Do You Really Need to Worm Your Horse This Month? Probably Not.

The Habit We Need to Break

For years, the advice was simple: worm every three months, rotate your products, job done. Most owners still follow that routine without a second thought. It feels responsible. It used to be responsible. Science has moved on.

UK vets and welfare organisations have quietly shifted their guidance over the past decade. Blanket, calendar-based worming — giving every horse a dose on a fixed schedule, regardless of whether they actually need it — is now considered outdated. Not because worms don't matter. They do. But giving unnecessary treatments speeds up resistance, and resistance is a problem we can't undo once it takes hold.

The good news is that the smarter approach isn't more complicated. In some ways, it's simpler. And it starts with one question most owners have never been asked: when did you last test?

What Are Worms, and Why Do They Matter?

Horses and donkeys pick up worm larvae while grazing. The larvae develop inside the animal, the adult worms produce eggs, and those eggs pass back out onto the pasture — where the cycle starts again. A low worm burden is normal and largely harmless. A high burden causes real problems: gut damage, colic, weight loss, and in serious cases, death.

Several types of worms can affect equines in the UK, but a few are worth knowing about:

• Small redworm: The most common and most dangerous parasite for adult horses and donkeys. What makes them particularly tricky is what they do in winter: larvae burrow into the gut wall and lie dormant — invisible, laying no eggs, completely undetectable by standard testing. In late winter or early spring, they can emerge in large numbers all at once. When that happens, animals can go downhill very fast.
• Tapeworm: Associated with a specific type of colic. They don't show up in a standard worm egg count, so they need their own test.
• Roundworm: Mainly a problem for foals. Adult horses develop natural immunity over time.
• Lungworm: Important if you have donkeys. Donkeys can carry lungworm without getting ill — but horses sharing their pasture can develop a persistent, serious cough. More on this in the donkey section.

Horses and donkeys grazing together on pasture — co-grazing is safe with the right worming programme

Why Testing First Makes Sense

A worm egg count (sometimes called a faecal worm egg count, or FWEC) is a simple test that counts the worm eggs in a fresh droppings sample. You collect a small amount, send it to a postal lab or use an at-home kit, and get a result back — usually within a day or two. That number tells you whether your horse is carrying enough worms to need treatment right now, or whether they're fine to leave alone.

Here's what changes things: in any group of horses or ponies, roughly 20% of the animals are carrying about 80% of the total worm burden. The rest are managing perfectly well. Testing shows you which is which — so you only treat the animals that actually need it, rather than everyone, every time.

Collecting a droppings sample for a worm egg count — a simple test that could save you money and slow resistance

How often should you test?

• Two to four times during the grazing season (roughly spring through autumn) covers most horses.
• Tapeworm needs a separate saliva test, done once or twice a year.
• There's no point testing in the middle of winter — the encysted larvae are dormant and won't show up in the results anyway.

What does it cost?

A postal worm egg count typically costs £5–£15. A wormer syringe costs £8–£20 or more. If testing shows that two-thirds of your horses don't need treating this month, you've already saved money — and you've helped slow down resistance at the same time.

The Two Treatments Every Horse Needs Each Year

Testing-led worming doesn't mean doing nothing. There are two times of year when treatment is recommended regardless of the worm count.

Winter — encysted redworm (October to December)

Because the dormant larvae hiding in the gut wall can't be detected by testing, this treatment is given based on the season rather than a test result. The product most vets recommend for this is moxidectin — it's the only single-dose treatment that reaches the larvae inside the gut lining. This is arguably the most important worming treatment of the year.

Autumn — tapeworm

Tapeworms can cause colic and don't show up on a standard worm count. A saliva test in autumn will tell you whether treatment is needed. If it is, it's usually given at the same time as the winter encysted redworm dose. Young horses, or those with a history of colic, may also need a tapeworm treatment in spring.

One thing to flag on moxidectin: it's stored in body fat, which means it shouldn't be used in very thin or underweight animals — they can be overdosed. If your horse or donkey is underweight, check with your vet before using it.

Donkeys Are Different — Here's What to Know

If you keep donkeys, or your horses share a field with donkeys, this section is for you. The approach is broadly similar to horses, but with a few important differences.

They hide it better

Donkeys are stoic animals. They can carry a significant worm burden with very few visible signs — a slight drop in condition, a small weight change, sometimes nothing obvious at all. This makes regular testing even more important for donkeys than for horses. By the time a donkey looks unwell, things are often already quite serious.

Different thresholds

The egg count that triggers treatment is generally set higher for healthy adult donkeys than it is for horses. Your vet or SQP will guide you on the right level for your animals.

Lungworm — and why donkeys and horses can still share a field

Donkeys are natural hosts for lungworm. They can carry it without showing any symptoms. Horses sharing the same pasture can develop a persistent, severe cough as a result. This sounds alarming — and plenty of people have been told donkeys and horses can't mix — but that's not the case. They can share grazing safely, provided the worming programme covers lungworm.

• Any donkey new to a property should be treated for lungworm immediately and kept off the pasture for 48 hours before turning out.
• Ivermectin and moxidectin are both effective against lungworm.
• Once you have a regular treatment programme in place, co-grazing is straightforward.

Products and dosing

Fewer wormers are licensed specifically for donkeys, so your vet may prescribe things slightly differently to horses. Getting an accurate weight before any treatment is especially important — donkey-specific weight tapes are available and worth having.

Don't Forget the Pasture

Wormers aren't the only tool available. Good pasture management reduces the number of worm eggs on the grass in the first place, which means less infection and less need for treatment.

Good pasture management — regular poo-picking breaks the worm lifecycle before it gets going

• Poo-pick two to three times a week. Worm eggs need time on the pasture to develop into infectious larvae. Remove the droppings regularly, and you break the cycle before it gets going.
• Rotate paddocks when you can. Resting a field gives any larvae left behind time to die off before animals graze it again.
• Cross-graze with sheep or cattle. They pick up equine larvae off the grass without being harmed by them, and horses won't pick up sheep or cattle parasites in return. It's a genuinely effective natural clean-up.
• Don't over-stock. Overcrowded pasture leads to more eggs, faster reinfection, and animals in poorer overall condition.

One thing worth knowing: some of the most commonly used wormers — particularly moxidectin and ivermectin — are highly toxic to dung beetles and harmful to waterways. Keeping treated animals away from streams and rivers for about a week after treatment is good practice.

A Simple Year-Round Plan

This is how a sensible, testing-led year looks for most adult horses and ponies.

Spring (March–May)

• First worm egg count of the year — treat only if the count is high.
• Consider a tapeworm saliva test if you didn't do one in autumn.

Summer (June–August)

• Second worm egg count — treat high shedders only.
• Keep on top of poo-picking; warm weather is when eggs develop fastest.

Autumn (September–November)

• Tapeworm saliva test — treat if positive.
• Plan the winter encysted redworm treatment.

Winter (December–February)

• Single moxidectin treatment — this one doesn't need a worm count first.
• No need to test mid-winter; dormant larvae won't show up in results.

For donkeys, the framework is the same, with the addition of ensuring lungworm is covered — particularly if co-grazing with horses — and checking product licensing with your vet.

Why the Handover Record Matters

When a horse or donkey moves to a new home, one of the first practical questions any owner needs to answer is: when was this animal last wormed, and with what?

Without that information, you're guessing. You either treat immediately — which might mean doubling up on a recent treatment, or using a product the worms on this animal are already resistant to — or you wait and test, which takes time and leaves a gap you can't see.

A good handover record for any horse or donkey should include:

• The date and name of the last wormer given.
• Any worm egg count results from the past 12 months.
• Whether a winter encysted redworm treatment was given this season.
• Whether lungworm has been treated — particularly relevant for donkeys.
• Any known resistance issues on the previous property.

The Bottom Line

Testing-led worming isn't more complicated than what most owners already do. It's just more considered. You test a couple of times during the grazing season, treat the animals that need it, do the winter treatment regardless, and keep a note of what you've done.

Test first. Treat when needed. Keep the record. That's it.

*Guudpet provides general guidance only. For specific worming advice for your animals, always consult a qualified veterinarian or SQP (Suitably Qualified Person).*