How often should you deworm a horse?

Most adult horses need one or two treatments a year. High shedders, identified by fecal egg counts, need more. Foals need a separate, more frequent program.

Is rotational deworming still recommended?

No. Rotating drug classes on a fixed schedule doesn't prevent resistance. Experts now recommend treating based on fecal egg counts and checking that your dewormers still work.

Do fecal egg counts detect tapeworms?

Not reliably. Tapeworm eggs show up inconsistently in routine counts. That's why many plans include praziquantel once or twice a year regardless of the count.

What is the best dewormer for horses?

There isn't one best drug. Ivermectin or moxidectin, often combined with praziquantel, are the backbone of most adult programs. Which one works best on your farm depends on resistance, which a fecal egg count reduction test can tell you.

What happens if you don't deworm a horse?

Heavy infections can cause weight loss, a dull coat, diarrhea and colic. A sudden emergence of dormant small strongyle larvae can cause severe illness. Most adult horses on a good plan need far less dewormer than they used to get, but not none.

Horse Deworming Schedule: Why Fecal Egg Counts Replaced Rotational Deworming

By Gayle Woods Editors Updated November 11, 2026 6 min read

A generation ago, the standard advice was to deworm every horse every six to eight weeks, rotating between drug types. It seemed sensible. It also helped breed parasites that many of those drugs no longer kill. Today, vets and parasite researchers recommend a different approach: test first, treat the horses that need it, and keep checking that your dewormers still work.

This guide explains why the advice changed and how to build a modern plan with your vet.

Why the old approach stopped working

Every time you deworm, the few parasites that happen to survive the drug go on to breed. Deworm often enough, and those survivors take over. That's drug resistance, and it's now widespread:

Small strongyles, the most important parasite of adult horses, are commonly resistant to the benzimidazole drugs, such as fenbendazole, and often to pyrantel.
Roundworms in foals are increasingly resistant to ivermectin and moxidectin.
No new classes of horse dewormers have reached the market in decades, and none are expected soon.

The goal now is to control parasites well enough to keep horses healthy, while preserving the drugs we have.

The parasites that matter

Parasite	Who it affects	Notes
Small strongyles (cyathostomins)	Adult horses	The main target. Larvae can hide in the gut wall in a dormant stage.
Tapeworms	Horses of all ages	Linked to some colic cases. Hard to detect with routine egg counts.
Roundworms (ascarids)	Foals and yearlings	Can cause impaction colic in foals. Horses become immune with age.
Large strongyles	All ages	Once a major killer, now uncommon on well-managed farms.
Bots	All ages	Fly larvae that live in the stomach. Treated in fall.
Pinworms	All ages	Cause tail rubbing. Mostly a nuisance.

Fecal egg counts, explained

A fecal egg count (FEC) counts how many parasite eggs are in a gram of manure. Your vet or a lab does it from a fresh sample, and it costs about as much as a tube of dewormer.

Researchers found that in most herds, roughly 20 percent of adult horses shed about 80 percent of the eggs that contaminate the pasture. Each horse tends to stay in the same group over time, so a few counts sort your horses into categories:

Low shedders: under about 200 eggs per gram. Most adult horses.
Moderate shedders: about 200 to 500 eggs per gram.
High shedders: over about 500 eggs per gram.

Low shedders still carry some worms, but their immune systems keep them in check. Treating them often adds little benefit and speeds up resistance.

To collect a sample, take a few fresh fecal balls, less than 12 hours old, put them in a sealed bag, squeeze out the air, and refrigerate them until you drop them off. Don't freeze them. Collect samples at least eight weeks after the last dewormer, or 12 to 16 weeks after moxidectin, so you measure the horse rather than the last treatment.

A basic plan for adult horses

This is a starting framework based on the AAEP's parasite control guidelines. Work out the details with your vet.

Every adult horse:

One treatment a year, usually in fall after the first frost, with ivermectin or moxidectin combined with praziquantel. This targets small strongyles, bots and tapeworms.
Many vets add a second treatment in spring for all horses.

Moderate shedders: one or two additional treatments in the grazing season.

High shedders: two or three additional treatments a year, timed to the parasites' active season, which is spring and fall in the North and fall through spring in the South.

Most horses end up with one to four treatments a year, and many with just one or two.

Check whether your dewormers still work

A fecal egg count reduction test tells you whether a drug is working on your farm. Take a sample right before deworming and another 10 to 14 days later. A working dewormer should cut the egg count by about 90 to 95 percent or more, depending on the drug. If it doesn't, that drug isn't reliable for your herd any more. Do this every few years for each drug you use.

Foals, yearlings and seniors

Foals and young horses need more frequent treatment, often starting at around two to three months old. Their main parasite, the roundworm, responds to different drugs than small strongyles, and many roundworm populations now resist ivermectin and moxidectin. Foals usually get four or more treatments in their first year. Your vet will plan this with fecal egg counts.

Senior horses, especially those with PPID, may become higher shedders as their immunity weakens. Keep checking their egg counts. See senior horse care.

New arrivals

A new horse can bring resistant parasites to your farm. Before turning it out with the herd:

Keep it separate for a few days.
Run a fecal egg count.
Deworm it, often with moxidectin plus praziquantel, and keep it off shared pasture for about three days while it passes the treated eggs.

Dosing correctly

Underdosing is one of the biggest drivers of resistance. Dewormer paste is dosed by body weight, and owners commonly guess low.

Weigh or tape the horse before each treatment. See how to measure a horse.
Set the syringe to the correct dose, rounding up rather than down.
Make sure it all goes in. Put the syringe tip deep into the corner of the mouth, onto the back of the tongue, and hold the horse's head up for a moment. Paste spat onto the ground is underdosing.

Pasture management cuts parasites too

Dewormer is only part of the picture. Most of a horse's parasite exposure comes from eggs and larvae on pasture.

Pick up manure from small paddocks at least twice a week.
Don't spread fresh manure on pastures horses are grazing. Compost it first; a hot pile kills eggs. See manure management.
Harrow pastures only in hot, dry weather, when spreading manure exposes eggs to sun and heat. Harrowing in cool, wet weather spreads larvae around.
Avoid overgrazing. Horses forced to graze close to manure piles pick up more larvae.
Graze with cattle, sheep or goats if you can. They eat horse parasite larvae without being infected.
Keep the number of horses per acre low. See how many acres a horse needs.

Sources

Frequently Asked Questions

: Most adult horses need one or two treatments a year. High shedders, identified by fecal egg counts, need more. Foals need a separate, more frequent program.
: No. Rotating drug classes on a fixed schedule doesn't prevent resistance. Experts now recommend treating based on fecal egg counts and checking that your dewormers still work.
: Not reliably. Tapeworm eggs show up inconsistently in routine counts. That's why many plans include praziquantel once or twice a year regardless of the count.
: There isn't one best drug. Ivermectin or moxidectin, often combined with praziquantel, are the backbone of most adult programs. Which one works best on your farm depends on resistance, which a fecal egg count reduction test can tell you.
: Heavy infections can cause weight loss, a dull coat, diarrhea and colic. A sudden emergence of dormant small strongyle larvae can cause severe illness. Most adult horses on a good plan need far less dewormer than they used to get, but not none.