Roundworms, bots, tapeworms, strongyles: there are many kinds of parasites ready to take up residence in your horse’s body. That hasn’t changed, but the strategies for keeping them under control have evolved significantly in recent years. Here’s what you should know about managing the parasite problem.

A History of Resistance

Parasite control was pretty simple for horse owners in 1978, when Dr. Wendy Duckett graduated from veterinary school. The recommendations were that all horses were given specific drugs via a stomach tube on a regular schedule, which varied a bit depending on where you lived. It seemed to work well at the time, but a serious problem was looming.

“Even in the 1940s and ’50s, some parasites were starting to develop resistance to some medications,” said Dr. Duckett, now a professor at the Atlantic Veterinary College in Charlottetown, PEI. “But this kind of repeated exposure to the same dewormers creates more resistance.” As the number of resistant parasites increases, the medications become less effective and horse health is more at risk.

By the 1990s, the concern about the problem was growing and researchers had begun to publish studies highlighting the issues.

Dr. Fernando J. Marqués, an associate professor of Internal Medicine of the Western College of Veterinary Medicine in Saskatoon, pointed out that the number of medications available to deal with equine parasites is fairly small, so resistance can be a major concern. It’s not easy to find another medication if one stops working. “The goal today is to protect the parasite refugia, the non-resistant parasites,” he explained. “To do that, we need to only use medications when necessary.”

The American Association of Equine Practitioners states in their guidelines that the goals of any parasite control program today are:

1. To minimize the risk of parasitic disease.

2. To control parasite egg shedding.

3. To maintain efficacious drugs and avoid further development of anthelmintic resistance as much as possible.

The Prevention Triad

The Parasite
The normal life cycle of a parasite is that the horse ingests parasite larvae (from parasite eggs present in manure in the pasture, typically), these mature in the horse’s gut or, in some cases, other organs. The mature parasites lay eggs in the horse’s gut and these are shed when the horse defecates. This new generation of eggs is now left in the pasture, where the larval stage develops and are ready for the next horse grazing to take them into his digestive system.

But that is just a general picture; some parasites have variations on this. For example, small strongyles – today’s most common parasite – can become encysted, remaining in the horse’s system for a year or so, having minimal if any effects on the horse.

Previous treatments focused on killing the parasite within the horse, leaving larvae and eggs in the pasture where the horse could easily become re-infected. Today, veterinarians work towards controlling the parasite outside the horse as well. Doing that effectively requires testing and developing a strategic parasite control specific for the individual farm. “You need to know what parasites you are dealing with, because each one is a bit different,” said Dr. Marqués. “You want a strategy that touches all the phases of that parasite’s life cycle.”

The Environment
Dr. Duckett teaches her veterinary students an entire lecture on preventing parasite infestations with an emphasis on pasture management. Reducing the opportunity for horses to ingest eggs will help keep the numbers down. She recommends that, if possible, the manure in the pasture should be picked up frequently and composted; the heat generated in composting will destroy both parasite eggs and larvae. Spreading fresh, non-composted manure on a pasture, though, will just spread the infection further and should be avoided.

A 1986 study found that twice-weekly cleaning of the pasture controlled parasites as effectively as using medication.

Rotating pastures is another strategy Dr. Duckett says can be very effective. Without horses to consume the eggs and allow the parasites to grow in their digestive tracts, the parasites will eventually die off. How long to keep horses off the pasture will depend on factors such as plant growth and weather – parasites live longer in humid, mild weather as compared to when it is hot and dry. As such, you should consult your veterinarian on the best course of action.

Dr. Duckett also said putting other types of grazing animals on the pasture can be helpful, although horse owners will need to be aware of parasites the other animals may be carrying. Fortunately, equine strongyle larvae are quite host-specific; they cannot infect cattle, sheep, goats, alpacas or llamas.

If a new horse is coming on to your property, Dr. Duckett said it is very important to keep him or her isolated and away from pastures the other horses use at first. Have a fecal egg count done, provide medication if needed, and do another fecal egg count two weeks later. “If the egg count doesn’t decrease after you’ve given the medication, that’s an indication of resistance,” she said. “Horses transported from a region where parasites have a high level of resistance will introduce those resistant genes into your property, unless you take these precautions.” If resistance is present, you should continue to keep the horse out of shared pastures until a different medication is found to treat the parasites.

The Horse
A healthy horse will be better able to resist parasites and deal with the negative effects of having parasites in his system. Paying attention to your horse’s nutrition and conditioning is an important part of minimizing parasite risk by helping strengthen his immune system.

Individual constitution may, however, be the main determinant of whether a horse tends to acquire high levels of parasites or not. Dr. Marqués explained, “There seems to be something inherent in each horse that determines whether they are high or low shedders of parasite eggs – it could be genetic, but we don’t know at this point.” He said researchers have found that horses that are high shedders at one point in their lives tend to be high shedders at other times as well. The same applies to those who are low shedders.

The Presence of Parasites

How can you tell if your horse is infected with parasites? Dr. Marqués said that at the most basic level, parasites steal valuable nutrients from the horse’s food. This causes many symptoms related to decreased nutrition: a dull hair coat, loss of energy, fatigue. Large numbers of parasites in the gut can also cause colic. Other parasites migrate from the gut to other organs such as the lungs and liver, often inducing severe damage and predisposing the horse to bacterial infections and organ dysfunction.

A horse’s immune system will respond to the presence of parasites, but a heavy load of parasites can overwhelm it making the horse more susceptible to other infections.

Unfortunately, signs of parasite infection and disease are often not very specific. A horse can be tired or colicky for many different reasons, and infections happen even to horses without parasites. Even worse, some of the most serious effects of parasites may go unnoticed. “Some parasites can cause extensive internal damage to the horse’s organs before the owner realizes it,” said Dr. Marqués. “In the worst cases, this can be fatal.”

This is why veterinarians today recommended horses be tested for parasites before treatment, so that the appropriate medications can be administered only when needed.

Instead of treating all horses on a farm with the same dewormer on a set schedule, veterinarians today recommend that horses be tested, through a fecal egg count process, to determine which parasites are present.

When should that testing be done? The American Association of Equine Practitioner Guidelines, updated in February 2016, point out: “Studies have illustrated that parasites reduce their egg shedding outside the grazing season, where conditions are less favorable for parasite transmission. This indicates that fecal egg counts may be less reliable in cold winter months (northern climates) and during hot, dry summers (southern climates).”

The guidelines also recommend that sufficient time be provided since the last time the horse was treated for parasites to ensure an accurate count. The length of time will depend on the medication used. The guidelines recommend:

  • After Moxidectin (ERP = 10-12 weeks): Wait ≥ 16 weeks to collect a fecal sample.
  • After Ivermectin (ERP = 6-8 weeks): Wait ≥ 12 weeks to collect a fecal sample.
  • After benzimidazoles (fenbendazole/oxibendazole) or pyrantel (ERP = 4-5 wks): Wait ≥ 9 weeks to collect a fecal sample.

Once you have the results for each horse in your herd, you and your veterinarian are prepared to make decisions about treatment. The most important point: only those horses with significant numbers of parasites (“high shedders”) should be treated. Horses with low numbers can go untreated, and, in most cases, these will be the vast majority of horses in the herd. This increases and protects the refugia (parasites that are not resistant to the medication used).

Fecal Collection Video
Not sure how to collect manure for testing? Visit where Equine Guelph’s Dr. Susan Raymond demonstrates what to do.

Dr. Ducket pointed out that this more individualized approach has benefits beyond reducing resistance. It tends to save money because fewer horses are being treated and are usually treated less often, for example. It also keeps any unnecessary anti-parasite drugs from contaminating the environment when excreted by horses.

The testing reveals specifically which parasites are present, so that the appropriate medication can be chosen. This is crucial because the parasite population can change over time and will also be different from one location to another.

“In the old days, we were mostly concerned with large strongyles,” Dr. Marqués explained. “Now the main parasites seen are small strongyles [cyathostomins] and Parascaris equorum [roundworms/ascarids] in foals.”

However, Dr. Marqués said he has recently seen an uptick in the number of cases of horses with heavy infestations of large strongyles in Saskatchewan. “I have seen cases in the hospital recently with these large strongyles at levels that I haven’t seen for years.”

Researchers have found that there can be a large difference in the types of parasites present between one farm and the next, both the species and the level of resistance to various medications. That reinforces the need to conduct testing before treating.

At least two weeks after the medication is given, fecal egg testing should be repeated to be sure that the number has dropped.

The AAEP guidelines remind veterinarians and horse owners that despite the value of fecal egg counts, it’s not a perfect system. Some challenges they mention:

  • They do not detect immature or larval stages of parasites including migrating large strongyles and roundworms (ascarids) and/or encysted small strongyles (cyathostomins).
  • They do not accurately reflect the total adult strongyle or roundworm burden of the horse.
  • Tapeworm infections are often missed or underestimated by fecal techniques.
  • Pinworm eggs are usually missed since they are adhered as egg packets around the anus rather than being shed in the feces.

Despite that, the fecal egg counts do provide much helpful information that can help guide treatment decisions.

Parasite control in 2016 and forward may not be as simple as the old approach, but the individualized approach recommended today offers better outcomes with lower costs and less resistance.

Types of Dewormers

Anthelmintics are medications used against parasites. Currently, the following ones are available in North America according to AAEP:

Benzimidazoles are available in paste, liquid and pellets.

Tetrahydropyrimidines (pyrantel pamoate and pyrantel tartrate); the first is available in suspension and paste, while pyrantel tartrate is made in pellets and given long-term to prevent (but not cure) parasite infections.

Heterocyclic compounds (piperazine) are used infrequently in horses and piperazine is only sold in Canada. It affects only the adult parasites and is given by a nasogastric tube.

Macrocyclic lactones are the most potent medications used with horses and can also kill external parasites such as lice and mites. They are given as pastes (ivermectin) and oral gels (moxidectin).

Praziquantel is sold in combination with macrocyclic lactones in North America, but can be purchased separately in Europe. It is effective only against tapeworms.

Special Cases: Foals and Young Horses

Horses under three years of age are at higher risk of parasites and tend to have different ones than more mature horses, especially roundworms (ascarids). These can cause poor growth and intestinal impaction, and may migrate into the airway causing inflammation and coughing. The eggs can remain on pasture for several years and still hatch once they are consumed by the foal.

The AAEP recommends scheduled deworming for foals during their first year, rather than fecal egg counts. The first should be at about two or three months, using a benzimidazole drug, the second just before weaning at around six months, the third at nine months and the fourth at one year. Tapeworm treatment should be added at nine months. A fecal egg count should be done at weaning to determine the type of parasites present at that point. From then on, an annual fecal check should help develop an effective plan.