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

March 2025: This article has been updated to reflect new data and recommendations in the 2024 American Association of Equine Practitioners (AAEP) Internal Parasite Control Guidelines. Please familiarize yourself with the new recommendations.

The Importance of Dewormers on Farms

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) that can be given to an animal to destroy or inhibit the development of internal parasites. All domestic horses are assumed to have some level of internal parasite load, and dewormers are administered to help manage these loads to avoid undesirable health issues that coincide with large parasite populations. Many dewormer products are available, so it is important that you consider the type of dewormer needed and current parasite resistance to that dewormer when selecting which product you will use. Additionally, horse owners need to consider their horses' age, egg shedding rate, and the time of year when deworming.

The goal of dewormer treatment is not to eradicate all parasites, which is impossible; instead, the goal is to (1) manage the internal parasite load at a level that your horse will remain healthy and (2) delay further development of parasite resistance to dewormers and maintain drug efficacy (the ability of the drug to be effective) for as long as possible.

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 the treatment of internal parasites in horses (Table 1). Each of the three drug classes targets specific internal parasites and prevents them from obtaining nutrients, causing them to starve to death and be expelled from the horse's body. One minor drug class, praziquantel, is also available. Praziquantel is listed as an active ingredient in combination with one of the three major drug classes and targets tapeworms.

Because there are only three main drug classes, maintaining drug efficacy is critical to ensure that they continue reducing parasite burden. If all three drugs become ineffective, we (horse caretakers) will have no more options to control these parasites. There are no new drugs on the horizon. Therefore, to best manage parasite loads in horses, we should be strategic about our deworming practices and use dewormers in combination with other farm management practices (such as removing manure from dry lots) to reduce parasite burden.

Parasite 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 these drugs less effective. This resistance occurs at the farm level, rather than at the horse level, meaning that resistance to certain drug classes varies from farm to farm and even country to country. This is not just a local problem but an international problem for the equine industry. Therefore, steps to slow the rate of anthelmintic resistance must be taken on each farm to ensure that dewormers 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 on 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 susceptible to the drug class are killed, and then only the resistant parasites are left to reproduce. The previous "standard" practice of deworming all horses every 8 weeks created strong selection pressure for several parasites to develop resistance to dewormers. If all parasites in a population become resistant to a drug class, then the drug class will be unable to reduce parasite burden in a horse. As such, it is important that deworming practices be adapted and strategically designed to avoid such selection pressure.

Our goal is to keep some susceptible parasites alive so that when we use a particular drug class, it will reduce the parasite load. To achieve this goal, we must ensure that there is a population of parasites that are not exposed to dewormers, called refugia. Any parasites not exposed to a dewormer (those on pasture, encysted in the gut, and in horses that are dewormed less frequently) will not benefit from developing resistance genes and will remain susceptible, which dilutes the proportion of 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 resistance? The best thing we can do is make strategic deworming decisions to increase refugia by reducing the frequency of dewormer treatments for horses that have good natural immunity, which is determined by fecal egg counts (FECs). Performing annual assessments of dewormer efficacy on your farm through an FEC reduction test (FECRT) can also help you make decisions about which drugs should be used.

Current Resistance

In many locations around the world, some level of anthelmintic resistance has been documented in small strongyles, ascarids, tapeworms, and pinworms. Small strongyles have widespread resistance to benzimidazoles and 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 (Table 2).

Source: Data from the AAEP (2024). Triple crosses (+++) indicate widespread resistance. whereas a single cross. (+) indicates early signs of resistance.

Small strongyles are the main parasite of concern in adult horses because they are the most common and because of growing anthelmintic resistance. It is important that horse owners take action to reduce incorrect use and/or overuse of dewormers to ensure that drug classes remain effective. The 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: (1) the horse's egg shedding rate and (2) 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 into the horse's current level of immunity and the amount of treatment needed; however, it does not indicate the size of the parasite burden (i.e., a horse with a low shedding rate does not necessarily have fewer parasites than a horse with a high shedding rate).

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, where 40–60% of adult horses are low shedders, 20–30% are moderate shedders, and 10–30% are 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 or moderate shedders. This allows for the overall population of small strongyles on your farm to be reduced while preserving refugia in horses with lower shedding rates.

There are a few limitations of FECs that should be noted. An FEC cannot detect parasite larvae, only eggs. For small strongyles, it cannot detect encysted larvae. For these reasons, an FEC result of 0 does not mean that a horse has no parasites. Additionally, one cannot tell the difference between large and small strongyle eggs under a microscope, although large strongyle infections are rare in the United States. Tapeworm and pinworm eggs do not reliably appear using the FEC technique commonly used to count small strongyle and ascarid eggs (the McMaster technique).

Age

A horse's immunity to parasites and egg shedding rate vary 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) develop some level of immunity to parasite infection that stays relatively consistent throughout their adult life. This acquired immunity will generally control ascarid infections and, although it will not prevent infection by 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 a senior (over 15 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 a horse's life to ensure that the dewormers remain effective in that horse and on that farm (using an FEC reduction test)

Recommended Deworming Frequency for Adult Horses

The AAEP recommends that all adult horses be treated with a dewormer at least one to two times a year, depending on climate. 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 does not always reveal the presence of other parasites, such as tapeworms and bots. Additionally, an FEC cannot account for encysted small strongyles that are dormant within the horse's gastrointestinal system. There is also a chance that 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 and reducing egg shedding in pastures. Therefore, the AAEP recommends more frequent treatments for horses with higher shedding rates. Targeting treatments toward horses shedding most of the eggs can help reduce the farm's overall small strongyle load and increase refugia (Table 3).

Source: Data from AAEP (2024).

Treatments should be administered during times of peak transmission, typically spring and late fall for those in the northeastern United States. Any additional treatments for high shedders should be given in between spring and fall. Peak transmission times can vary by region in the United States. 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, 5–6 months (specifically just before weaning), 9 months, and 12 months of age. The first two treatments should target ascarids, with the AAEP advising benzimidazoles 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- and 12-month treatments. The 9-month treatment should also include tapeworm control.

Yearlings and 2-year-olds should receive three to four treatments yearly. A 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, yearly deworming 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 the fall (for northern climates), a dewormer should be administered that specifically treats encysted small strongyles, as this is when the burden of encysted small strongyles will be highest. Additionally, you should treat your horse for tapeworms once per year if the horse has access to green pastures.

Table 4 describes which parasites each drug class treats. 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 that the seasons listed are recommendations for northern climates in the United States (this includes Pennsylvania). Any seasonal recommendations are due to the known life cycle of that internal parasite and temperatures/humidity levels when populations are expected to be highest. Talk with your local veterinarian, as they can guide you on which drug class you should give based on your horse, farm, and region.

Source: Data from AAEP (2024). Note  Praziquantel is only available in combination with macrocyclic lactones in the United States.
Current concerns regarding resistance to chemical class are indicated by "+++" and "+". Three crosses (+++) indicate widespread parasite resistance to the chemical class on multiple continents with high farm prevalences often above 80%, and one cross (+) indicates early signs of parasite resistance to chemical class from a few single-farm cases of reduced efficacy..

Monitoring Efficacy with Fecal Egg Count Reduction Tests

After you have selected the dewormer and treated the horse, it is important that you continue to monitor for resistance through FECRTs. 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 testing whether the dewormer you are using is effective.

Resistance occurs at the pasture or farm level (not the individual horse level), so it is recommended that an FECRT be performed annually on at least five horses that have moderate to high shedding rates (over 300 EPG) and that graze together (if possible). 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 five horses, an FECRT will not be as accurate, but you should still monitor resistance. (Please note the updates to this procedure from the 2024 AAEP Internal Parasite Control Guidelines at the end of this section.)

To conduct an FECRT, have an FEC performed before deworming (at least 8 weeks since the last deworming) and again after deworming (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 percent 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 annually 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). Table 5 shows example results from an FECRT performed on a farm with six horses to check for resistance to the benzimidazoles drug class (using fenbendazole).

We would expect a reduction of at least 90% if there was no resistance. As such, in this example, the FECRT suggests that there is some resistance to fenbendazole on this farm. Now, 87% is not much lower than 90%, so your veterinarian 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.

2024 Changes to the FECRT Procedure

The AAEP Internal Parasite Control Guidelines were revised in 2024. Specifically, they recommend a few changes to this procedure.

First, the AAEP recommends counting a minimum of 40 eggs per horse used in the pretreatment FEC. This means that you should continue making slides until at least 40 eggs have been counted per horse; make note of how many slides were needed for each horse. For the post-treatment FEC, make the same number of slides for each horse as before. There is no need to calculate EPG for this version of the reduction test. Second, a statistical program can be used to calculate the "90% confidence interval" for the percent reduction, which accounts for the variation in egg reduction between horses. The AAEP Guidelines do provide an online calculator for this purpose. Although this method provides a more meaningful result, interpretation is tricky. Instead, you can still use the same percent reduction formula, substituting total eggs counted for EPG. Any dewormer should reduce egg shedding by at least 95% to be considered effective.

There may be times when a manager needs to perform a treatment check on a single horse, such as a new arrival to the farm. Although it would not be a true test for resistance, an FECRT with 40+ counted eggs would still provide information on the treatment efficacy of that drug.

Conclusion

Resistance to deworming treatment is a growing concern on horse farms. To slow the development of resistance, 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

• Anthelmintic resistance to all three drug classes is documented in small strongyles, ascarids, tapeworms, and pinworms. To reduce resistance, use targeted deworming to increase refugia.
• Anthelmintic resistance occurs at the farm level, not at the level of individual horses; therefore, the entire farm should be considered when making internal parasite management plans.
• FECs should be conducted on each horse to determine their egg shedding category, and a plan should be made in collaboration with your veterinarian regarding farm deworming strategies, including frequency and chemical class.
• Drug efficacy should be evaluated on the whole-farm level. An FECRT should be conducted annually for each drug class being used on the farm.

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

Prepared by Brittani Kirkland, former Penn State Extension educator. Updated by Laura Kenny.

Further Reading

AAEP. 2024. “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, Smarsh D. 2021. "Prevalent Parasites: Common Types of Equine Internal Parasites." Penn State Extension.

Lane T. 2014. "Parasite Control in Horses." Merck Veterinary Manual.

Neilsen M, Reinemeyer CR. 2018. Handbook of Equine Parasite Control, 2nd edition. John Wiley & Sons, Inc.

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.