The answer to how often you should deworm a horse is it depends heavily on the individual horse, its living situation, the local parasite load, and results from fecal egg count testing. In the past, horses were dewormed on a fixed schedule, often every 6-8 weeks. Today, modern equine parasite control focuses on strategic deworming to reduce the risk of anthelmintic resistance.
The Shift Away from Routine Deworming
For decades, horse owners followed a set horse deworming schedule—a blanket approach treating all horses the same way, usually every two months. This method killed most worms but also wiped out the few worms that naturally resisted the medicine. These resistant worms survived and reproduced, leading to widespread resistance across many common parasite types, especially small strongyles.
Now, veterinarians promote targeted deworming. This modern approach means testing horses first. It ensures you only treat horses that truly need it and use the right drug for the parasites present.
Deciphering Parasite Threats in Horses
To set the right schedule, you must know what you are fighting. Horses face several types of internal parasites. The most common and concerning include:
- Small Strongyles (Cyathostomins): These are the most common worms. Larvae can burrow deep into the horse’s gut walls. If many emerge at once, they can cause severe colic.
- Large Strongyles (Bloodworms): Once a major threat, they are now less common due to effective modern drugs. They migrate through arteries, which can cause serious issues.
- Tapeworms (Anoplocephala perfoliata): These live in the small intestine. They are linked to some types of colic, especially in the ileocecal junction.
- Pinworms (Oxyuris equi): These cause tail itching and irritation.
- Bots (Gastrophilus species): Fly larvae attach to the stomach lining. They cause irritation and sometimes ulcers.
The Importance of Fecal Egg Count Testing
Fecal egg count testing (FEC) is the cornerstone of modern equine parasite control. This simple manure test tells your vet how many parasite eggs are in a sample of your horse’s manure.
What FEC Results Mean
Vets use FEC results to classify horses into categories. This classification helps decide the deworming frequency.
| FEC Result Category | Eggs Per Gram (EPG) | Treatment Strategy Implied |
|---|---|---|
| Low Shedders | Less than 200 EPG | Treat infrequently (1-2 times per year). |
| Moderate Shedders | 200 – 500 EPG | Treat moderately (2-3 times per year). |
| High Shedders | Greater than 500 EPG | Treat often (3-4+ times per year). |
High shedders are the most important group to monitor. They pass the most eggs, contaminating pastures more heavily. Treating them effectively protects the entire herd. Low shedders often only need treatment once or twice a year, based on seasonal risks.
Limitations of FEC Testing
FEC tests are great, but they have limits.
- Tapeworms: Standard FEC tests often do not reliably detect tapeworm eggs. A special washing technique might be needed, or treatment might be given based on risk rather than testing.
- Bots: FEC tests do not count bot eggs.
- Larval Stages: The test only counts eggs from egg-laying adults. It misses developing larvae migrating in tissues, like encysted small strongyles.
Determining Your Horse’s Deworming Frequency
A one-size-fits-all approach fails in equine parasite control. Your ideal horse deworming schedule depends on several factors.
Individual Horse Factors
- Age: Foals and older horses often need more focused management. Young horses are highly susceptible. Very old horses may have weaker immune systems.
- Health Status: Sick or underweight horses might need closer monitoring.
- Past FEC Results: This is the most crucial factor. A horse that consistently tests low needs less medication than one that tests high.
Management and Environment Factors
- Pasture Density: The more horses you keep in a small area, the faster the pasture gets contaminated. High stocking rates mean higher parasite exposure.
- Climate: Warm, wet weather helps parasite larvae survive and develop on pasture grass. Cold, dry areas reduce parasite activity.
- Co-Grazing: If horses share fields with other species (like sheep or goats), the risk profile can change.
Creating a Strategic Deworming Plan
Strategic deworming uses FEC results to tailor treatment. The goal is to preserve the effectiveness of available dewormers by minimizing overuse. This is critical to fighting anthelmintic resistance.
The Recommended Approach: Targeted Deworming
Targeted deworming involves:
- Testing manure (FEC) two to four times a year.
- Treating only based on the test results and the time of year.
- Rotating dewormers to prevent worms from becoming resistant to any single drug class.
If your horse is a low shedder, you might only deworm twice a year. If they are a high shedder, you might need four treatments, but each treatment must use a different class of drug.
Best Time to Deworm Horse: Seasonal Considerations
When you treat is as important as how often. You must treat at times when parasite transmission is highest or when the largest burden of adult worms needs removal. This dictates the best time to deworm horse throughout the year.
Spring Treatment (Early Season)
Spring is vital. After the pasture season starts, adult worms lay many eggs. A key goal here is often treating for encysted small strongyles that survived winter within the gut wall.
- Action: Use a drug effective against encysted larvae (like Fenbendazole or Moxidectin).
- Timing: Just before or as you turn horses out onto new spring grass, or late spring (May/June).
Summer Treatment (Mid-Season)
If your horse is a moderate or high shedder, a mid-season treatment is necessary to knock down the current population before they massively contaminate the pasture.
- Action: Use a drug from a different class than the spring treatment.
- Timing: Mid-July to early August, depending on your climate.
Fall Treatment (Late Season and Tapeworms)
The fall treatment is often the most important for overall herd health. You want to clear the horses of parasites before they enter winter housing.
- Action: Use a macrocyclic lactone (like Moxidectin) and a Benzimidazole or Pyrantel (like Pyrantel Pamoate) to cover tapeworms. This is often called the “power pack” treatment.
- Timing: Late October to early December, before the first hard frost if possible.
Winter Treatment (Deworming for Bots)
If you live in a cold climate, most larvae are dormant. The primary target here is bots.
- Action: Use a bot-specific product (often Ivermectin or Moxidectin).
- Timing: After the first few hard frosts kill off the adult bot flies.
Rotating Dewormers: Fighting Resistance
Anthelmintic resistance means the drug no longer kills the target worms. The only way to slow this down is by rotating dewormers. You must alternate the chemical class used for each treatment.
There are four main classes of dewormers used in horses. Never use the same class twice in a row without an FEC test proving it was necessary.
| Chemical Class | Example Drug Name (Active Ingredient) | Primary Targets | Notes on Resistance |
|---|---|---|---|
| Macrocyclic Lactones (MLs) | Ivermectin, Moxidectin | Small Strongyles, Pinworms, Bots | Resistance is growing, especially to Ivermectin for small strongyles. Moxidectin is generally still effective against encysted small strongyles. |
| Benzimidazoles (BZ) | Fenbendazole (Panacur), Albendazole | Small Strongyles, Some Tapeworms | Resistance is common. Use carefully. |
| Tetrahydropyrimidines | Pyrantel Pamoate | Large Strongyles, Pinworms | Generally considered the least effective against small strongyles now. Good for tapeworms in combination. |
| Praziquantel | Praziquantel (Often added to combination pastes) | Tapeworms | Highly effective against tapeworms. |
Crucial Rule: If you are using a combination dewormer (e.g., one that contains Ivermectin AND Pyrantel), you have only used two classes. The next treatment must be from a different class, like a Benzimidazole.
Managing High Risk Groups Separately
Some horses cannot be managed like low shedders, even if their last FEC was low. These horses require a more aggressive horse deworming schedule.
Foals and Weanlings
Foals need more frequent monitoring. Their immune systems are still developing. They are often treated more often early in life but should be tested by 6 months old to start establishing their shedding category.
Horses with Known Worm Issues
If you suspect worms due to symptoms like poor coat, weight loss, or diarrhea, you should test immediately. If signs are severe, your vet may advise a power flush treatment (using a drug known to kill encysted larvae) followed by an FEC test a few weeks later to see how effective the treatment was.
Signs of worms in horses can include:
- Dull, rough coat.
- Weight loss despite good feed intake.
- Diarrhea or sometimes impaction colic.
- Tail rubbing (pinworms).
- Swelling around the jaw (rare, due to large strongyles).
Pasture Management: Reducing the Need to Deworm
The best way to reduce the deworming frequency is to reduce the number of infective larvae on your pasture. This supports strategic deworming.
Steps for Healthier Pastures
- Remove Manure: Scoop manure piles at least twice a week. This stops eggs from hatching into larvae.
- Use Harrowing or Dragging: Spreading manure out makes it dry faster, killing larvae. Do this when the weather is dry, not when it’s wet.
- Paddock Rotation: If possible, rest pastures for several months. Larvae usually die off after 6–12 months without a host.
- Mowing: Keep grass short. Larvae cannot survive high up on tall grass blades.
- Separate Areas: Keep foaling paddocks or sick pens separate from main grazing areas.
The Role of Moxidectin and Tapeworm Treatment
Moxidectin is often the most potent drug currently available for killing encysted small strongyles. Because of this, many protocols suggest using it in the late fall treatment.
However, it is crucial to note that resistance to Moxidectin is beginning to appear in some areas. Therefore, targeted deworming means you should not use Moxidectin year after year without confirming its effectiveness through a fecal egg count testing follow-up.
Tapeworms need special mention. Since FECs are unreliable for them, most vets recommend treating for tapeworms at least once, sometimes twice, yearly, usually in the fall/early winter. Pyrantel Pamoate or Praziquantel are the drugs of choice.
Putting It All Together: A Sample Strategy Framework
This is an example framework. Always confirm specifics with your veterinarian based on your location and local parasite resistance patterns.
| Season | Horse Shedder Category (Based on FEC) | Recommended Action | Drug Class Focus |
|---|---|---|---|
| Late Winter/Early Spring | All Horses | Treatment for encysted larvae (before turnout). | Benzimidazole (e.g., Fenbendazole) |
| Late Spring/Early Summer | High Shedders Only | FEC retest; treat if EPG > 500. | Macrocyclic Lactone (e.g., Ivermectin) |
| Mid-Summer | Moderate/High Shedders | FEC retest; treat if EPG is high. | Pyrantel/Benzimidazole (if not used in Spring) |
| Late Fall | All Horses | Comprehensive treatment targeting tapeworms and strongyles before winter. | Macrocyclic Lactone (Moxidectin) PLUS Tapeworm drug |
This system reduces drug use for low shedders while intensely managing the small percentage of horses creating the most pasture contamination. This is the heart of strategic deworming.
Fathoming Drug Efficacy After Treatment
After you treat a horse, how do you know the drug worked? You perform a Fecal Egg Count Reduction Test (FECRT).
- Perform an FEC before treatment (Baseline FEC).
- Deworm the horse using the drug you chose.
- Wait 10 to 14 days.
- Perform a second FEC (Post-Treatment FEC).
If the post-treatment count is less than 5% of the baseline count, the drug was effective. If the reduction is 50% or less, it means the worms present were resistant to that drug. This information is vital for your next rotation decision, helping you avoid using a failing drug and thus combatting anthelmintic resistance.
Conclusion on Deworming Frequency
Modern equine parasite control moves away from guesswork. The key to setting the correct deworming frequency is regular testing. By using fecal egg count testing to define your horse as a low, moderate, or high shedder, you can implement targeted deworming. This approach protects your horse’s health, manages environmental contamination, and preserves the effectiveness of the medications we rely on for sound equine parasite control. Following good pasture management and rotating dewormers will ensure you have the best chance of winning the fight against parasites and anthelmintic resistance.
Frequently Asked Questions (FAQ)
H5: How often should a “low shedder” horse be dewormed?
A low shedder horse (testing under 200 EPG consistently) usually only needs deworming 1 to 2 times per year. This treatment should target tapeworms and any larvae surviving winter, usually done in the late fall and perhaps one other strategic time based on your vet’s advice.
H5: Can I stop deworming entirely if my horse tests zero on the FEC?
No. You should never stop deworming completely. A zero result only means no adult egg-laying worms were detected in that specific sample. It does not account for bots, tapeworms, or encysted larvae. You still need at least one annual treatment aimed at these resistant stages.
H5: What is the biggest risk if I over-deworm my horse?
The biggest risk of over-deworming is accelerating anthelmintic resistance. This means the drugs will stop working, leaving you defenseless against severe worm burdens, which can lead to fatal colic or illness.
H5: Are combination pastes better than single-drug pastes?
Combination pastes treat multiple parasite types at once, which can be useful for the main fall treatment. However, if you use a combination paste every time, you increase the chance of resistance to both active ingredients faster than if you used single drugs strategically. Always consult your vet on which combinations fit your horse deworming schedule.