring parasites from other horses because parasites’ external stages like moisture and live on blades of grass. However, if your horse ever leaves the property and goes somewhere he might have access to shared grazing space, you need to be mindful of parasite transmission.

Are cleaning pastures and spreading manure effective methods for reducing parasite populations?

You’ll manage shared pastures differently than you will shared drylot space. Again, the likelihood of parasites causing a problem on a drylots is not very high, even if they’re shared. While it’s important to keep those areas clean for other reasons, from a parasite control perspective you need to prioritize keeping your pastures clean.

“If you can consistently remove manure from pastures, that does a tremendous amount to minimize parasite problems,” says Mittel.

On the other hand, if you leave the feces out in the field for months, the eggs have time to hatch and the larvae have time to develop to the third, infective stage. “These migrate out of the fecal piles and onto the grass, where they can be picked up by grazing horses,” says Nielsen. “If feces are removed before the larvae reach the third stage, the transmission will have been prevented.”

While it can be challenging to manage a large-acreage property, you can take steps to minimize horses’ parasite exposure by focusing your cleaning efforts on high-traffic areas. “If there’s a way to pick up these areas, that can be helpful,” says Mittel. “In general, if there is any way to keep pastures cleaned, we’d see a major improvement in parasite control everywhere.”

She advises against spreading noncomposted manure on pastures. If you do this, horses have no way of selective grazing, which generally helps them avoid picking up parasites.

How should I handle deworming a horse with an unknown history?

“The guideline for these horses is not any different,” says Nielsen. “You need to test the horse’s FEC regardless of history, and then you need to test again to see if the dewormer worked.”

Is the goal of deworming to rid my horse of all internal parasites?

“We do not deworm to keep our horses parasite-free,” says Nielsen. “That is neither an attainable nor a desirable goal. It is completely natural and normal for horses to be harboring worms, and there is nothing wrong about it. Our goal with deworming is to minimize the risk of parasitic disease.”

When should I use a daily dewormer?

Your horse should never be on a daily dewormer, says Nielsen. “Not a single horse in the world needs to be administered the same dewormer medication day after day. These products were developed to falsely lead people to believe that they could actually keep their horses parasite-free. They cannot and they should not. Besides, daily dewormers are ineffective due to all of the resistance developed to their active ingredient.”

Does fecal testing detect all worm species?

Tapeworms and pinworms are two parasites that regular FEC testing does not easily detect. “Currently, one of the best ways to identify tapeworms is via ELISA tests, which are mostly available in Europe and is not widely used in North America,” says Nielsen. “Parasitologists in the United Kingdom recommend testing for tapeworms once a year.”

In general, tapeworms don’t produce as many eggs as other types of worms, such as strongyles. Because their numbers aren’t as high, they don’t always appear in fecal samples.

If your horse seems to constantly suffer from “itchy butt,” have your veterinarian check for pinworms using a Scotch tape test. “Pinworms live in the (dorsal, or back part of) the colon, and they crawl out of the anus to lay their eggs around (it), which can cause horses to itch their tail,” says Mittel.

The act of tail rubbing only increases pinworm egg transmission throughout the environment. Carefully washing these horses’ perineum and perianal regions can help remove parasite eggs and relieve horses’ urge to scratch. Pinworms are widely resistant to ivermectin and moxidectin, and Nielsen says there are anecdotal reports of resistance to other dewormer classes, as well.

Take-Home Message

In the end, the most important parasite control steps to take are FEC testing and creating an appropriate deworming program in consultation with your veterinarian. Without routine testing and careful administration of the correct dewormer, you can end up wasting your money on ineffective products and contributing to the growing issue of parasite drug resistance, putting the global equine population at greater risk of parasitic disease.