Almost every horse carries worms, but far from every horse needs a deworming treatment. The advice has shifted from fixed treatments on the calendar to targeted treatment based on fecal egg counts. That is also what the Dutch guideline on horses and parasites (Leidraad Paard en Parasieten, by the KNMvD, the Dutch veterinary association, and Royal GD, the Dutch animal health service, October 2024) says; it is a handbook for vets. Here's why that difference matters and how to put it into practice.
Why the Fixed Schedule Is Outdated
The guideline writes that some owners still think horses should be dewormed at least four times a year, up to every eight weeks. That fixed schedule is outdated, because only about a third of adult horses shed so many eggs (more than 200 eggs per gram of manure) that treatment is warranted. On top of that, egg shedding and pasture contamination differ by season.
Then there is resistance: worms that become insensitive to a product. Only three groups of products against roundworms are available, and according to the guideline, on 80 to 90 percent of Dutch farms small strongyles are resistant to one of those groups, the benzimidazoles. The more often you use a product, the faster that grows. That's why the rule now is: treat the horse that needs it, and let the vet choose the right product at the right time.
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. The guideline calls a result positive, and treatment therefore indicated, for:
- an individual manure sample with more than 200 EPG of the strongyle type (stomach and intestinal worms);
- a pooled sample (manure from several horses together) with more than 100 EPG;
- roundworm eggs (Parascaris) in the manure of a foal, always.
A low EPG usually means: don't treat. A horse with fewer than 200 EPG keeps part of the worm population out of treatment, which slows resistance, and builds up immunity through low exposure. In the first year, the guideline advises having manure tested three times: two weeks before the horse goes out (or from March if it's already in the pasture), in summer, and in September. If the result stays under 200 on at least three tests, testing can be done less often afterward. If a result is above 200 once, or roundworm eggs are found, you keep testing three times a year. The choice of product is up to the vet, who also considers 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.
A count doesn't catch everything. After the first months of the first grazing season, the EPG says little to nothing about the total worm burden, according to the guideline, because larvae can withdraw into the gut wall. Tapeworm is better detected with a saliva test (EquiSal), and bot flies you recognize by the yellow eggs on the hairs of the legs.
Checking whether a product works. The guideline describes the FECRT (fecal egg count reduction test): you count the EPG on the day of treatment and fourteen days later. That shows whether the product lowers the egg count enough. The guideline advises doing this every two years, so you know whether there is resistance on the farm.
The Worms That Matter
| Worm | What the guideline says |
|---|---|
| Small strongyles (stomach and intestinal worms) | Occur almost everywhere on horse pastures; in principle all grazing horses become infected. Larvae can withdraw into the gut wall and become active again months to years later |
| Large strongyles | As of 2024, found almost only occasionally in extensively or semi-wild kept horses and imported horses |
| Roundworms (Parascaris) | Mainly in foals and yearlings; coughing is the first and most common sign. The eggs can survive in the environment for up to 10 years |
| Tapeworm | Horses develop no immunity to it, so all ages can be infected. Treat only with a proven or very strongly suspected infection |
| Bot flies | Usually no treatment needed; removing the eggs from the hairs helps |
| Liver fluke | Only on pastures infected by cattle, sheep, or goats, among others; many infections cause no signs |
A Workable Rhythm Through the Year
The guideline gives no fixed schedule, because worm control is custom work for each farm. It does give a direction for adult horses:
| Period | Approach |
|---|---|
| Spring, before turnout | Fecal egg count; treat only with an EPG above 200 or with roundworm eggs |
| Summer | Fecal egg count; treat only with an EPG above 200 |
| September | Fecal egg count. Don't deworm if the EPG was low, the saliva test for tapeworm is negative, and there are no bot fly eggs on the skin. Do treat if there was no fecal testing all year, the history is unknown, or the EPG is high |
| Fall, encysted larvae | Only if monitoring was insufficient in young horses (younger than 4 to 5 years) or a heavy infection is suspected, or in older horses with a raised EPG |
| Tapeworm and bot flies | Only when indicated |
A treatment in fall or winter is not without risk. According to the guideline, acute larval cyathostominosis, a serious bowel inflammation with diarrhea and fever, regularly occurs after a deworming product in that period, because encysted larvae are released in large numbers. So a fixed fall treatment without a reason isn't wise. If your horse has diarrhea or a fever after treatment, call the vet right away.
Foals, young horses, and newcomers to the yard call for a stricter regimen. The guideline advises having foals tested monthly. Roundworm eggs appear in the manure from about 9 to 16 weeks. After a treatment against roundworms, a foal can develop a bowel blockage from dead worms, usually within 24 hours. According to the guideline, you keep new horses separate for at least two weeks after they have been dewormed with an effective product. Then comes a fecal test on arrival and at the end of that period, and only when there is no meaningful egg shedding does the horse join the herd.
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 guideline names these measures:
- Picking up manure: once a week in spring up to twice a week in summer, and mucking out stalls daily.
- Preventing overgrazing: with enough grass, horses don't eat right next to the manure spots, where contamination is highest. Feed supplements from a raised feeder, not from the ground.
- Mowing: mowing regularly, for example to make hay, takes larvae off the grass tips.
- Rotating and mixed grazing: foals and yearlings on the cleanest plots, where no young horses grazed the year before. Grazing with ruminants (cattle, sheep) lowers contamination pressure, but watch for liver fluke with cattle.
- Composting: only spread horse manure if it is fully composted, with a core temperature of at least 70 degrees Celsius throughout the heap for at least a week. Manure from foals and yearlings never belongs on a horse pasture.
Good pasture management lowers worm burden so much that many horses can get by with fewer treatments. It's a core part of the approach, not a side issue.
Preventing Resistance
A few habits keep your products effective:
- Treat based on testing, not the calendar. The guideline considers "blind" treatment defensible only in young horses being raised, and then on a plan.
- Let the vet choose the product, and check every two years whether it works.
- Keep new horses separate until they've been tested.
For a tailored schedule, 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.
In Short
- Nearly every horse carries worms; not every horse needs a treatment. About a third of adult horses shed so many eggs that treatment is warranted.
- Let fecal egg counts (EPG, threshold 200) determine who you treat, instead of deworming everyone on fixed dates.
- A count says little about larvae in the gut wall. You detect tapeworm with a saliva test and treat it only with a proven infection.
- A treatment in fall or winter can itself trigger a serious bowel inflammation. Only do it on the vet's advice.
- Picking up manure, preventing overgrazing, and rotating pastures lower worm burden at least as much as the treatment itself.
- Check every two years whether your product still works: count the EPG on the day of treatment and fourteen days later.
Sources (verified September 2, 2026, worm types, schedule, and intervals November 2, 2026): KNMvD and Royal GD: Leidraad Paard en Parasieten (horses and parasites guideline), extended version October 21, 2024 · Royal GD: Parasitology for horses
See also: Glossary: health and care terms · Level: Considering Your Own Horse