Updating How We Deworm Horses

It’s Hard to Change Old Habits

I’ve probably lost a few readers already by merely suggesting a change, so congratulations to those who have made it this far! Although an equine veterinarian, I’m also an owner who has been ‘blanket deworming’ horses on a rotational schedule for the last 40 years, the same way most of you have. This ‘old school’ deworming style has accomplished what parasitologists set out for it to do in the 1970’s, which were to wipe out large strongyles (bloodworms) and obliterate verminous arteritis; now a rare and deadly disease of the past, caused by large strongyles. Fast forward, this kind of deworming program is not working well against small strongyles (cyathostomes), which along with tapeworms, are the most important parasites currently affecting adult horses. In light of this change, we need to re-strategize our deworming habits. Ascarids (roundworms) remain the most important parasite affecting younger horses.

The New Problem of Superbugs

Another reason for reform is the emergence of ‘resistant parasites.’ These are worms that remain unaffected by treatment with specific anthelmintics (dewormers) that once killed them (much like bacteria that have become resistant to certain antibiotics). There is known resistance to all available dewormers, less so with the avermectins. This is a particularly big problem since there are no new anthelmintics on the horizon.

How Does Resistance Occur?

After repeated exposure worms, similar to bacteria and viruses, learn to outsmart medications. Take ivermectin for example, at one time it worked reliably killing off both ascarids (roundworms) and strongyles. Unfortunately, now most ascarids are completely unaffected by it, and small strongyles are not far behind. So, by continuing to use the traditional approach where ALL the horses on a farm are treated at one time, we are making matters worse by knocking out all remaining sensitive worms and leaving behind resistant ones to repopulate. Resistance is then inherited in future generations of worms. Further, because we have been deworming at regular intervals regardless of worm burdens, parasites were exposed to medications unnecessarily and this is how we seem to have succeeded in exponentially increasing populations of resistant worms. It is now emphasized that horses with small numbers of worms do not need to be treated, as having some worms is a normal state of affairs in horses for which they have natural immunity.

Our Goal

Ultimately, we want to limit infections with parasites, so that horses stay healthy. To this end, we aim to significantly slow down the spread of resistant worms by treating less frequently, and by keeping a ‘healthy’ number of the non-resistant worms in the population.

Important Tools

Fecal Egg Count (FEC): A test used to group horses based on their innate ability to shed strongyle or ascarid eggs in their feces. Although horses may live in the same herd, they do not shed the same numbers of parasite eggs. Horses develop some degree of natural resistance to parasites and the more immune they are, the fewer eggs they shed. In fact, only a small number of horses may be responsible for most of the parasite eggs in the pasture (those are the ones we want to target for treatment). We, therefore, use FECs to determine numbers of strongyle and/or ascarid eggs in the feces, and based on this information, determine whether or not to deworm each horse (versus just deworming everybody). Limitations to the FEC include infection with pinworms, tapeworms and bots.

Fecal Egg Count Reduction Test (FECRT): A variation of the Fecal Egg Count (FEC) that is useful when resistance is suspected on a farm. The FECRT utilizes fecal egg counts performed before AND after deworming, to determine if a specific dewormer is still effective in killing strongyles or ascarids (or if there is resistance); a 90% reduction in egg counts are expected seven days following treatment with Pyrantel, ten days with Benzimidazoles, or 14 days with Avermectins. It is always most informative to do FECRTs on high shedders. Once resistance is positively identified, adjustments can be made to the arsenal of dewormers used for ALL horses on that farm.

Horse Feces

The Plan

Perform Fecal Egg Counts (FECs): Start by figuring out shedding patterns of each horse by performing three or four FECs in the first year of testing, avoiding periods of extreme temperatures. It is important to collect a fecal sample and perform a FEC, after the effects of the last dewormer administered are completely gone. FECs should be done at least 9 weeks after deworming with Pyrantel, Oxibendazole, or Fenbendazole; 12 weeks after deworming with Ivermectin; and 16 weeks after deworming with Moxidectin. The number of tests can be reduced after the first year, when there is strong enough evidence that a horse has a particular shedding pattern. Periodic testing is still advised, however, as egg shedding categories can occasionally change particularly in horses with values near the cutoff points FECs should only be performed on fresh feces and can generally be analyzed right at your veterinary hospital.

Get a Number: FEC results will yield a number in units of “EPG” (Eggs Per Gram of Feces). This number will help determine if your horse fits in the low, moderate, or high shedding group and ultimately, tell you how often to deworm them. The current recommendation is that only adult horses with heavy or moderate worm burdens be treated.

Keep your vet involved: Keep in mind that your veterinarian should oversee any changes to your deworming program and that the information given here is just a guideline.
Deworm every spring and fall at a minimum: All horses should be dewormed spring and fall, including low shedders. Spring treatment should target encysted small strongyles (for which there may be little or no egg shedding), with a fall treatment to target bots (which don’t produce eggs) and tapeworm (where the presence of eggs in the feces is unreliable). For horses that live alone, or in severe climates, a fall treatment may be all that is necessary. Deworm on an as-needed basis for pinworms.

What Do FEC Results Mean for My Horse?
Low Shedders (<200 EPG) – Generally horses with FEC reading at less than 200 EPG do not need to be dewormed, unless it is spring or fall. Once again, however, we strongly suggest low shedders have several FECs performed during the first year of testing to support a ‘diagnosis of low shedder,’ before jumping from three or more treatments a year to just one or two treatments per year.

Spring: FEC +/- treatment with Moxidectin, Ivermectin, or double dose Fenbendazole for five days.

Late Fall: Use Moxidectin + Praziquantel, or Ivermectin + Praziquantel. Do not use same anthelmintic (dewormer) used in spring.

Moderate Shedders (200 – 500 EPG) – These horses may require an additional deworming mid-summer or winter with the appropriate anthelmintic.

Spring: Use Moxidectin, Ivermectin, or double dose Fenbendazole for 5 days.

Summer: FEC +/- treatment with Pyrantel Pamoate, Fenbendazole, or Oxibendazole.
Fall: Use Moxidectin + Praziquantel, or Ivermectin + Praziquantel. Do not use same anthelmintic (dewormer) used in spring.

Winter: FEC +/- treatment with Pyrantel Pamoate, Fenbendazole, or Oxibendazole

High Shedders (> 500 EPG) – Horses that have a FEC of greater than 500 eggs per gram are deemed “high shedders,” and therefore have a high likelihood of earning this title again and again. These horses are thought to have compromised immunity. It is not uncommon to find horses with numbers in the thousands. It is important to seek out and deworm these horses carefully for their benefit, but also in consideration of the infection risk they pose to other horses. High shedders will require more frequent FECs to monitor shedding and more frequent anthelmintic treatments to control it; daily pyrantel tartrate (if not resistant) or moxidectin can be considered for suppression of egg shedding in consistent high strongyle shedders. Ultimately, a FECRT (to determine resistance patterns of strongyles and/or ascarids) should be performed at least once, for each kind of dewormer used on the premises.

Perform FECs prior to deworming Spring & Fall.

At any point, incorporate FECRTs for each kind of dewormer used.

Spring: Use Moxidectin, Ivermectin, or double dose Fenbendazole for 5 days.
Summer: Use Pyrantel Pamoate, Fenbendazole, or Oxibendazole.
Fall: Use Moxidectin + Praziquantel, or Ivermectin + Praziquantel.
Winter: Use Pyrantel Pamoate, Fenbendazole, or Oxibendazole.

Close up of bacteria

A Class by Themselves


Begin deworming at two months of age using Fenbendazole.

From 4 to 12 months of age, alternate use of Ivermectin and Pyrantel Pamoate every eight weeks. Tapeworm treatment (Ivermectin + Praziquantel or double dose Pyrantel Pamoate) can be included in one of the latter/fall treatments.

Horses less than three years old are more prone to damaging disease and colic from parasites. Therefore, they require more frequent FECs to monitor worm burdens. They should be treated as high shedders and should be subjected to FECs and 3 to 4 yearly treatments.


Broodmares should be dewormed as per any other adult horse, based on FECs incorporating a treatment of Ivermectin 4-6 weeks prior to foaling.

The practice of deworming broodmares with ivermectin the day of foaling has fallen by the wayside due to lack of evidence that threadworms (passed in the milk) are a problem in foals anymore.

Large Herds

In large herds where individual fecal exams are truly impractical not to mention impossible to obtain, FECs from a just handful of the horses 3 or 4 times a year may provide useful information about herd parasite status. Focus on those that appear to be skinny or have rough hair coats.

Dollars and Cents

Realistically, at first, you’re not going to save any money by testing in place of deworming. In the long run, however, once we have a handle on each horse in the barn, and provided your horse isn’t a high shedder, you likely will save money.

Take-Home Message

The best way to determine the deworming schedule for your horse is to perform fecal egg counts to determine the propensity of your horse to carry high, medium, or low worm burdens.
Individualized deworming programs created with your veterinarian’s help, allow the use of less dewormer.

Resistance in small strongyles and ascarids needs to be factored into decision making about what dewormers are to be used on your horse.
Bottom line, if we don’t change how we do things, we will soon be left with worms for which we have no effective treatment.

What Are You Using? Legend of Common Anthelmintic Products

The Avermectin Dewormers:
Ivermectin = Eqvalan®, Zimectrin®, Bimectin®, DuraMectin®, Agri-Mectin®, IverCare®, Ivercide®
Ivermectin + Praziquantel = Eqvalan Gold®, EquiMax®
Moxidectin = Quest®
Moxidectin + Praziquantel = Quest Plus®

Pyrantel Dewormers:
Pyrantel Pamoate = Strongid®
Pyrantel Pamoate Double Dose = Exodus®
Pyrantel Tartrate = Strongid C 2X® (Daily Dewormer)

Benzimadzole Dewormers:
Fenbendazole = Panacur®, Safe-Guard®
Fenbendazole Double Dose 5 Day Treatment = Panacur Power Pack®
Oxibendazole = Anthelcide EQ®