Worm infection in your horse: symptoms and approach

Which worms are most common?

In the Netherlands, large strongyles, small strongyles (cyathostomins), roundworms, pinworms, and bot flies are the most common parasites in horses. Small strongyles are by far the biggest culprit: they can encyst in the intestinal wall and emerge en masse in spring. This is known as cyathostominosis and can cause serious intestinal damage. Young horses up to three years of age are particularly susceptible to roundworms, while adult horses are more affected by strongyles. Knowing which worm your horse has is essential for the right treatment.

Common symptoms of a worm infection

Symptoms vary greatly depending on the type of worm and the severity of the infection. Watch for the following signs:

• Weight loss or a poor body condition score despite adequate feeding
• A dull, bloated, or distended belly, especially in young horses (roundworms)
• Persistent or recurring diarrhoea, sometimes with mucus
• Itching around the anus and tail rubbing (pinworms)
• Colic-like complaints without a clear other cause
• Reduced energy and decreased performance
• Visible worms or worm segments in the manure

When are symptoms a red flag?

Some symptoms call for immediate action. Acute cyathostominosis is accompanied by severe watery diarrhoea, rapid weight loss, and lethargy. This can become life-threatening within a few days, especially in horses older than fifteen years or horses that have recently gone through a stressful period. Colic combined with a high worm burden — established via a faecal egg count — is also a reason to call your vet the same day. Do not wait if multiple symptoms occur simultaneously.

Faecal egg count: the foundation of a targeted approach

Deworming without data is outdated. Resistance to deworming agents is increasing because horses were preventively treated for years without checking whether it was necessary. A faecal examination (EPG count, eggs per gram of faeces) provides insight into the worm burden. A result above 200–300 EPG calls for treatment. Also request a larval cyathostomin test if your horse shows symptoms in autumn or winter. Discuss the results with your vet and record the data — in EquiSight you can track results and treatments in the horse profile, so you build a clear picture over the years.

Deworming: medications and timing

Three active substances are available in the Netherlands: ivermectin, moxidectin, and fenbendazole. Ivermectin works well against most adult worms, but not against encysted larvae. Moxidectin is more effective against those larvae and is often used when the risk of cyathostominosis is elevated. Fenbendazole is used less frequently due to increasing resistance. Treat preferably on the basis of a faecal examination and check the effect 14 days after deworming with a follow-up test. Use the calendar in EquiSight to set reminders for follow-up tests.

• Ivermectin: broad spectrum, available as paste or gel
• Moxidectin: effective against encysted larvae, not for foals under 4 months
• Fenbendazole: check for resistance via a follow-up test after 14 days
• Praziquantel: specifically for tapeworms, often combined with ivermectin

Pasture management significantly reduces worm burden

Medication alone is not enough. Worm eggs survive for months in the pasture. Remove manure from small paddocks at least three times a week. Rotate pastures and leave fields empty for at least six weeks, preferably in summer when UV light and drought break down eggs. Limit stocking density to a maximum of one horse per hectare on permanent pasture. Co-grazing with sheep or cattle helps: they ingest horse worm larvae as a dead-end host. Record your pasture planning and manure management so you can compare year on year what works for your situation.