In a study of 786 horses across 44 farms in the southern United States, fenbendazole failed on 97.7% of them. Average reduction in worm eggs: 24.8%.

That is the tube plenty of owners still reach for every spring.

The every-eight-weeks rotation most of us were taught was built in the 1970s to fight a worm that is now rare. It did not simply go out of fashion. It is a large part of why the drugs stopped working.

The short answer: a modern horse deworming schedule gives every adult horse one or two treatments a year as a baseline, then adds more only for the horses a fecal egg count proves are shedding heavily.

Most horses need two. Some need four.

Below you get the actual calendars for low, moderate, and high shedders, the two treatments no test will ever tell you to skip, and why the standard March and October plan is close to backwards if you live in the Deep South.

Horse Deworming Schedule: Key Takeaway 

Stop deworming every horse on the same fixed schedule. Test fecal egg counts twice a year, treat low, moderate, and high shedders according to their actual risk, and keep essential annual coverage. The goal is not more dewormer. It is smarter treatment that controls parasites without accelerating drug resistance.

Why the Every 8 Weeks Rotation Stopped Working

The old rotation was not stupid. It worked, and that is exactly why it stopped working.

In 1966, vets recommended treating horses every six to eight weeks. Paste dewormers arrived in the 1970s and replaced the stomach tube, putting deworming in owners’ hands for the first time.

The target was Strongylus vulgaris, the large strongyle. It migrated through the arteries feeding the gut, and it killed horses.

The plan worked. Large strongyles became rare in managed horses, and small strongyles took over as the main threat to adult horses.

Now the part most articles skip: how the schedule itself failed.

Treating every horse every eight weeks means nearly every generation of worms meets the drug. The only ones left to breed are the ones that survived it.

Rotating between drug classes does not fix that. It spreads the pressure across all three classes instead of concentrating it on one.

Which is why the American Association of Equine Practitioners now says to stop treating at fixed intervals year-round and stop blindly rotating classes.

And no replacement drug is coming. Moxidectin, released in 1997, is still the newest broad-spectrum dewormer on the US market, and new drugs have historically taken five to 25 years to lose ground to resistance.

The answer is not a different deworming schedule. It is treating fewer horses, less often, and testing to find out which ones.

The 20/80 Rule: Most Horses Barely Contaminate the Pasture

About 20% of horses shed roughly 80% of the strongyle eggs on a farm.

The exact figure moves by study. An Italian survey found 12.8% of positive horses producing 80% of the eggs, a Hungarian study found about 22%, and a Canadian study averaged 32%. The principle holds every time.

Worm eggs are counted per gram of manure, written as EPG. That one number sorts your horse into a group.

• Low shedder: under 200 EPG
• Moderate shedder: 200 to 500 EPG
• High shedder: over 500 EPG

Before you even test, the odds favor you. Across studies, low shedders make up 50% to 75% of a typical herd, moderate shedders 10% to 20%, and high shedders 15% to 30%.

Read that again if you are currently deworming everything six times a year. Most of those horses were never the problem.

One thing almost nobody explains: shedding level is a trait of the individual horse, driven by its own immunity. It is not a snapshot of how dirty the paddock is.

That stability is why two counts a year is enough. You are measuring a characteristic, not chasing a moving number.

It does drift with age and illness, which the special cases section covers.

And there is a practical payoff. The high shedder in your herd is seeding the pasture your low shedder grazes, so aiming treatment at that one horse does more for everyone than treating all of them equally.

Six blanket treatments spread thin across every horse, or four aimed at the one that matters. The second costs less and works better.

Refugia: Why Leaving Some Worms Alive Protects Your Horse

The worms you deliberately do not kill are the ones keeping your dewormer working.

That idea is called refugia, and it is the piece missing from nearly every deworming guide online.

Refugia means the share of the worm population that never meets the drug. Larvae sitting out on pasture. Worms inside horses you chose not to treat.

Those worms are never tested against the chemical, so they keep their ordinary, drug-susceptible genes. Then they breed with the survivors and dilute resistance back down.

The AAEP puts it plainly: the higher the proportion of worms in refugia, the more slowly resistance develops. It also suggests leaving some horses untreated at every deworming.

Now flip it to see the failure mode.

Deworm every horse during a hard freeze, or in the middle of a hot dry summer, and almost nothing is alive out on the pasture. Nearly the entire worm population on your farm is inside the horses you just treated.

Every survivor is resistant, and it has nothing to breed with except other survivors. That is the fastest possible route to a useless drug.

So timing matters as much as frequency.

One honest caveat, because this can be pushed too far. On 42 Danish farms, Strongylus vulgaris turned up on 83.3% of farms using selective treatment, against 38.9% of farms that were not, and occurrence was linked to the last deworming being more than six months earlier.

Targeted does not mean optional. That is why the baseline one or two treatments a year stay in the plan even for a horse that tests at zero.

Start Here: The Fecal Egg Count

Two manure balls and about $25 replaces the guesswork.

Cost is the objection that stops most owners, so let’s deal with it first. Mail-in kits from Horsemen’s Laboratory run $28 for a single kit, $25 each for two to five, and $23 each for six or more. That covers postage both ways, the collection kit, lab processing and emailed results.

Compare that to $10 to $20 for every tube of dewormer.

A low shedder dropping from six blanket treatments a year to two saves the cost of the test in year one, and every year after.

How to collect a sample that is actually usable

The test is only as good as what goes in the bag.

• Take one or two fresh manure balls, ideally passed within the last 12 hours
• Seal them in a plastic bag and squeeze the air out
• Label it with the horse’s name and the date
• Refrigerate right away, and never freeze it, because freezing ruptures the eggs and gives a falsely low count
• Get it to your vet or into the mail within 48 hours, though some labs allow up to seven days
• Skip diarrhea, which is not a usable sample

When to test

Test before you deworm, not after.

And only once enough time has passed since the last treatment for eggs to reappear. Test too soon and you get a low number that files a high shedder in the wrong box.

Ask your vet how long to wait based on the product you last used. This is the trap that quietly ruins a first attempt.

What the number can and cannot tell you

The modified McMaster test uses four grams of manure and detects down to 25 eggs per gram. Counting done too quickly cuts accuracy by 50% to 60%, so lab quality is not a small detail.

A zero is not a clean bill of health. It means no detectable strongyle eggs, which is a much narrower claim than “no worms.”

The next section covers exactly what it is blind to.

Run counts at least twice a year on adult horses, spring and fall.

Order a kit now, pull the sample before your spring treatment, then use the tables below.

The Actual Schedules for Low, Moderate, and High Shedders

Find your horse’s egg count, then use the matching table. Each deworming schedule below follows the one published by Colorado State University’s equine field service and edited by parasitologist Dr. Martin Nielsen.

Low shedder: under 200 EPG

Two treatments a year.

Run a fecal egg count before deworming in spring, and ideally in fall too.

Moderate shedder: 200 to 500 EPG

Three treatments a year.

High shedder: over 500 EPG

Four treatments a year.

Run counts before deworming in both spring and fall on these horses. Resistance shows up here first, so high shedders are your early warning system.

Reading the calendar properly

Those months assume a northern, temperate climate. If you are in the Gulf states or the Deep South, keep reading, because the regional section moves them.

The shape is what matters. Spring and fall bracket the transmission season, and the extra treatments for moderate and high shedders slot into the middle of it.

If you have never run a count

You are not stuck waiting.

Treat once with a macrocyclic lactone plus praziquantel, which covers the baseline every adult horse needs. Then wait out the egg reappearance period and test before the next treatment.

One note on the ceiling. Four treatments is the standard for a high shedder, not a hard cap, and Merck puts some high shedders at up to six a year.

These tables fit a healthy adult horse on ordinary pasture. If your horse is under three, pregnant, elderly or brand new to the farm, the special cases section overrides them.

The Two Treatments No Test Will Tell You to Skip

A clean fecal egg count is the most dangerous result on the page.

Not because it is wrong, but because of what the test cannot see.

Encysted small strongyles

Small strongyle larvae burrow into the wall of the large intestine and go dormant. They can sit in those nodules for as long as two years, producing no eggs at all.

No eggs means no detection. Your horse can test at zero while carrying thousands of them.

The danger is what happens when large numbers emerge at once. That is larval cyathostominosis: diarrhea, swelling along the belly, fever, colic and fast weight loss.

Mortality runs 50% to 60%. In a 2023 review of 38 hospitalized horses, 21 of them died, on average within 2.9 days.

It hits horses aged one to four hardest, though any age can get it.

Moxidectin is the drug of choice for encysted larvae, given at or near the end of the grazing season when the burden in the gut wall peaks.

That single fact is the entire reason the fall treatment exists.

Tapeworms

Tapeworms attach where the small intestine meets the cecum, and they are associated with colic.

Routine fecal exams detect them poorly.

Praziquantel is the only dedicated tapeworm drug in the US, and it sells only in combination with ivermectin (Equimax, Zimectrin Gold) or moxidectin (Quest Plus). That is why the fall treatment is a combination product.

Once a year is standard.

There is a test if you would rather not treat blind. The saliva ELISA measures antibodies instead of eggs, with 83% sensitivity and 85% specificity, close to the blood test, and you collect the swab yourself.

One honest note: 2023 research found efficacy failures for both praziquantel and pyrantel, the only two licensed tapeworm treatments. If you use the saliva test, retest after treating.

The verdict is simple. One macrocyclic lactone plus praziquantel every fall, count or no count.

Which Dewormer Does What, and Which Ones Stopped Working

Against small strongyles on 44 southern US farms, using the strict standard of under 80% egg reduction, resistance broke down like this:

• Fenbendazole: resistant on 97.7% of farms
• Oxibendazole: 53.5%
• Pyrantel pamoate: 40.5%
• Ivermectin: 0%

Average reductions tell the same story. Fenbendazole managed 24.8%, oxibendazole 73.8%, pyrantel 78.6%, ivermectin 99.9%.

One of those tubes is doing about a quarter of the job you paid for.

Sources: Penn State Extension and SmartPak’s efficacy matrix.

Which parasite matters depends on the horse. Small strongyles are the main threat to adults, ascarids (roundworms) are a foal problem, large strongyles are now rare, and bots and pinworms are mostly a nuisance.

The ascarid inversion

This is the part that trips people up.

Ivermectin and moxidectin have widespread resistance in ascarids while still working well on strongyles. The benzimidazoles are the exact reverse.

That is why foals get different drugs than adults. It is not about being gentle with a young gut. It is about which drug still kills the worm they actually have.

Egg reappearance period, your early warning

Egg reappearance period, or ERP, is how long after treatment eggs stay absent from the manure.

Ivermectin’s ERP was originally 8 to 10 weeks and moxidectin’s was 12 to 16. Recent studies now put both at around 4 to 5 weeks.

The drug still kills the adult worms. But egg output rebounds in half the time, which shortens your protection between treatments and is widely read as the first stage of resistance.

If you are still buying fenbendazole to treat strongyles in an adult horse, stop. And run a reduction test before you trust any product you have used for years.

Your Region Changes the Calendar

If you are on the Gulf Coast, the calendar above points at the wrong months.

The mechanism is simple, and it happens out on the pasture rather than inside the horse. Strongyle eggs die at sustained temperatures above 104F, and winter freezing halts larval development.

Weather decides when your pasture is genuinely dangerous.

In a northern, temperate climate, hard winter kills off pasture larvae. Transmission runs spring through fall, so you treat at the two boundaries, and the larvicidal dose lands at the end of the grazing season in autumn.

In the South it inverts.

Infection risk is lowest during the hottest weeks of summer, because heat sterilizes the pasture, while mild winters let transmission carry on.

The larvicidal treatment still belongs at the end of the grazing season. In a subtropical climate, that season ends in spring rather than fall.

Drought does the same job as heat, suppressing small strongyles on pasture.

The rule to carry away: do not treat when the weather has already suppressed transmission. Merck’s guidance is to avoid deworming during cold winters or hot dry seasons.

That ties straight back to refugia. Treating when pasture larvae are already dead is the worst possible moment, because the entire surviving worm population is inside the horses you just dosed.

Work out when your region’s transmission season runs, then place the treatments at its edges. The egg count tells you how many. The climate tells you when.

Horses That Need a Different Schedule

The shedder tables assume a healthy adult with a known history. Five common situations break that assumption.

Foals, birth to 12 months. Different worm, different drugs. Foals fight ascarids rather than strongyles, and their immune systems are still building.

At 2 months, fenbendazole or oxibendazole. At 4 to 5 months, run a count to tell ascarids from strongyles, then treat ascarids with fenbendazole or oxibendazole. Around 5 months, ivermectin for strongyles. Before the year ends, ivermectin plus praziquantel.

Those drug choices trace straight back to the ascarid inversion above.

Yearlings and two-year-olds. Penn State recommends treating them as high shedders by default, three to four times a year, whatever the count says.

Roughly three ivermectin treatments for strongyles, plus one moxidectin with praziquantel by the end of the grazing season. Their immunity is still under construction.

Pregnant and lactating mares. Most dewormers are labeled safe in pregnancy, but check the specific label rather than assuming.

The reason to treat is not only the mare. Keeping her egg output down limits what the foal picks up off the ground. And do not run mares and foals on the same paddocks year after year.

Seniors and horses with PPID (Cushing’s). PPID weakens the immune response to parasites, so these horses start shedding again sooner after treatment.

A third of PPID horses passed 200 EPG by day 42 after deworming, and half by day 56.

Once a year, check counts at 45 and 90 days post-treatment. If actionable counts appear at intervals shorter than six months, treat at that interval all year. A horse that used to be a low shedder can stop being one.

New arrivals. Buying a horse is the standard way resistant worms walk onto a clean farm.

Quarantine for 7 to 30 days depending on risk, commonly 21. Give a moxidectin-based treatment on arrival and hold the horse in a dedicated quarantine paddock for at least 72 hours before it touches your pasture, so resistant eggs are not deposited on clean ground.

Run a count at about 10 weeks to establish shedder status. Do not share equipment, and see to your resident horses before the new one.

If your horse fits none of these five, the tables above stand.

The Mistakes That Undo a Good Schedule

Owners underestimate their horse’s bodyweight by around 20% on average.

That habit does more damage to your dewormer than any calendar mistake.

Underdosing

Weight tapes systematically read low, more so on heavier horses, and the reading shifts with breed, bone density and fat cover.

Stack tape error on owner error and an 1,150 lb horse routinely gets a 900 lb dose.

Worms that survive a sub-lethal dose are precisely the ones carrying resistance genes, and they go on to breed.

Use a livestock scale if you can reach one. Otherwise read a weight tape the same way every time and round up to the label’s increments, never down. Never guess by eye.

Moxidectin specifically

With moxidectin, dosing accuracy stops being about resistance and becomes about safety.

Overdose happens at three times the label dose, with nervous system signs within 8 to 12 hours. It is not labeled for foals under 4 months, where coma has been reported, and not recommended for debilitated or miniature horses.

Dosed correctly in a healthy adult it is safe, and it is the encysted larvae drug. Do not let nerves push you off the fall treatment.

Losing the dose

Check the mouth is empty of hay or feed first.

Insert the syringe at a 45 degree angle into the toothless corner of the mouth. Wiggle it as you press so the paste spreads across the back of the tongue instead of sitting in one lump they can spit out.

Release small amounts at a time, letting the horse swallow between each, then lift the head briefly.

If they spit part of it out, do not guess at a top-up. Call your vet or follow the label.

Skipping the pasture work

Chemicals are only half the program.

• Pick out paddocks often and remove manure
• Avoid overstocking and overgrazing
• Rotate pastures with real rest periods
• Co-graze cattle, sheep, goats or camelids, which eat around manure and do not carry equine strongyles
• Harrow only in hot dry weather, then keep horses off for two months
• Compost manure properly and never spread it uncomposted

Every larva your horse does not swallow is a treatment you never have to give.

Herbal dewormers

Herbal and natural dewormers are not classified as drugs, so they never had to prove they work or are safe.

Using one instead of an anthelmintic leaves the horse untreated.

A mediocre schedule executed precisely beats a perfect schedule executed by eye.

FAQs

How often should I deworm my horse?

Every adult horse needs one to two treatments a year as a baseline. A fecal egg count decides whether it needs more: two a year for low shedders (under 200 EPG), three for moderate shedders (200 to 500 EPG), and four for high shedders (over 500 EPG). The every-eight-weeks rotation is no longer recommended.

Find out more on our other blog: How Often do You Deworm a Horse? 

My horse’s fecal egg count came back zero. Do I still deworm?

Yes. A zero means no detectable strongyle eggs, not a worm-free horse. Encysted small strongyle larvae produce no eggs for up to two years, and tapeworms are poorly detected on fecal exams. Every adult horse still needs at least one annual treatment with a macrocyclic lactone plus praziquantel.

How much does a fecal egg count cost?

Mail-in kits cost roughly $23 to $28 per test, including postage both ways, the collection kit, lab processing and emailed results. Multi-horse discounts drop the price further. Against $10 to $20 per tube of dewormer, the test usually pays for itself in the first year by cutting unnecessary treatments.

What does it mean if my horse’s egg count only dropped 50% after deworming?

That points to resistance or underdosing. Acceptable reductions are 85% for pyrantel, 90% for fenbendazole and oxibendazole, and 95% for ivermectin and moxidectin. Check the dose against your horse’s actual weight before blaming the drug, because guessed bodyweight is the more common cause.

Do I still need to rotate horse dewormers?

No. The AAEP advises against blindly rotating anthelmintic classes, because rotation spreads selection pressure across all of them instead of preserving any. Use the drug that a fecal egg count reduction test proves still works on your farm.

What if my whole barn deworms on the same schedule?

Pasture contamination is a herd-level problem, so your individual plan is capped by what everyone else’s horses shed onto shared ground. Bring your fecal egg count results and a written plan to the barn manager. Herd-wide testing usually reduces total drug spend, which is generally the argument that lands. Test twice a year. Treat every horse once or twice with a macrocyclic lactone plus praziquantel, add treatments only for the horses whose counts earn them, and dose to real weight instead of a guess. Then build the specifics with a vet who knows your region and has walked your pasture. They are the one who can tell you when your transmission season actually starts.

Horse Deworming Schedule: Conclusion

The old deworming calendar is not protecting horses the way owners once thought. Modern parasite control starts with testing, treating according to shedding level, and timing treatments around your local climate. 

Use the drugs that still work, dose for your horse’s real weight, and involve your vet. Fewer, smarter treatments protect both your horse and future dewormer effectiveness.