Deworming Your Horse: Why a Faecal Egg Count Beats a Fixed Schedule

For a long time the rule was simple: every horse got a wormer a few times a year, whether it needed one or not. We are now paying the price for that. Many worms no longer respond to the drugs we have. A plan based on faecal egg counts therefore works better: you treat the horses that really need it, and you know whether the drug still works.

Have you just bought a horse or are you still looking? Also read the checklist for buying a horse, recognising colic in horses and what does a horse cost, which lists faecal testing and deworming as a regular expense.

Why the fixed schedule no longer works

According to the MSD Veterinary Manual, schedules with frequent, fixed treatments have led to very high levels of resistance. Small strongyles (cyathostomins) are widely resistant to two of the three major groups of wormers. Resistance to the third group, which includes ivermectin and moxidectin, is also spreading rapidly. Every unnecessary treatment makes the problem bigger.

The LICG writes that regular faecal testing or preventive deworming is needed to prevent worm infections. Doing nothing is not an option. The point is to choose in a targeted way.

Not every horse carries the same number of worms

In a group of horses, worms are very unevenly spread. A study (available via PubMed Central) describes that only 15 to 30 percent of the horses in a group carry a large worm burden. Together those horses shed about 80 percent of the worm eggs that end up on the pasture. Half to three quarters of the horses in a group are low shedders.

The same study gives the classification many vets use, based on the number of eggs per gram of faeces (EPG):

• low shedder: 0 to 200 EPG;
• moderate shedder: 201 to 500 EPG;
• high shedder: more than 500 EPG.

The approach the study describes: treat high shedders in a targeted way and leave low shedders untreated. The MSD Veterinary Manual also advises tailoring deworming to the yard and to the individual horse, and above all identifying the high shedders. Where your vet draws the line depends on the situation at your yard. Discuss it together.

How faecal testing works in practice

1. Having a sample tested: your vet sends the sample to a lab, for example GD Animal Health in the Netherlands. Ask beforehand how to take and store the sample.
2. Discussing the result: based on the number of eggs, your vet decides whether this horse needs a treatment now, and with which drug.
3. Checking whether the drug works: the MSD Veterinary Manual advises a faecal egg count before treatment and again 14 days later. If the number of eggs does not drop enough, the drug no longer works well at your yard. GD advises repeating the faecal test two and five weeks after a treatment with ivermectin, or two and six weeks after moxidectin.

Note: an ordinary egg count does not find every worm. According to GD, tapeworm eggs are not counted; the lab only reports whether they are present. According to GD, tapeworm infection occurs at all ages, but can only be detected once the horse has been out to grass. The MSD Veterinary Manual advises also treating against tapeworms and bots once or twice a year. Tapeworms are common in grazing horses and can cause colic.

Foals and young horses are a separate group

According to the MSD Veterinary Manual, the advice for foals and young horses is very different from that for adult horses. If you plan to breed or buy a foal, also read buying and raising a foal.

• Roundworms: according to GD, foals in particular can become seriously ill from roundworms (Parascaris). They may show signs of colic, constipation or diarrhoea, sometimes after a period of nasal discharge and coughing. According to GD, faecal testing for roundworms is useful from about ten weeks of age.
• Resistance: according to the MSD Veterinary Manual, roundworms are widely resistant to ivermectin and related drugs. Leave the choice of drug to your vet.
• Blockage after a wormer: a foal with many roundworms can develop an intestinal blockage after an effective wormer. According to the MSD Veterinary Manual this happens most often around five months of age. It is an emergency.
• Young horses: according to GD, horses aged one to two years show signs of gastrointestinal worms more often than older horses.

When to call the vet

Worms often cause no clear signs for a long time. Call your vet in any case if your horse:

• shows signs of colic (see recognising colic);
• suddenly develops watery diarrhoea. According to the MSD Veterinary Manual this can point to larval cyathostominosis, a disease caused by encysted small strongyle larvae in the gut wall. It mainly affects horses aged one to four, comes with dehydration and weight loss, and about half of affected horses do not survive. According to the MSD Veterinary Manual the number of encysted larvae peaks in autumn and winter;
• loses weight for no clear reason;
• as a foal starts coughing with nasal discharge and then develops abdominal problems.

Do not give a wormer yourself when you see such signs. According to the MSD Veterinary Manual, both the intestinal blockage in foals and larval cyathostominosis often develop shortly after a wormer. Let the vet look first.

Fewer worms on the pasture

Deworming is only half the job. The MSD Veterinary Manual gives these measures:

• Remove dung: clear manure from stables, paddocks and pastures every 24 to 72 hours, before the eggs hatch and become infective larvae.
• New horses: keep a new horse apart first and have its dung tested before it joins the group.

Pasture management also matters for other problems, such as too much sugary grass. Read more in laminitis and pasture management. GD calls regular, well-performed faecal testing, together with a well-considered plan for deworming and pasture management, crucial to prevent problems with gastrointestinal worms at a yard.

What to ask when buying a horse

Ask the seller when the horse was last dewormed, with which drug, and whether there are recent faecal test results. If those are missing, that is no reason to walk away, but plan a faecal test straight after arrival, while the horse is still kept apart.

Looking for a horse or pony? Browse the horse page on Nestjes and always ask about the health history.

Sources

1. LICG – Paard (horse) (accessed October 2026)
2. GD Animal Health – Maagdarmwormen (gastrointestinal worms, horse) (accessed October 2026)
3. GD Animal Health – Spoelwormen (roundworms, horse) (accessed October 2026)
4. GD Animal Health – Lintworm (tapeworm, horse) (accessed October 2026)
5. MSD Veterinary Manual – Parasite Control in Horses (accessed October 2026)
6. MSD Veterinary Manual – Parasite Control in Horses (Management and Nutrition) (accessed October 2026)
7. MSD Veterinary Manual – Diseases Resulting From Gastrointestinal Parasites in Horses (accessed October 2026)
8. PubMed Central – Evaluation of Fecal Egg Count Tests for Effective Control of Equine Intestinal Strongyles (accessed October 2026)