For decades, horse owners were told to rotate dewormers every 8 weeks. Today, veterinarians know that approach has led to widespread drug resistance in parasites. Modern deworming for horses is more targeted, based on testing each horse. Here’s how fecal egg count strategies work.
Quick Answer: Current guidelines from the American Association of Equine Practitioners recommend deworming adult horses based on fecal egg counts (FECs). Most adults need only 1 to 2 treatments a year, with more for “high shedders.” Testing helps protect dewormers’ effectiveness. Foals and young horses need a different, more frequent plan.
Why the Old Approach Changed
Frequent, routine deworming of every horse has led to parasites that resist common dewormers. The AAEP Internal Parasite Control Guidelines recommend strategic, test-based deworming instead.
Key facts:
- A small number of horses shed most of the parasite eggs on a property.
- Many adult horses naturally keep low parasite burdens.
- No dewormer kills 100% of parasites, and resistant worms survive and multiply.
Main Parasites in Horses
| Parasite | Main concern |
|---|---|
| Small strongyles (cyathostomins) | Most common in adults; can cause colic and weight loss |
| Large strongyles | Less common now; can damage blood vessels |
| Tapeworms | Linked to some types of colic |
| Roundworms (ascarids) | Mainly in foals; can cause intestinal blockage |
| Pinworms | Tail rubbing |
| Bots | Larvae in the stomach |
What Is a Fecal Egg Count?
A fecal egg count measures how many parasite eggs are in a manure sample, reported as eggs per gram (EPG).
| Category | Typical EPG range |
|---|---|
| Low shedder | Under 200 EPG |
| Moderate shedder | 200–500 EPG |
| High shedder | Over 500 EPG |
Ranges may vary slightly by lab and vet.
How Strategic Deworming Works
- Test each adult horse with a fecal egg count, usually in spring or as your vet advises.
- Classify each horse as a low, moderate or high shedder.
- Deworm everyone 1 to 2 times a year for parasites FECs don’t detect well, like tapeworms and bots, often in spring and fall.
- Add extra treatments for moderate and high shedders.
- Do a fecal egg count reduction test (FECRT) periodically to check if a dewormer is working.
Fecal Egg Count Reduction Test
Your vet tests manure before deworming and again about 10 to 14 days after. If eggs don’t drop enough, parasites on your farm may be resistant to that dewormer.
Deworming Foals and Young Horses
Young horses are more vulnerable, especially to roundworms. A typical approach includes:
- First deworming around 2 to 3 months of age
- Additional treatments as advised during the first year
- Fecal testing to guide which drugs to use
Follow your vet’s specific plan.
Pasture Management
Good pasture management reduces parasite exposure:
- Pick up manure regularly from small pastures and paddocks.
- Avoid overcrowding.
- Rotate pastures or graze with other species, like cattle.
- Don’t spread fresh manure on horse pastures.
- Harrow pastures only in hot, dry weather when eggs and larvae die quickly.
Tips for Giving Dewormer
- Weigh or tape your horse to dose correctly. Underdosing contributes to resistance.
- Make sure the mouth is empty before dosing.
- Place the syringe at the back of the tongue.
- Hold the head up briefly to help your horse swallow.
Signs of a Heavy Parasite Burden
- Weight loss or poor condition
- Dull coat
- Pot belly, especially in young horses
- Diarrhea
- Colic. See colic in horses.
- Tail rubbing (pinworms)
When to Call Your Vet
Call your vet if your horse:
- Is losing weight despite good feed
- Has diarrhea or colic
- Is a young horse with a pot belly and poor growth
- Needs a parasite control plan
Frequently Asked Questions
Is rotating dewormers still recommended?
Routine rotation on a fixed schedule is no longer recommended. Choose drugs based on testing and resistance results.
How much does a fecal egg count cost?
Usually much less than a tube of dewormer, and it can save money by avoiding unnecessary treatments.
Can I skip deworming if my horse is a low shedder?
Most low shedders still need 1 to 2 treatments a year to target tapeworms and bots. Your vet will advise.
Do senior horses need more deworming?
Some older horses, especially those with PPID, may shed more eggs. Testing guides treatment.
When is the best time to do a fecal egg count?
Many vets test in spring and again later in the grazing season, but don’t test within about 8 to 12 weeks of the last deworming, depending on the drug used.
Should all horses on a farm be dewormed at the same time?
Not necessarily. Plans are based on each horse’s shedding status, though some treatments, like a fall tapeworm and bot dose, are often given to the whole herd.
Can natural or herbal dewormers replace drugs?
There’s little scientific evidence they work. Rely on proven dewormers chosen with fecal testing.
This article is for general information only and isn’t a substitute for professional veterinary advice. If you’re worried about your pet’s health, contact your veterinarian or an emergency animal hospital.
