Almost every horse carries worms, but far from every horse needs a deworming treatment. Over the past few years, the advice has shifted from fixed treatments on the calendar to targeted treatment based on fecal egg counts. Here's why that difference matters and how to put it into practice.
Why the Fixed Schedule Is Outdated
Until recently, it was normal to deworm every horse four times a year, regardless of whether the animal had worms. That worked for a while, but it has a downside: the more often you use a product, the faster worms become insensitive to it. That insensitivity is called resistance.
The problem is real. Almost no new deworming products are in development, so the handful of active substances that exist need to keep working for decades to come. Every unnecessary treatment speeds up the moment when a product stops working. That's why the guidance now is: only treat the horse that truly needs it, and then choose the right product at the right time.
On top of that, worm burden is unevenly distributed across a herd. A well-known rule of thumb is that a small proportion of horses shed the vast majority of the eggs. Identifying and treating those shedders tackles the problem at the source, while the rest can be left alone.
What a Fecal Egg Count Tells You
With a fecal egg count, a laboratory counts the number of worm eggs in a manure sample. The result is expressed in EPG: eggs per gram of manure. Based on that count, you determine whether treatment is needed.
- Low count — usually no deworming is needed. The horse carries worms, but in a quantity it can handle on its own.
- Higher count — a shedder; deworming is then worthwhile, partly to limit contamination of the pasture.
- Resistance test — a second count a few weeks after treatment shows whether the product used actually worked.
Leave the precise threshold values and the choice of product to your vet or a deworming consultant. They also factor in what type of horse is involved: a foal, an old horse, or a horse with weaker resistance needs a different regimen than a healthy adult horse.
Important: a standard fecal count doesn't catch everything. Tapeworm and bot fly larvae are poorly detected or not detected at all this way, and encysted redworm larvae (the small strongyles) go undetected. Separate tests or targeted, one-off treatments exist for those, independent of the count.
A Workable Rhythm Through the Year
A commonly used approach combines fecal egg counts during the growing season with one or two fixed treatment moments for the worms you don't count. Broadly, that looks like this; always tailor it to your own situation.
| Period | Approach |
|---|---|
| Spring through fall | Fecal egg count every few months; treat only shedders |
| Fall / early winter | Often a targeted treatment against encysted redworm larvae and tapeworm |
| If in doubt or showing symptoms | Extra fecal egg count or consult the vet |
Foals, young horses, and newcomers to the yard call for a stricter regimen, because they're more susceptible and shed more eggs. Ideally, you only turn out a new horse with the rest in the pasture after testing (and treatment if needed), so you don't import resistant worms.
Pasture Management Counts Just as Much
Deworming fights the worms inside the horse; pasture management fights the contamination around it. Without that second part, you're mopping the floor with the tap still running. The main measures:
- Picking up manure — clearing the pasture once or twice a week removes by far the most eggs and larvae.
- Not overstocking — more space per horse means less contamination pressure.
- Rotating and mixed grazing — alternating between plots, and where possible grazing with cattle or sheep, which ingest horse worms without becoming infected themselves.
- Composting — don't spread fresh manure directly over the pasture; well-heated compost kills most of the eggs.
Good pasture management lowers worm burden so much that many horses can get by with fewer treatments. It's therefore not a side issue, but a core part of the approach.
Recognizing and Preventing Resistance
A product is no longer working well if the egg count barely drops after treatment. You only notice this if you test for it, hence the follow-up test a few weeks after treatment. A few habits keep your products effective:
- Treat based on testing, not the calendar.
- Give the correct dose based on actual body weight — underdosing breeds resistance. Estimate the weight with a weigh tape or livestock scale instead of by eye.
- Don't switch products at random; do so in consultation with your vet, since some substances are matched to specific worms or seasons.
- Keep new horses separate until they've been tested.
In the Netherlands, deworming products are classified as veterinary medicines; the vet or pharmacy helps determine what's appropriate. Don't blindly trust random advice found online.
In Short
- Nearly every horse carries worms; not every horse needs a treatment.
- Let fecal egg counts (EPG) determine who you treat, instead of deworming everyone on fixed dates.
- Account for tapeworm, bot fly larvae, and encysted redworm larvae that a count misses — separate, targeted treatments fit those.
- Picking up manure, not overstocking, and rotating pastures lower worm burden at least as much as the treatment itself.
- Check whether your product still works and dose based on actual body weight, so resistance doesn't get a chance.
For a tailored schedule and choice of the right product, consult your vet; a persistently heavy worm burden can cause symptoms such as weight loss or colic (abdominal pain in the horse — always call the vet), and should therefore be assessed by the vet as well.
See also: Glossary: health and care terms · Level: Considering Your Own Horse