Decoding Dewormers: Types, Resistance Concerns, and Use for Horses

Dewormers are a common treatment for internal parasites in horses. It is important that horse owners use the correct type of dewormer at the appropriate dosage to ensure dewormers remain effective on their farm.

A dewormer is an antiparasitic drug (known as an anthelmintic) given to an animal to destroy or inhibit the development of internal parasites. All domestic horses have some level of internal parasite load, and dewormers are given to help manage these loads to avoid undesirable health issues that can accompany large parasite populations. Many dewormer products are available for treatment, so it is important that you consider the type of dewormer needed and current resistance to that dewormer when selecting which product you will use. The goal of treatment with dewormer is not to eradicate the parasites, which is almost impossible, but instead to manage the internal parasite load at a level that your horse will remain healthy.

This article will explain the types of dewormers currently available, why we are concerned about parasite resistance to dewormers, and the current veterinary recommended protocol for deworming horses.

Types of Dewormers

There are many brand names of dewormer products available for horses, but the listed active ingredient is the key component to consider when determining which type to purchase. There are only three main drug classes used for internal parasite treatment in horses:

Each of these drug classes targets specific internal parasites to kill them and cause expulsion from the horse's body. The drugs do this by preventing the parasites from obtaining nutrients, causing them to starve to death.

An additional minor class drug, praziquantel, is also available. Praziquantel is listed as an active ingredient in combination with one of the three classes above. It targets tapeworms.

Because there are only three main drug classes, maintaining efficacy (ability of the drug to be effective) of these drugs is critical to ensure they keep working to reduce parasite burdens. If all three become ineffective on parasites, we (horse caretakers) will have no more options to control these parasites. Therefore, to best manage parasite loads of horses, we should be strategic about our deworming practices and use them in combination with other farm management practices (such as removing manure from dry lots or rotating pastures) to reduce parasite burdens.

Concerns about Resistance to Dewormers

Anthelmintic resistance, or the ability of parasites in a population to survive treatment with a dewormer, is a growing concern in internal parasite management. Some parasites have become highly resistant to current drug classes, making the drugs less effective. This resistance occurs at the farm level, rather than the horse level, meaning that resistance to particular drug classes varies from farm to farm and even country to country. This is not just a local problem, but an international one for the equine industry. Therefore, steps to slow the rate of anthelmintic resistance must be implemented on each farm to ensure drugs remain effective.

The rate of anthelmintic resistance in a parasite population is determined by how frequently the parasites are exposed to dewormers (selection pressure) and how effectively surviving parasites pass their genes to the next generation. In other words, the more you use a drug class on a parasite population, the faster resistance develops. This is because parasites that are still susceptible to the drug class are killed and only the resistant parasites are left to reproduce. The previous "standard" practice of deworming all horses every 8 weeks created strong selection pressure for small strongyles to develop resistance to the drugs.

If all parasites in a population become resistant to a drug class, then the drug class will not be able to reduce the parasite burden in a horse. Our goal then is to keep susceptible parasites alive so that when we use the drug class, it will have an effect on the horse's parasite load. To do this, we have to make sure there is a population of parasites that are not exposed to dewormer, called refugia. Any parasites not exposed to the dewormer (those on pasture, encysted in the gut, and in horses that are dewormed less frequently) will not benefit from developing resistant genes and will remain susceptible, which dilutes the resistant parasites within the population. This susceptible parasite population is maintained by deworming some horses less than others and allowing them to shed a small number of susceptible eggs back into the environment. Once resistance is present in a parasite population on a farm, it will not go away, but the rate of developing resistance can be slowed using refugia.

If parasites develop widespread resistance to all three drug classes, we will be left with no effective options for controlling parasites. So what can we do to slow down resistance? The best thing you can do is make strategic deworming decisions to increase refugia by reducing the dewormer treatments for horses that have good natural immunity, which is determined by fecal egg counts (FEC). Consistently monitoring the efficacy of dewormers used on your farm through a fecal egg count reduction test (FECRT) every 3 years can also help you make decisions on which treatments should be used

Current Resistance

In many locations worldwide, some level of anthelmintic resistance has been documented in small strongyles and ascarids. Small strongyles have widespread resistance to benzimidazoles, moderate resistance to pyrimidines, and early indications of resistance to macrocyclic lactones. Ascarids have widespread resistance to macrocyclic lactones and early indications of resistance to both benzimidazoles and pyrimidines.

(+++) indicates widespread resistance. (++) indicates moderate or common resistance. (+) indicates early resistance. (-) indicates no known resistance at this time.

Note. Information in table gathered from the American Association of Equine Practitioners Internal Parasite Guidelines.

Because horses develop immunity to ascarids at a young age, small strongyles are the main parasite of concern in adult horses because of the growing anthelmintic resistance problem. It is important that horse owners take action to reduce incorrect use and/or overuse of dewormers to ensure drug classes remain effective. The American Association of Equine Practitioners (AAEP) has created guidelines for deworming that focus on increasing refugia, and these should be consulted when creating your deworming strategy with your veterinarian.

How frequently does my horse need to be treated with dewormer?

When creating a whole-farm deworming plan, you must evaluate each individual horse and treat it specifically. Horses vary in immunity to internal parasites. Therefore, two key factors should be assessed: the horse's shedding rate and age. These two factors can guide treatment plans and frequency based on the horse's current parasite load and risk for developing health problems from the parasite load.

Shedding Rate

A horse's shedding rate is the number of parasite eggs it releases into the environment through its manure. This rate provides insight on the horse’s current level of immunity and the amount of treatment needed. To evaluate a horse's shedding rate, an FEC is conducted, which analyzes the horse’s manure to identify the type and number of parasites within it. FECs reveal primarily small strongyle and ascarid eggs; it is rare to find other parasite eggs in an FEC, even if the horse is infected. Horses are then placed in one of three categories (low, moderate, or high shedders) based on the number of eggs per gram (EPG) of feces (small strongyle and ascarid eggs are counted separately).

Most adult horses have good immunity against small strongyles, with 40–60% of adult horses being low shedders, 20–30% moderate shedders, and 10–30% high shedders. Therefore, about 80% of eggs shed on a horse farm will come from only 20% of the horses. Horses that are high shedders should be targeted with more frequent treatments than horses that are low shedders. This allows for the overall population of small strongyles on your farm to be reduced, while reducing exposure to dewormers in horses with lower shedding rates, thus promoting refugia.

Age

A horse's immunity to parasites varies with age, due to acquired immunity. Young horses typically have a greater risk for parasite infection (most commonly ascarids and small strongyles) and therefore require more frequent deworming treatments. Adult horses (those that are 5 years or older) tend to develop higher immunity that stays relatively consistent throughout their adult life. This acquired immunity will control ascarid infections and, while it won't prevent infection from small strongyles, it will regulate the horse's risk of disease and egg shedding. In most cases, for mature adult horses, low shedders often remain low, moderate shedders remain moderate, and high shedders remain high. However, as a horse becomes geriatric (over 20 years old), it may start to shed a higher number of strongyle eggs. This may be associated with lower immunity. Therefore, it is important to continue shedding rate evaluations at various ages in adult horses.

FECs should continue throughout horses' life stages to ensure the dewormer drug classes remain effective on that horse and farm (using an FECRT). Foals should not have FECs until they are at least 6 months of age.

Recommended Deworming Frequency for Adult Horses

The AAEP recommends that all adult horses be treated with dewormer at least one to two times a year. Even if a horse's FEC reveals 0 EPG, they should still receive at least one treatment because the test focuses on strongyle eggs and doesn't always reveal the presence of other parasites, such as tapeworms and bots. Additionally, an FEC cannot account for encysted small strongyles that are within the horse's intestinal system. There is also a chance the test may be inaccurate if the manure sample was not fresh enough or if the horse was dewormed recently.

Deworming programs for adult horses are typically focused on controlling small strongyles. Therefore, the AAEP recommends more frequent treatments for horses with higher shedding rates. Targeting treatments on horses shedding the majority of the eggs can help reduce the farm's overall small strongyle load and increase refugia.

Note. Information in table gathered from the AAEP Internal Parasite Guidelines.

Treatments should be given during times of peak transmission, typically spring and late fall for those in the northeastern US. Any additional treatments for moderate and high shedders should be given in between spring and fall. Peak transmission times can vary by region of the US. Your local veterinarian should be aware of recommendations in your region. Always consult with them when making your internal parasite management plan.

Recommended Deworming Frequency for Foals, Weanlings, Yearlings, and Two-Year-Olds

The AAEP has specific guidelines for foals, weanlings, and yearlings that focus on controlling ascarids and small strongyles. Foals should be dewormed at 2–3 months of age, 4–6 months of age (specifically just before weaning), 9 months of age, and 12 months of age. The first treatment should target ascarids, with the AAEP advising benzimidazole as the drug class to use. After weaning, an FEC should be conducted to evaluate small strongyle and ascarid parasite loads. The FEC results should inform the drug classes used for both the 9-month and 12-month treatments. Treatments should focus on targeting small strongyles, and the 9-month treatment should also include tapeworm control.

Yearlings and 2-year-olds should receive three to four treatments yearly. Drug class should be selected based on the time of year and parasites found in FECs. Even if FECs reveal yearlings and 2-year-olds to have a lower shedding rate, the AAEP recommends they be treated as high shedders. Always work with a licensed veterinarian when creating an internal parasite plan for young horses.

Which dewormer should I give?

According to the AAEP, treatment programs for adult horses should ensure treatment for large strongyles, tapeworms, bots, and small strongyles. Typically, this is achieved with one or two treatments per year. However, certain drug classes are recommended during specific seasons. It is recommended that in fall (for northern climates), a dewormer be given that treats encysted small strongyles specifically, as this is when the burden will be highest. Additionally, you should treat your horse for tapeworms once per year.

The table below describes which parasites each drug class will treat. Additionally, if the AAEP has recommended a specific season for this drug class to be given, it is listed in the recommended time of year column. Please note the times listed are recommendations for northern climates in the US (this includes Pennsylvania). Any seasonal recommendations are due to the known life cycle of that internal parasite and times when populations are expected to be highest. It isn't recommended that the same drug class be used every time you treat your horses with dewormer. Talk with your local veterinarian, as they can guide you on which drug class you should give based on your horse, farm, and region.

Note: Praziquantel is only available in combination with macrocyclic lactones in the United States.
Current concerns with resistance to chemical class indicated by (+++), (++), and (+). (+++) indicates widespread resistance by parasite to chemical class, (++) indicates moderate/common resistance by parasite to chemical class, (+) indicates early resistance by parasite to chemical class.

Note. Table information is derived from the American Association of Equine Practitioners Internal Parasite Guidelines.

* Where trade names appear, no discrimination is intended, and no endorsement by Penn State Extension is implied.

Monitoring Efficacy

After you have selected the dewormer and treated the horse, it is important that you continue to monitor for resistance through FECRT. This test helps determine if resistance is building to a specific drug class, which may warrant changes in dewormer treatment type on your farm. Again, to reduce anthelmintic resistance, it is important to continue making sure the dewormer you are using is effective.

Resistance occurs at the farm level (not the horse level), so it is recommended that an FECRT be performed every 3 years on at least six horses that have moderate to high shedding rates (over 300 EPG). Those with higher shedding rates are preferred for this test, and the more horses evaluated, the more accurate your results will be. If you have fewer than six horses, an FECRT will not be as accurate, but you should still monitor resistance.

To conduct an FECRT, have an FEC conducted before deworming (at least 8 weeks since last deworming, 12 weeks if moxidectin was used) and again after deworming (at least 14 days after the dewormer is administered). Use the results from these FECs to calculate the percent fecal egg count reduction (FECR) for each horse. To calculate the % FECR per horse, use the following formula:

Each horse's percentage FECR should then be averaged to determine the farm s overall reduction. To calculate the average FECR for the farm, use the following formula:

An FECRT should be conducted for each drug class used on the farm to determine current resistance and drug efficacy. Results lower than 85% for pyrantel, 90% for fenbendazole/oxibendazole, or 95% for ivermectin or moxidectin indicate resistance or incorrect dosage (i.e., the dose was measured wrong or the horse spit most of it out). Below is an example FECRT for a farm with six horses to check for resistance to the drug class benzimidazoles (fenbendazole).

In this example, the FECRT reveals that there is some resistance to fenbendazole on this farm. We would expect at least a 90% reduction if there was no resistance. Now, 87.27% is not much lower than 90%, so your vet may recommend trying this drug class again and performing another FECRT to ensure its accuracy. If the FECRT is below 90% again, resistance is suspected and you should stop using that drug on the farm.

Conclusion

It is recommended that deworming products with proven efficacy be administered at the appropriate time of year based on the parasite burdens of individual horses. Age and immunity levels can help in determining each horse's need for dewormer treatment and can direct which type of drug should be used. Consult with your veterinarian and continue to assess drug efficacy on your farm for best results.

Take Home Messages

• Anthelmentic resistance against all three drug classes is documented in small strongyles and some resistance has also been identified by ascarids. To reduce this resistance, use targeted deworming to increase refugia.
• Anthelmentic resistance occurs at the farm level, not with individual horses, so the entire farm should be considered when making internal parasite management plans.
• FECs should be conducted on each horse to determine egg shedding category, and a plan should be made in collaboration with your veterinarian on farm deworming strategies, including frequency and chemical class.
• Drug efficacy should be evaluated on the whole-farm level. A FECRT should be conducted every 3 years for each drug class being used on the farm.

For more detailed information about dewormer treatment and internal parasite management, check out the online course on Controlling Parasite Resistance on Your Equine Farm.

Resources

American Association of Equine Practitioners. 2019. Internal Parasite Control Guidelines.

Extension Horses. 2020. Management and Control of Internal Parasites in Horses.

Kirkland, B. 2021. Whole-Farm Management Strategies for Equine Internal Parasites. Penn State Extension.

Kirkland, B., and D. Smarsh. 2021. Prevalent Parasites: Common Types of Equine Internal Parasites. Penn State Extension.
Lane, T. 2014. Parasite Control in Horses. Merck Veterinary Manual.

Neilsen, M., and C. R. Reinemeyer. 2018. Handbook of Equine Parasite Control, 2nd ed.

The content of this publication, including text, graphics, and images, is educational only and not intended to be a substitute for veterinary medical advice, diagnosis, or treatment. Always seek the advice of a licensed doctor of veterinary medicine or other licensed or certified veterinary medical professional with any questions you may have regarding a veterinary medical condition or symptom.