How often should you worm a horse in the UK?

There is no fixed interval any more. Under targeted worming you take faecal worm egg counts through the grazing season, from roughly early spring to late autumn, and treat only the horses shedding significant numbers of eggs — most mature horses turn out to be low shedders. Separately, most horses are treated for encysted small redworm in winter, and tapeworm is decided by a saliva or blood test rather than an egg count.

Does a worm egg count show encysted small redworm?

No. Encysted small redworm larvae are walled into the gut wall and do not lay eggs, so they are invisible to a faecal worm egg count. Tapeworm is also detected unreliably by a standard count and needs a saliva or blood test instead. A clean egg count therefore does not mean a horse is free of worms.

When should a horse be treated for encysted small redworm?

Typically in winter, with December or January cited as good strategic points, because the larvae are dormant in the cold and can be removed before they emerge. The treatments carrying the indication are a single dose of moxidectin or a five-day course of fenbendazole. Moxidectin should not be used more than once or twice a year, since over-exposure is how resistance to it developed.

Guide · UK

Horse worming schedule (UK): worm egg counts, tapeworm tests, and winter redworm

Last reviewed 4 August 2026

Interval worming — a tube every eight weeks, rotating the brand — is the strategy that produced the resistance we now have. Targeted worming replaced it. Here is how it works in the UK, and the two things a worm egg count will never show you.

This is a summary of published guidance, not veterinary advice. This page covers the UK. A worming programme depends on your grazing, your horse's age and history, and what resistance looks like on your yard — decide it with your vet or a qualified prescriber.

Resistance is the whole reason the advice changed

Small redworm resistance to fenbendazole and pyrantel has been documented in the scientific literature for around twenty-five years. More recently, resistance to moxidectin — the drug the industry has leaned on hardest — has been demonstrated in the UK. There is now resistance reported to every wormer class available for small redworm, and no new class is coming.

That is why blanket interval worming was abandoned. Treating horses that don't need it doesn't just waste a tube; it selects for resistant worms and burns through the drugs that still work. The modern approach is test, then treat what the test justifies, deliberately leaving low-shedding horses untreated so a population of unexposed worms survives to dilute the resistant ones.

Worm egg counts: what they sort

A faecal worm egg count (FWEC) counts redworm eggs per gram of droppings. It sorts horses by how much they contaminate the pasture, not by how ill they are. Counts through the grazing season — roughly early spring to late autumn — identify the minority of horses shedding most of the eggs, and those are the ones to treat.

Most mature horses turn out to be low shedders. Done properly, targeted worming usually means fewer tubes across a yard than the old calendar, not more.

The two things a worm egg count cannot see. First, encysted small redworm — larvae walled into the gut wall don't lay eggs, so they are invisible to a FWEC. Second, tapeworm, which a standard count detects unreliably; a saliva or blood test is used instead. A clean egg count is not a clean bill of health, and treating it as one is the single most common mistake in an otherwise well-run programme.

Encysted small redworm and the winter treatment

This is the parasite that kills. When large numbers of encysted larvae emerge from the gut wall at once, the result is larval cyathostominosis: rapid-onset watery diarrhoea, dramatic weight loss and colic, with mortality commonly cited around 50%. It arrives without warning in a horse whose egg counts looked fine.

Because the larvae are dormant through the cold months, treating in winter — December or January are the usual strategic points — removes the burden before it can emerge. Two treatments carry the indication: moxidectin as a single dose, or a five-day course of fenbendazole. Moxidectin is the more convenient and is generally considered the most effective single dose, but it should not be used more than once or twice a year: over-exposure is precisely how resistance to it developed.

How wormers are actually supplied here

UK equine wormers are classified POM-VPS — prescription-only medicines that may be prescribed by a vet, a pharmacist or a Suitably Qualified Person (SQP). A physical examination by the prescriber isn't required, which is why buying from an SQP at a merchant or online is routine and entirely legitimate.

That convenience is double-edged. It is easy to buy a tube without anyone asking what the last count said, which is how yards drift back into interval worming by habit. The prescriber can only advise on what you tell them — so what you can tell them is the constraint.

Why the plan drifts

A worming programme is harder to hold together than a vaccination schedule, because it isn't one date repeating. It is a count per horse through the season, a treatment only where the count justifies it, a tapeworm test on its own cycle, and a winter encysted treatment on another — times every horse on the yard.

Which is how yards quietly slide back to "everyone gets a tube at Christmas": not by decision, but because nobody could say who was counted when, or what they were last given.

Keep the counts where the horse is

EquiDiary keeps each horse's egg counts, tests and treatments in one history — product, date and all — so last season's numbers are there when you need to compare them. Log it once and the record does the remembering; your vet, your yard and your sharer all see the same page.

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Sources

Westgate Labs — Resistance to wormers: where are we now?
Westgate Labs — Small redworm and encysted stages
Redwings Horse Sanctuary — Worming advice
GOV.UK — Retail of veterinary medicines (POM-VPS supply and SQPs)
AAEP — Internal Parasite Control Guidelines (shedding thresholds and efficacy testing)

Guidance and product indications change; check the current advice and confirm the programme with your vet or prescriber.

In the US? The US deworming guide follows the AAEP guidelines and their shedding categories.

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See also: horse vaccination schedules — flu boosters and the rules by discipline.