How often should you deworm a horse?

It depends on the individual horse, which is the point of the modern approach. Under AAEP guidelines, most mature horses are low shedders (under 200 strongyle eggs per gram) and need only baseline treatment once or twice a year. Additional treatments target moderate shedders (200-500 epg) and especially high shedders (over 500 epg). Deworming every horse every eight weeks is the obsolete rotational approach that drove today's resistance.

What is a good fecal egg count for a horse?

Under 200 eggs per gram classifies a horse as a low shedder, 200 to 500 as moderate, and over 500 as high. But a fecal egg count sorts horses by how much they contaminate pasture, not by how sick they are — the AAEP notes there is no correlation between egg count and the disease-causing larval stages, so it should not be used to diagnose disease.

Does a fecal egg count detect encysted small strongyles?

No. Egg counts detect adult egg-laying worms, so encysted small strongyle larvae in the gut wall do not show up, and counts are unreliable for tapeworm and bots. A horse can return a zero count and still carry a heavy encysted burden, which is why treating for encysted larvae is a separate decision made with your vet rather than something read off the count.

Guide

Horse deworming schedule: fecal egg counts, thresholds, and what rotation got wrong

Last reviewed 4 August 2026

If you were taught to rotate dewormers every eight weeks, you were taught a strategy that is now understood to have created the problem it was meant to solve. Here is what replaced it, and the one thing a clean egg count still doesn't tell you.

This is a summary of published guidance, not veterinary advice. Parasite control depends on your region, your pasture, your horse's age and what resistance looks like on your farm. The thresholds below are the AAEP's; your vet decides the program.

Why the calendar approach was abandoned

Rotational deworming — treating every horse every two months, cycling through drug classes — made sense when it was devised in the 1960s, and it worked against the parasite it targeted. The problem is what it did over sixty years: treating every horse whether or not it needed it applied relentless selection pressure, and the worms adapted.

The result is documented resistance across every drug class available. Benzimidazole and pyrantel resistance in small strongyles is widespread, and resistance to the macrocyclic lactones — ivermectin and moxidectin, the drugs that still work best — is now being reported too. There are no new classes coming. What we have is what we get.

Hence the modern principle, which inverts the old one: test first, then treat the horses that need it. Leaving low-shedding horses untreated is not negligence — it deliberately preserves a population of worms that has never met the drug, which keeps the drug working for the horses that genuinely need it.

What a fecal egg count actually sorts

An FEC counts strongyle eggs per gram of manure. Its job is not to diagnose disease — it is to sort horses by how much they contaminate the pasture, because shedding is remarkably consistent and unequal: a minority of horses produce most of the eggs.

Category	Strongyle eggs per gram	What it means
Low shedder	under 200 epg	Most mature horses. Baseline treatment once or twice a year is generally all that is indicated.
Moderate shedder	200–500 epg	May warrant additional treatment during the year.
High shedder	over 500 epg	The target for further treatments — these horses seed the pasture.

Two practical points follow. First, most mature horses turn out to be low shedders, so a well-run program usually involves less deworming than the calendar demanded, not more. Second, the categories are stable enough to be worth knowing per horse — which is exactly the sort of thing that lives in a record rather than a memory.

Check that your dewormer still works

Resistance is farm-specific, so the only way to know whether a drug still works on your place is to measure it. A fecal egg count reduction test — count, treat, count again roughly two weeks later — shows whether the drug did its job. The AAEP recommends running one annually for every herd or barn.

This matters more than choosing a brand. A dewormer that has quietly stopped working on your farm looks exactly like one that works, right up until you count.

The thing a zero egg count does not mean. Fecal egg counts detect adult egg-laying strongyles. They do not detect encysted small strongyle larvae walled into the gut wall, and they are unreliable for tapeworm and bots. A horse can return a clean count and still carry a substantial encysted burden. When those larvae emerge en masse, the result is larval cyathostominosis — sudden diarrhea, dramatic weight loss and colic, with mortality reported around 50%. Treating for encysted larvae is a separate decision from the egg-count result, and one to make with your vet.

Foals are a different animal

Everything above concerns mature horses. Foals and young stock are treated on a schedule rather than by testing, because their parasite picture is different: Parascaris (roundworm) rather than small strongyles, with a genuine risk of impaction, and immunity that hasn't developed yet. Ascarid resistance to the macrocyclic lactones is now widespread, which makes an efficacy check in youngstock particularly worth doing.

Why the plan drifts

A parasite program is harder to keep straight than a vaccination schedule, because it isn't one date repeating. It is a count per horse, a category that follows from it, a treatment that may or may not be indicated, an efficacy check two weeks later, and a separate encysted-larvae decision on its own cycle — multiplied by every horse on the place.

Which is why it tends to collapse back into "everyone gets a tube in spring" — not because anyone decided that, but because nobody could reconstruct who was counted when.

Keep the counts where the horse is

EquiDiary keeps each horse's egg counts, treatments and products in one history, so last spring's number is there when you need to compare it to this spring's. Log the count, log the tube — with the product and date — and the record does the remembering. Your vet, your barn manager and you all see the same page.

Start free for 3 months

Sources

AAEP — Internal Parasite Control Guidelines (shedding categories, testing intervals, FECRT)
AAEP — Internal Parasite Control Guidelines, full document (2024 update)
AVMA — AAEP updates internal parasite control guidelines

Guidance is revised periodically; check the current edition before relying on it, and confirm the program with your vet.

In the UK? The UK worming guide covers SQP supply, the tapeworm saliva test and winter encysted treatment.

Also available in Español (España) · Español (México) · Deutsch · Français

See also: horse vaccination schedules — core vaccines and USEF show rules.