The correct answer to how often you should worm your horse is: it depends entirely on your horse, your location, and the results of regular testing. Modern equine parasite control has moved away from fixed, calendar-based schedules. Instead, it relies on testing and targeted treatment to keep your horse healthy and slow down drug resistance.
Why Fixed Deworming Schedules Fail
For many years, horse owners followed a set schedule. They would deworm every 6 or 8 weeks, often rotating products. This system had major flaws. It treated every horse the same. It also led to a huge problem: ivermectin resistance horses and resistance to other common drugs.
The Problem with Over-Deworming
Giving medicine when it is not needed causes two main issues:
- Wasting Money: You spend money on pastes or solutions your horse does not need.
- Drug Resistance: Parasites that survive treatment multiply. If you treat too often, the tough worms survive. Soon, the medicine stops working for almost all worms. This is the biggest threat to equine parasite control.
The Problem with Under-Deworming
If you do not treat when needed, your horse can become sick. High parasite loads cause poor coat quality, weight loss, and colic. They also shed massive numbers of eggs onto the pasture. This raises the risk for all horses sharing the field.
The Modern Approach: Fecal Egg Count Testing
The cornerstone of current recommendations is fecal egg count testing (FEC). This simple manure test tells you exactly how many parasite eggs (mostly strongyle eggs) your horse is shedding.
What FEC Testing Tells You
FEC tests give you a number, reported as Eggs Per Gram (EPG). This number sorts your horse into a shedding category. This grouping lets you tailor the deworming frequency horses need.
Horse Shedding Categories
| Shedding Category | EPG Count | Treatment Focus |
|---|---|---|
| Low Shedders | Under 200 EPG | Treat only 1-2 times per year. |
| Moderate Shedders | 200 to 500 EPG | Treat 2-3 times per year. |
| High Shedders | Over 500 EPG | Treat 3-4 times per year, and monitor closely. |
Key Point: About 80% of the parasite burden in a herd usually comes from about 20% of the horses—these are the high shedders who need the most attention. Low shedders may need treatment only once a year, or sometimes not at all, depending on their risk level.
When to Get an FEC Test Done
You must perform an FEC test before you treat. This is called a pre-treatment count. After treatment, a re-test (post-treatment count) is needed about 10–14 days later. This second test confirms the dewormer worked. If the EPG count remains high after treatment, it signals drug resistance, which requires a change in product.
Crafting a Strategic Deworming Horses Plan
Strategic deworming horses means using FEC tests and targeted drug rotation. It is no longer about treating every 6 weeks. It is about treating only when the test shows a high need, and using the right drug for the right parasite at the right time of year.
The Importance of Rotating Horse Wormers
Even with FEC testing, you must still rotate the classes of drugs you use. This is vital for managing resistance, especially against small strongyles in horses. These worms are very common and can become encysted (hidden) in the gut wall, surviving standard treatments.
There are four main classes of dewormers used today:
- Benzimidazoles (e.g., Fenbendazole): Good for general strongyles. Resistance is common.
- Pyrantel (e.g., Pyrantel Pamoate): Good for small strongyles and pinworms. Often used for maintenance.
- Macrocyclic Lactones (MLs): Includes Ivermectin and Moxidectin. These are the most potent. Moxidectin is often better for targeting encysted small strongyles.
Warning: If you find resistance to one drug class (your post-treatment FEC is still high), stop using that drug immediately. Switch to a different class that has been proven effective in your area.
Moxidectin and Encysted Small Strongyles
Small strongyles in horses (Cyathostomins) are the biggest headache for horse owners. They burrow into the lining of the large intestine. When conditions get tough (like winter), they go dormant (encyst). Many standard dewormers do not kill these dormant larvae.
Moxidectin (found in products like Quest) is currently the only drug recommended to effectively kill these encysted larvae. Therefore, most equine worming program schedules include Moxidectin or a product containing an active ingredient that targets these stages, usually once or twice a year, regardless of the FEC result, to clear the encysted stages.
Timing Your Treatments: When to Worm a Horse
When to worm a horse depends on the climate and the stage of parasite life cycles in your region. Generally, treatment should focus on two key times: late fall and early spring.
Late Fall Treatment (The Big Clean-Out)
This is perhaps the most crucial treatment of the year.
- Goal: Kill all internal parasites, especially the encysted small strongyles in horses, before winter sets in.
- Timing: About 5–7 days after the first hard frost, or around November/December.
- Drug Choice: This treatment should use a drug known to kill encysted larvae (Moxidectin) or a combination product if prescribed by your vet.
Early Spring Treatment
- Goal: Clear out any remaining worms before the pasture season begins and eggs start hatching rapidly.
- Timing: Usually February or March, before you start turnout for the season.
- Drug Choice: Use a drug effective against tapeworms (if tapeworm testing was positive) and general strongyles (like a Pyrantel or Benzimidazole if resistance tests allow).
Summer Treatments
Summer treatments are dictated almost entirely by the fecal egg count testing. If your horse is a moderate or high shedder, you might need a second or third treatment during the peak summer months (e.g., July). If your horse is a low shedder, you might skip summer treatments altogether.
Addressing Specific Parasites
A good equine worming program must account for more than just the common roundworms and strongyles.
Tapeworms in Horses
Tapeworms (Anoplocephala perfoliata) are often missed because standard FEC tests do not detect their eggs well. They are linked to gut inflammation and sometimes colic.
- Diagnosis: Standard FEC tests are unreliable for tapeworms. You need specialized tapeworm testing horses can undergo. This usually involves a tapeworm-specific test (like a saliva test) or treating empirically (treating just in case) during the late fall treatment when using Moxidectin or a double dose of Pyrantel.
- Treatment: Use Praziquantel (often included in combination pastes) or Moxidectin during the late fall clean-out.
Pinworms (Oxyuris equi)
Pinworms primarily affect the horse’s rear end, causing tail rubbing and skin irritation. Their eggs are laid outside the anus, making them invisible on standard FEC tests.
- Diagnosis: Visual inspection or using clear tape around the anus at night to collect eggs.
- Treatment: Regular treatment with Pyranel Pamoate is often effective. Keep the stall and bedding very clean.
Bots (Gasterophilus species)
Bot flies lay eggs on the horse’s hair, usually on the legs and shoulder. The larvae hatch and migrate through the mouth, stomach, and intestines.
- Treatment: Ivermectin or Moxidectin are effective against bot larvae in the stomach. The late fall treatment is the best time to eliminate bots before they hibernate. Scraping eggs off the hair daily during the fall season also helps reduce pasture contamination.
Pasture Management: The Non-Chemical Side of Control
Deworming medicine is only one part of the battle. Effective equine parasite control means managing the environment where parasites thrive.
Keeping Pastures Clean
Parasite larvae climb up blades of grass to wait for your horse to eat them. Shorter grass means the horse ingests more larvae.
- Mowing: Keep pastures mowed to a reasonable height (around 3 inches). This exposes the manure to more sunlight and dries it out, killing larvae.
- Manure Removal: Pick up manure daily or every other day. This stops larvae from maturing. Focus on high-traffic areas like around water troughs and gates.
- Stocking Density: Do not keep too many horses on a small piece of land. Overcrowding leads to high egg counts on the ground. A general rule suggests 1 to 2 acres per horse, though this varies greatly by climate and pasture quality.
- Separating Grazers: If possible, let cattle or sheep graze pastures after horses. They act as “clean-up” grazers, eating grass contaminated with horse parasite larvae, which do not harm them but reduce the load for the next horse.
Quarantine New Horses
Any new horse coming into your facility must be treated as a potential parasite risk until proven otherwise.
- Isolate the new horse for 3–4 weeks.
- Perform an FEC test immediately upon arrival.
- Treat based on the FEC result, focusing on clearing any possible encysted larvae.
- Perform a second FEC test 10–14 days after treatment.
- Only introduce the new horse to the main herd once their parasite load is confirmed low.
Navigating Drug Resistance: The Threat to Ivermectin
The widespread overuse of dewormers has led to significant resistance, especially against Ivermectin. Many veterinarians now note that Ivermectin may no longer be fully effective against adult small strongyles in horses in many regions.
How Resistance Happens
When a horse has a high parasite load, and you use a drug that only kills 99% of the worms, the 1% that survive are naturally resistant. These resistant worms survive to reproduce. Over several years, the entire population of worms in that environment becomes resistant to that drug.
What to Do About Resistance
If you suspect ivermectin resistance horses in your herd, the proof is in the post-treatment test.
- If your post-treatment FEC is high (over 200 EPG) after using an Ivermectin product, that product is likely failing.
- Switch your rotation to a different class, most likely Moxidectin, which often still works against resistant populations.
- Always consult your equine veterinarian to confirm which drugs are still effective in your local area, as resistance patterns change geographically.
Developing Your Custom Equine Worming Program
A successful equine worming program is highly individualized. It requires collaboration between you, your veterinarian, and your local lab.
Summary of Annual Plan Steps
| Season | Action | Purpose | Drug Focus (If Needed) |
|---|---|---|---|
| Spring (March/April) | FEC Test | Establish baseline for the grazing season. | Treat only if high EPG. Check for tapeworms. |
| Summer (July/August) | FEC Test | Check high shedders. | Target only moderate/high shedders. |
| Fall (November/Dec) | FEC Test & Strategic Treatment | The “Big Clean-Out.” Mandatory treatment. | Moxidectin (or equivalent) to kill encysted stages. |
| Winter (Jan/Feb) | Optional FEC Test | Check if any worms survived winter dosing. | N/A—no dosing unless vet advises. |
Age Considerations
Foals and Young Horses: Young horses have less developed immune systems and often carry higher parasite loads. They typically require more frequent monitoring and treatment than healthy adult horses, often needing treatment every 8-10 weeks until they are about 18 months old, after baseline FEC testing.
Senior Horses: Older horses may have immune systems that decline, making them more vulnerable. They need regular monitoring, as their sensitivity to parasite burdens may increase later in life.
Comprehending the Role of Fecal Egg Count Testing
The FEC test is not just a number; it is a management tool. It helps you apply the right treatment at the right time.
Reading the Report
When your lab returns the report, focus on the EPG number.
- Low EPG (<200): This horse is doing a great job controlling its own parasite load naturally. Focus on environmental control and treat maybe once a year in the fall with a highly effective drug like Moxidectin.
- High EPG (>500): This horse is a major source of contamination. You must treat this horse, retest two weeks later to confirm the drug worked, and investigate why their immunity is lower.
By focusing on the few horses that truly need treatment, strategic deworming horses keeps the majority of your herd safe, reduces drug use, and preserves the effectiveness of the medicines we have.
Frequently Asked Questions (FAQ)
Q1: Can I skip deworming entirely if my horse’s FEC is zero?
A1: While a zero EPG is fantastic, skipping treatment entirely is risky. A zero reading means the horse is not currently shedding strongyle eggs. However, it does not mean the horse has zero parasites. It might have tapeworms, bots, or encysted small strongyles in horses, which standard FECs miss. Most experts recommend at least one targeted treatment per year in the late fall using a drug effective against these hidden stages (like Moxidectin).
Q2: How soon after treating should I retest my horse?
A2: You should perform a post-treatment fecal egg count testing 10 to 14 days after administering the dewormer. This short window allows the treatment to clear the adult worms from the gut. If the EPG count remains high, the drug used was not effective against the parasites present, indicating resistance.
Q3: What is the best dewormer to use for pinworms?
A3: Standard FEC tests do not show pinworm eggs. Products containing Pyranel Pamoate are often effective against adult pinworms. Due to their lifecycle, cleaning the environment (stall, tail area) is just as important as medication. Some vets recommend treating all horses when one is found positive.
Q4: Do I need to worry about resistance to Moxidectin?
A4: Currently, Moxidectin (Quest, Quest Plus) remains highly effective against most internal parasites, including encysted small strongyles in horses. However, resistance is the goal of every parasite population. Following a strict strategic deworming horses plan, using Moxidectin only when necessary based on testing, and always following up with a retest is the best way to preserve its efficacy for future use.
Q5: Are pastes or granules better for deworming?
A5: When discussing the actual effectiveness, the formulation (paste, liquid, or granule) does not matter, provided the horse receives the correct, full dose. The most important thing is ensuring the horse consumes the entire dose. Pastes are often preferred because the dose is administered directly into the mouth, ensuring the full medication is ingested.