The answer to how often to deworm a horse is not a simple, fixed number; it depends heavily on the horse’s age, environment, health, and, most importantly, the results of fecal egg count testing. Modern parasite control in horses emphasizes tailored treatment over blanket schedules.
The Shift Away from Fixed Schedules
For many years, horse owners followed a strict, calendar-based equine deworming schedule. Every horse got the same drug, perhaps every eight weeks, year-round. This approach made things simple but created a major problem: anthelmintic resistance horses now face widespread. Worms become strong because we kill the weak ones repeatedly. Smart horse care today means testing first.
Why Testing Replaces Guesswork
Relying on a fixed schedule is like treating every sick person with the same strong antibiotic without knowing what is causing the illness. We now know this causes drug resistance.
The Role of Fecal Egg Count Testing
Fecal egg count testing (FEC) is the cornerstone of modern parasite control. This simple manure test shows how many strongyle eggs (usually small strongyles treatment targets) are present in a single gram of manure (EPG).
- High EPG: The horse sheds many parasite eggs. This horse needs immediate, targeted deworming.
- Low EPG: The horse sheds few eggs. This horse needs less frequent treatment to preserve drug effectiveness.
This process is key to strategic deworming. We only treat the horses that truly need it.
Deciphering Your Horse’s Parasite Load
To create a sensible deworming frequency for horses, you must group your horses based on their egg counts. This is often called FEC categorization.
FEC Categories and Implications
| Category | EPG Count (Eggs Per Gram) | Recommended Action |
|---|---|---|
| Low Shedders | Under 200 EPG | Treated 1–2 times per year. |
| Moderate Shedders | 200–500 EPG | Treated 2–3 times per year. |
| High Shedders | Over 500 EPG | Treated 3–4 times per year, plus regular monitoring. |
Important Note: These numbers are general guidelines. Your veterinarian will set the specific targets for your herd based on local resistance patterns and your environment.
Fecal Egg Culture Testing
While the standard FEC tells you how many strongyle eggs are present, it does not tell you which parasite laid them. For a complete picture, some vets recommend a fecal egg culture. This test grows the eggs in the lab to identify the specific parasite species present. This is vital for tracking large strongyles deworming success.
Developing an Equine Deworming Schedule
A successful equine deworming schedule follows the testing results. It is no longer just about the calendar date; it is about the test result date.
Timing Treatments Based on Results
- Test First: Collect fresh manure samples (usually 2 to 5 horses per group, if testing the whole herd). Send them to the lab.
- Wait for Results: The results take a few days.
- Treat Based on EPG: Only treat the horses whose EPG exceeds the established threshold for your farm. Low shedders might skip a treatment.
- Retest After Treatment: About 10–14 days after administering dewormer, you should retest the treated horses. This is called a Fecal Egg Count Reduction Test (FECRT).
The Fecal Egg Count Reduction Test (FECRT)
The FECRT is crucial for checking if the drugs you used actually worked.
- What it shows: It compares the EPG before treatment to the EPG after treatment.
- Goal: A successful treatment should reduce the egg count by 95% or more.
- Failure: If the count only drops by, say, 60%, the worms on your farm are likely resistant to that drug. You must change the drug class used next time.
Managing Major Parasite Threats
Different parasites need different drugs and different timing. Parasite control in horses must target the most dangerous threats first.
Small Strongyles (Cyathostomins)
These are the most common internal parasites. They cause few visible signs when present in low numbers. However, mass encystment (hiding in the gut wall) followed by mass emergence causes severe damage, diarrhea, and colic.
- Treatment Focus: Most FECs measure small strongyle eggs.
- Drug Sensitivity: Resistance to older drugs (like Fenbendazole) is very high in many areas. Modern small strongyles treatment usually relies on Macrocyclic Lactones (e.g., ivermectin, moxidectin) or Pyrantel.
Large Strongyles (Bloodworms or Strongylus vulgaris)
These are the historical scourges of horses. They migrate through the arteries, causing blockages, colic, and death.
- Modern Status: Routine deworming with effective drugs has made them rare in well-managed herds.
- Deworming Strategy: Because they are so dangerous, veterinarians often recommend treating all horses for large strongyles deworming at least twice a year, even if their FEC is low, using a drug known to kill them (like a Benzimidazole or a Macrocyclic Lactone).
Tapeworms and Pinworms
FEC tests do not reliably find tapeworm or pinworm eggs. You must rely on other signs or routine timing for these parasites.
- Tapeworms: Often treated once or twice a year, usually in the fall or spring, using Praziquantel or a double dose of Pyrantel (some vets prefer the latter, as resistance data is clearer).
- Pinworms: Cause tail rubbing and irritation. Treatment usually involves specialized products or cleaning the tail area frequently.
Implementing Rotational Deworming Horses (The Old vs. New Way)
Rotational deworming horses used to mean cycling through different drug classes every few treatments. This helped prevent resistance to a single drug. Now, the focus is on strategic deworming using FEC results, but rotation remains a tool.
The Modern Rotation Strategy
Instead of strict calendar rotation, modern strategies use the FECRT to inform the next choice.
- Identify Drug Failure: If your FECRT shows resistance to Drug A, you switch to Drug B (a different chemical class).
- Annual Rotation: Many vets still suggest rotating the main drug class used for the two most critical treatments of the year (usually spring and fall) to reduce selection pressure.
Example of a common, FEC-guided schedule:
| Season | Focus | Drug Class Used (Based on FECRT) |
|---|---|---|
| Spring (Early) | Targeting emerging worms/Large Strongyles | Macrocyclic Lactone (e.g., Moxidectin) |
| Summer | Low Shedders skip; High Shedders test/treat | Pyrantel or Benzimidazole |
| Fall (Early) | Critical treatment before winter; Tapeworm coverage | Benzimidazole (perhaps double dose, if treating tapeworms) |
| Winter | Targeting encysted Small Strongyles | Macrocyclic Lactone (e.g., Ivermectin) |
This approach ensures horses are treated when necessary, and the drug used is one that has proven effective against the local parasite population.
Veterinary Deworming Guidelines and Drug Classes
It is essential to follow veterinary deworming guidelines because drug availability and local resistance change constantly. Always consult your veterinarian before purchasing and administering dewormers.
There are four main classes of dewormers used in horses:
- Benzimidazoles (e.g., Fenbendazole, Albendazole): Very common, but resistance to small strongyles is widespread. They are effective against tapeworms in large doses.
- Macrocyclic Lactones (e.g., Ivermectin, Moxidectin): Highly effective against many worms, including encysted small strongyles (Moxidectin is generally considered best for this). Resistance is growing but less common than with Benzimidazoles.
- Pyrantel Salts (e.g., Pyrantel Pamoate): Useful for small strongyles and tapeworms (at double dose). Resistance is common in some areas, but it is often the drug of choice for treating low shedders due to its shorter persistence in the environment.
- Praziquantel: Used specifically for tapeworms. Often combined with another drug or given as a double dose of Pyrantel.
Why Moxidectin is Often Preferred Annually
Moxidectin (a Macrocyclic Lactone) is often favored for one of the primary annual treatments because studies show it is highly effective at killing the encysted larval stages of small strongyles treatment—stages that Ivermectin might miss. Killing these hidden larvae prevents a huge surge of adult worms later.
Environmental Management: Beyond the Paste Tube
Deworming frequency is only half the battle. Poor pasture management allows eggs to survive and re-infect your horses, no matter how often you treat them.
Pasture Hygiene Practices
- Remove Manure Daily: This is the single best thing you can do. Removing manure removes the eggs before they hatch.
- Mow Pastures: Mowing keeps grass short, encouraging horses to graze widely rather than fouling the same small areas.
- Drag/Harrow Fields: Spreading manure out in the sun helps dry and kill the larvae. (Do this only when the soil is dry to prevent spreading mud).
- Use Sacrifice Areas: During wet or heavily used seasons, keep horses off main pastures, feeding them hay in dry lots or paddocks. This breaks the parasite life cycle.
- Quarantine New Horses: Any new horse should be dewormed based on an FEC test and then ideally kept separate for a few weeks while retesting to ensure they are not bringing in resistant strains.
Special Considerations for Deworming Frequency
Not all horses have the same needs. Deworming frequency for horses must adapt to the individual animal.
Foals and Young Horses
Foals are highly susceptible to parasites, especially Large Strongyles, early in life. They cannot build natural immunity quickly.
- Early Treatment: Foals often need treatment starting around 2 to 3 months of age, following veterinary advice.
- Frequent Testing: They require closer monitoring via FEC testing throughout their first year or two until their immune system matures enough to become low or moderate shedders.
Geriatric Horses (Older Horses)
Very old horses may have weaker immune systems, making them more susceptible, or they may have developed strong natural immunity over decades.
- Assessment: Older horses should always be FEC tested regularly. They may suddenly become moderate or high shedders as their natural defenses wane.
Horses on Restricted Pasture
A horse kept in a small paddock or dry lot might seem low-risk, but these areas often become highly contaminated because manure piles up and isn’t diluted by new grass.
- Risk Assessment: Paradoxically, a horse in a small, heavily used area might need more frequent treatment than a horse on vast, well-managed pastures. Always test!
Comprehending Anthelmintic Resistance Horses
The biggest threat to effective parasite control in horses today is resistance. When we overuse or misuse dewormers, we select for worms that can survive the treatment.
How Resistance Spreads
When a horse with resistant worms is dewormed, the susceptible worms die. The resistant worms survive, breed, and pass on their resistance genes. If this horse then goes out into the pasture, its manure is full of resistant eggs.
Best Practices to Slow Resistance
- Never Use Daily Dewormers: Drugs given daily (like continuous low-dose Ivermectin) rapidly create resistance. These are now largely discouraged by veterinary groups.
- Use Strategic Deworming: Only treat based on FEC results. This protects the low shedders from unnecessary drug exposure.
- Use High-Dose Treatments for Encysted Larvae: Ensure your annual treatment for encysted small strongyles treatment uses a drug known to effectively target those larvae (usually Moxidectin).
- Conduct FECRTs Regularly: This is your report card. It tells you if the drugs you are using are still working.
Practical Steps for Your Deworming Program
To set up a safe and effective equine deworming schedule, follow these concrete steps, always in partnership with your veterinarian.
Step 1: Initial Assessment (Baseline)
Perform FEC testing on all horses in the herd, ideally during high-risk seasons (spring/fall).
Step 2: Categorize and Treat High Shedders
- Horses over 500 EPG receive an appropriate dewormer based on veterinary deworming guidelines and local drug sensitivity (determined by FECRT history).
- These horses are retested 10–14 days later to confirm efficacy.
Step 3: Implement Annual Target Treatments
Even low shedders need treatment for parasites that FEC tests miss or for those that are considered too dangerous to ignore (large strongyles deworming).
- Target 1 (Spring): Treat for tapeworms and large strongyles using an effective drug (often Moxidectin or a Praziquantel combination).
- Target 2 (Late Fall/Early Winter): Treat for encysted small strongyles and large strongyles using a drug that targets larvae (usually Moxidectin).
Step 4: Manage Low Shedders
Low shedders (under 200 EPG) might only need the two targeted treatments listed above. They skip the summer treatments, reducing overall drug use dramatically.
Summary of Deworming Frequency
How often to deworm depends on the horse:
- High Shedders: May need treatment 3 to 4 times per year, guided closely by testing.
- Low Shedders: May need only 2 targeted treatments per year, provided their FEC remains low.
- All Horses: Should receive at least two powerful, rotating treatments annually targeting large strongyles and encysted small strongyles.
By moving to strategic deworming powered by data from fecal egg count testing, you protect your horse’s health while ensuring the deworming drugs remain effective for the future.
Frequently Asked Questions (FAQ)
What is the best time of year to deworm a horse?
The best times are usually early spring (around March/April) and late fall (around October/November). Spring targets worms emerging from dormancy, and fall targets worms before winter sets in. However, exact timing must align with when you collect samples for testing.
Can I just use the same dewormer every time?
No. Using the same drug repeatedly is the fastest way to select for anthelmintic resistance horses. You must rotate the chemical class used for major treatments, guided by the results of your FECRT.
What is the goal of a Fecal Egg Count Reduction Test (FECRT)?
The goal of the FECRT is to measure how well the dewormer you just gave actually killed the parasite eggs. A good result means the egg count dropped by 95% or more. A poor result means the worms are resistant to that drug.
Are deworming pastes more effective than feed-through products?
Pastes and pastes allow you to confirm the horse consumed the full dose, which is crucial. Feed-through products are often less reliable because not every horse in the herd consumes the same amount, leading to inconsistent dosing. For targeted treatment of high shedders, pastes are usually preferred.
How do I treat for tapeworms if my fecal egg count is low?
Since FEC tests often miss tapeworm eggs, most veterinary deworming guidelines suggest treating all horses for tapeworms once or twice a year (usually spring and fall) using Praziquantel or a double dose of Pyrantel, regardless of the strongyle EPG result.