Equine Parasites & Deworming

The way that we approach “worms” and other gastrointestinal parasites in horses has changed dramatically in recent years. Previously, we tried to eliminate all parasites from our horses, and in the process overused the few deworming products we have available, creating parasite resistance. We now realize that healthy adult horses do perfectly well with a low level of parasites, much like how normal flora bacteria coexist in our bodies are don’t do us harm. Our aim now is to deworm most horses, termed “low shedders” as infrequently as possible, and save more frequent deworming protocols for “high shedders” – generally very old, very young, or otherwise immunocompromised horses. We also strategically time when the medications are given, to kill parasites at specific points in their life cycle and thus reduce the number of eggs in the horse’s environment over time. We’ll explain common equine parasites and the process of how we determine high and low shedders below.

Roundworms (Parascaris equorum)- Roundworms primarily affect young horses, especially foals. Foals become infected after ingesting eggs, which can survive for years in the soil. Infected foals do not grow as quickly or look as robust as foals who don’t have roundworms. As the roundworm larvae migrate through the foal’s body, they can cause airway irritation, which manifests as coughing and nasal discharge. Foals with heavy parasite burdens are at risk of small intestinal impactions and even intestinal rupture.  Overuse of certain deworming medications has led to parasite resistance in roundworm populations. Checking fecal samples before and after deworming is essential to ensure foals are being medicated appropriately.


Large strongyles (Strongylus vulgaris, S. edentatus, S. equinus)- Large strongyles are luckily much rarer today than they were years ago, thanks to better deworming medications and pasture management. Horses become infected when they ingest larvae living on grass contaminated with manure. The larvae can migrate within and outside of the intestinal tract, causing damage to tissue, including blood vessels. The signs of large strongyle infestation can be severe and include blood loss, weakness, weight loss, diarrhea, colic, and even sudden death.

Small strongyles (40+ different Cyathostome species!)- Small strongyles are the most common parasites in adult horses. Horses become infected by eating larvae living on manure contaminated grass. Unlike large strongyles, the small strongyles stay within the intestines and intestinal wall, and generally cause less damage. Healthy adult horses generally show no signs of small strongyle infestation unless the parasite burden is very high. In highly parasitized or immunocompromised animals, weight loss, intestinal inflammation, and diarrhea can be noted.

Tapeworms (Anaplocephala perfoliata)- Horses get tapeworms by eating their intermediate host, orbatid mites, which live in the grass. Tapeworms are difficult to detect on routine fecal testing, due to the intermittent shedding of tapeworm eggs into the horse’s manure. Tapeworms cause mucosal damage where they attach to the intestinal wall, and have been implicated in certain types of colic (ileocecal impaction, spasmodic colic). Because tapeworms are shed and transmitted seasonally (warm weather), we recommend deworming all horses against tapeworms at least once a year in the fall- after the first hard frost. This greatly reduces tapeworm numbers in your pasture during the next grazing season. 

Pinworms (Oxyuris equi)- Pinworms are an infrequent parasite or horses but can be a great annoyance to both horses and their owners. These small worms deposit their eggs just outside the horse’s anus. The secretions released by the worm in this process can cause an intensely itchy sensation and can sometimes cause horses to scratch their behinds furiously, causing damage to their tail hair as well as their surroundings! Pinworms are not as easily contagious as other parasites, but can be spread from horse to horse by the objects they itch themselves on.

Bots (Gasterophilus species)- Bots are the larval stage of bot flies. The adult flies lay eggs on the horse’s coat. When the horse licks or grooms itself, the eggs are carried into the mouth, where the larvae hatch. The larvae then bury themselves in the horse’s gums or tongue for about a month, then slide down the esophagus into the stomach. They attach to and live in the horse’s stomach using small hook-like mouth parts. This attachment causes small erosions on the stomach lining.  It is speculated that these erosions may increase the risk of stomach ulcers or colic (especially around mealtime) in a small population of horses. Most horses, even those heavily infested with bots, never show any signs. Mostly they are a cosmetic problem, easily controlled with common deworming medications.


We recommend that all horses in our practice have a fecal egg count test (FEC) at least once a year, in the early spring. This should be done before any deworming medications are given. This test measures how many eggs (generally small strongyles) are present in a gram of manure- and gives us an idea about how many worms your horse has. Low shedders are those horses who have 200 eggs per gram (EPG) or less. High shedders are those with greater than 500 EPG. Moderate shedders are in the 200-500 EPG range. Low shedders are generally healthy horses that are either residing on a well-managed farm or are naturally able to keep their parasite numbers in check (or both!). These horses only need to be dewormed twice a year, with a combination product that treats strongyles, bots, and tapeworms. The number of eggs present at a “low shedder” level is acceptable for most horses. The high shedders are those horses that are not as able to keep the parasites in check. Many high shedders show no signs of being heavily parasitized, but we know that high of a parasite burden makes them more vulnerable. These horses should have their egg count checked more frequently, and generally require being dewormed more frequently as well. Because these horses can’t keep their own parasite numbers low, they shed more eggs into the environment and are often the source of infection for moderate and low shedders on the same farm. Picking manure out of your pastures every few days, keeping all of the high shedders in a separate turnout from low shedders, and using a strategic FEC and deworming plan all play a role in keeping your farm and horses healthy and with a low overall parasite burden. The moderate shedders are usually horses that can keep their parasite numbers down, but are faced with a higher parasite population on their farm. This can be due to high shedders contaminating the pastures, or parasites that have become resistant to the dewormers used on the farm. In these situations we recommend a series of fecal egg counts. The first (prior to deworming) establishes the baseline parasite load. A second FEC (a week after deworming) makes sure the dewormer used actually did its job- if not, the parasites are likely resistant and we should change medications. An effective dewormer should reduce the number of eggs by at least 90%. A third fecal is performed after the egg reappearance period- the time it takes after a deworming for your horse to become re-infected and pass eggs in his manure, if he is being continually exposed to eggs in his environment. This time period depends on which deworming product was used, and can be 8-16 weeks after deworming. If this third FEC shows a high parasite load, then it is safe to assume that the horse’s environment is to blame. In moderate temperatures and moist conditions, eggs can live for many months. Hot, dry weather, and below freezing temperatures will both kill eggs. The best way to keep your horse’s pastures clear of eggs is to remove all the manure piles at least every 3 days- year round. In the summer, you can also rake the manure piles to dry them out, but this is only effective if the weather cooperates! A strategic deworming plan seems expensive, but only the first year it is implemented. Once you have a plan in place- you can get all horses on the farm into the low shedding category and use a lot less medication in the long run! 


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