How often should I deworm my horse? The answer today is not a simple, set schedule. Modern equine parasite control relies on fecal egg count testing to figure out the best deworming frequency for each horse. This approach helps stop anthelmintic resistance and keeps your horse healthy.
Why A Set Deworming Schedule Is Outdated
For many years, horse owners gave dewormer on a strict timeline, maybe every two or three months. This was the standard horse deworming schedule. However, we now know this method causes big problems. Giving medicine when it is not needed helps worms become strong against the drugs. This is called anthelmintic resistance.
Imagine giving an antibiotic for a cold when you only have a mild cough. Eventually, the germs might ignore the antibiotic. Worms work the same way. The goal now is strategic deworming. This means using the right medicine at the right time, based on real testing.
Recognizing the Problem: Signs of Horse Worms
Before changing your plan, it is good to know what a worm problem looks like. Many horses can carry a light worm load and show no obvious signs. But heavy infections can cause trouble. Look out for these signs of horse worms:
- Poor coat quality, looking dull or rough.
- Weight loss, even when eating well.
- Diarrhea or soft stools.
- Colic, or belly pain, which can be severe.
- A pot-bellied appearance, especially in young horses.
- In older horses, poor performance or weakness.
If you see these signs, talk to your vet right away.
The Core of Modern Parasite Control: Fecal Egg Count Testing
The most important tool for setting your deworming frequency is the fecal egg count testing (FEC). This simple test counts the number of strongyle eggs in a small sample of your horse’s manure.
What Does the Test Tell Us?
The FEC result helps sort horses into groups based on how well their bodies fight off worms naturally. This is key to strategic deworming.
| FEC Result (Eggs Per Gram – EPG) | Horse Group | Suggested Action |
|---|---|---|
| Low Shedder (Under 200 EPG) | Low risk | Treat less often (maybe 2-3 times a year). |
| Moderate Shedder (200–500 EPG) | Medium risk | Treat as directed by the vet. |
| High Shedder (Over 500 EPG) | High risk | Treat more often and monitor closely. |
Horses that shed few eggs usually have strong natural immunity. They need less medicine. Horses that shed many eggs need closer attention and more frequent treatment.
When to Test?
Your veterinarian will give you veterinary deworming recommendations, but generally, you should test:
- Before Spring Treatment: To see what population survived the winter.
- 8 to 12 Weeks After Major Treatment: To check if the dewormer worked well. This is often called a fecal egg count reduction test (FECRT).
- Before Fall Treatment: To decide on the final treatment for the year, especially the drug needed to kill encysted small strongyles.
Developing a Strategic Deworming Plan
Strategic deworming moves away from guesswork. It uses FEC results, the horse’s age, farm management, and local parasite risk to create a plan.
Factors Influencing Deworming Frequency
The right schedule is not the same for every horse. Consider these factors when setting your horse deworming schedule:
- Age: Foals and yearlings need more frequent attention. Older horses might need fewer treatments if they are low shedders.
- Risk Level: Horses kept in small paddocks with lots of horses or high stocking density have higher exposure to worms.
- Geography: Warmer, wetter climates usually mean year-round parasite risk. Colder, drier climates might allow for more gaps in treatment.
- Past FEC Results: A horse that consistently tests high needs a different approach than a consistent low shedder.
Essential Treatments That Don’t Rely on FEC
Even with good testing, some treatments are necessary regardless of the current FEC. This is because FEC testing mainly counts adult worms in the manure. It does not easily detect all types of parasites.
Small Strongyles (Cyathostomins): These are the most common worms. The larvae can hide in the gut wall over winter (encyst). Standard FEC tests miss these encysted larvae. A drug that kills the encysted stages must be used, usually once a year in the fall or early winter.
Tapeworms: FEC tests often miss tapeworms completely. Your vet will likely recommend treating for tapeworms at least once a year, usually in the late fall.
Pinworms: These are usually seen by owners (white segments around the tail) but are not counted in standard FEC tests. They are usually managed by good hygiene and sometimes specific treatments.
Comprehending Types of Horse Dewormers
Knowing the types of horse dewormers available is crucial for strategic deworming. Different drugs target different parasites. Resistance builds when the same drug class is used repeatedly.
There are four main classes of dewormers used in horses today:
1. Benzimidazoles (e.g., Fenbendazole, Albendazole)
These are broad-spectrum drugs. They are often used for routine deworming. However, resistance is very common, especially in small strongyles. They are generally not the first choice for the main treatments anymore.
2. Pyrantel (e.g., Pyrantel Pamoate)
This targets small strongyles and large strongyles. It is often used for a “clean-out” treatment between the main drug applications. It has a short egg withdrawal time, meaning you can often test sooner after giving it.
3. Macrocyclic Lactones (MLs)
This is a powerful group of types of horse dewormers. It includes Ivermectin and Moxidectin. These drugs are highly effective against many worms, including the encysted small strongyles when dosed correctly.
- Ivermectin: A common, effective broad-spectrum drug.
- Moxidectin (Quest®): Often considered the best choice for targeting encysted small strongyles. It stays in the horse’s system longer, offering a bit more residual protection.
Caution with MLs: Because they are so effective, using them too often accelerates anthelmintic resistance. They should be rotated carefully or used based on clear need (like treating encysted larvae).
4. Praziquantel
This drug is specifically used to kill tapeworms. It is often combined with an ML product for convenience, as it targets a parasite the MLs do not cover well.
Dewormer Rotation vs. Targeted Treatment
The old way was rotating: use Drug A in January, Drug B in April, Drug C in July, etc. Now, the veterinary deworming recommendations favor targeted treatment based on FEC results, using the most potent drug (like an ML) only when necessary (e.g., for high shedders or the mandatory fall treatment).
The Ideal Horse Deworming Schedule: A Sample Year
While every horse is different, here is a framework based on modern strategic deworming principles, assuming a moderate climate:
Early Spring (March/April) – The “Wake Up” Treatment
- Perform FEC Test: See where your horse stands after winter.
- Treat Based on Results:
- Low Shedders: May only need a treatment for tapeworms if they have not had one recently.
- Moderate/High Shedders: Treat with a standard broad-spectrum drug (often a Benzimidazole or Pyrantel, depending on your vet’s resistance profile). If you suspect tapeworms, use a product containing Praziquantel.
Summer (June/July) – Mid-Season Check
- Monitor Environment: Are conditions wet or very dry?
- Repeat FEC Test: This check ensures the spring treatment was effective and reveals if new infections are building up.
- Targeted Treatment: Treat based on the new FEC results. Do not use the same drug class used in the spring if the horse is still shedding many eggs. This is where resistance management comes in.
Fall (October/November) – The Critical Treatment
This treatment is vital. It aims to kill off the parasites before winter sets in and, most importantly, eliminate encysted small strongyles.
- Treat for Encysted Larvae: Use a product containing Moxidectin or a double dose of Fenbendazole (consult your vet for the exact protocol for Fenbendazole, as dosage is specific). This addresses the larvae hiding in the gut wall.
- Treat for Tapeworms: Use Praziquantel if you did not treat in the spring, or follow your vet’s advice.
- Perform FECRT: 10–14 days after this treatment, perform a follow-up FECRT. This tells you if the Moxidectin or Fenbendazole worked against the adult worms and sets the baseline for the next year.
Winter (January/February) – Maintenance
If your horse is a low shedder and you managed the fall treatment well, you might skip chemical deworming during the coldest months. Focus instead on good pasture management.
Pasture Management: Non-Chemical Parasite Control
A good equine parasite control plan uses tools other than medicine. This reduces the pressure on dewormers and slows the development of anthelmintic resistance.
Pasture Rotation and Stocking Density
- Keep Horses Off Overgrazed Areas: Larvae live in the top layer of grass. Keeping horses off pastures that have been heavily grazed forces them to ingest fewer infective larvae.
- Manage Stocking Density: Too many horses in one small area overwhelms the pasture’s ability to “cleanse” itself. Fewer horses per acre means fewer worms shared among the herd.
Manure Removal
Pick up manure daily, or at least several times a week. This removes worm eggs before they hatch into infective larvae. Composting manure piles to high temperatures can also kill larvae.
Using Other Animals
Grazing sheep or cattle on horse pastures can help. They eat grass shorter and are not as susceptible to horse parasites, helping to graze down the larval stage.
Special Considerations in Equine Parasite Control
Certain situations demand special attention, regardless of the general horse deworming schedule.
Foals and Young Horses
Foals are highly vulnerable. They are often treated more frequently early in life to prevent severe anemia or gut damage from worms like tapeworms or roundworms. Veterinary deworming recommendations are essential here, often involving monthly or bi-monthly treatment for the first few months of life, followed by FEC testing as they age.
The Problem of Encysted Larvae
We mentioned this before, but it bears repeating. Small strongyle larvae can burrow into the intestinal lining and wait for months or years. This dormant stage is tough to kill. If you do not use a drug effective against them (like Moxidectin or double-dose Fenbendazole) at least once a year, those larvae can emerge later, causing severe disease, even if your horse has been tested regularly for adult worms.
Dealing with Resistance: The FECRT
The fecal egg count reduction test (FECRT) is crucial for monitoring if your chosen dewormer is still working.
How the FECRT Works:
1. Test the horse’s manure (FEC 1).
2. Deworm the horse with the suspected drug.
3. Wait 10–14 days (the time needed for the drug to clear the system).
4. Test the manure again (FEC 2).
If FEC 2 is nearly as high as FEC 1, it means the worms in that horse are resistant to that specific type of horse dewormer. You must immediately switch to a different drug class as advised by your vet.
Consulting Your Veterinarian for the Best Plan
While this guide provides the framework for modern equine parasite control, you cannot automate this process safely. Your veterinarian is the final authority on your horse deworming schedule.
Why is the vet essential?
- Local Resistance Patterns: Your vet knows which dewormers are failing in your specific region based on data collected from local farms.
- Accurate Dosing: Drugs must be given at the correct dose based on the horse’s weight. Underdosing is the fastest way to cause resistance.
- Prescription Needs: In many areas, the most effective drugs, especially Moxidectin, require a veterinary prescription.
Relying solely on over-the-counter products without incorporating fecal egg count testing and professional guidance leads back to the failed, fixed schedule of the past. Good equine parasite control is dynamic, not static.
Frequently Asked Questions (FAQ)
How long does it take for worms to develop resistance to dewormers?
Resistance develops over years of repeated, improper use. The faster you use the same drug class over and over, the faster the resistant worms survive and multiply. This is why strategic deworming is so important—it minimizes drug use overall.
Can I see tapeworms or pinworms on a regular fecal egg count test?
No. A standard fecal egg count testing only looks for strongyle eggs. Tapeworms and pinworms are not easily detected this way. You need specific tests (like a tape impression for pinworms) or routine yearly treatment with a drug like Praziquantel to manage them.
What are the common macrocyclic lactones used in horses?
The two main macrocyclic lactones used in horses are Ivermectin and Moxidectin. Moxidectin is usually preferred for targeting the encysted stages of small strongyles.
Are there any natural dewormers that work?
While many natural supplements like diatomaceous earth or herbal mixes are marketed for deworming, there is no scientific proof they can eliminate established worm burdens as effectively as approved chemical dewormers. They should not replace your medically sound horse deworming schedule.
When should I stop deworming my horse?
You should never stop deworming completely, but you can significantly reduce the frequency. If your horse is consistently a low shedder (under 200 EPG) and you have confirmed tapeworm and encysted larvae have been treated, you might only need 2-3 chemical treatments per year, alongside excellent pasture management. Always follow veterinary deworming recommendations.