Dewormer Resistance in Horses: Fecal Testing, Smart Deworming and Prevention

By Dr Duncan Houston

Deworming horses is no longer about giving a tube every few months and hoping for the best.

For decades, many horse owners were told to deworm every 6 to 8 weeks and rotate between products. At the time, that advice seemed sensible. The problem is that frequent blanket deworming placed enormous pressure on parasite populations. Susceptible worms were killed. Resistant worms survived. Then those resistant worms reproduced.

Now we have a different problem.

In many horse populations, some dewormers no longer work reliably against the parasites they used to control. That does not mean deworming is useless. It means the old routine is outdated.

Modern parasite control is smarter, more targeted and more veterinary-led. It uses fecal egg counts, fecal egg count reduction testing, pasture management, age-specific protocols and the right drug at the right time.

The goal is not to eliminate every parasite from every horse. That is not realistic, and trying to do it makes resistance worse. The goal is to keep horses healthy while preserving the dewormers we still have.

Quick Answer

Dewormer resistance means parasites survive a treatment that should normally kill them. In horses, resistance is now widespread in several important parasites, especially small strongyles against benzimidazoles and pyrantel-type drugs, while reduced performance concerns are also emerging with other drug classes and parasite groups. Modern parasite control should use fecal egg counts once or twice yearly, annual fecal egg count reduction testing, baseline treatments for all horses where appropriate, and extra treatments only for horses that need them.

The old strategy of deworming every horse every 6 to 8 weeks and blindly rotating products is no longer recommended. AAEP specifically recommends discontinuing fixed-interval year-round deworming and abandoning blind rotation.

What Is Dewormer Resistance?

Dewormer resistance, also called anthelmintic resistance, occurs when parasites survive treatment with a drug that should normally be effective.

The surviving parasites then pass resistance traits to the next generation. Over time, the parasite population on that property becomes harder to control.

This is not just a theoretical issue. Merck Veterinary Manual states that equine parasites have developed resistance to all anthelmintic classes available for horses, and that no single product should be assumed to work broadly across all important equine parasites without routine efficacy testing. (Merck Veterinary Manual)

In practice, this means the label on the tube is not enough.

You need to know:

• Which parasite you are targeting
• Whether that horse actually needs treatment
• Whether that product still works on your property
• Whether the timing makes sense for your climate and pasture system
• Whether the horse is an adult, foal, yearling, senior or high-risk patient

The wormer aisle makes it look simple.

The worms have made it irritatingly complicated.

Why the Old Rotational Deworming Plan Failed

The old plan was usually something like this:

Deworm every 6 to 8 weeks.
Rotate drug classes.
Treat every horse the same.
Repeat forever.

That approach is now considered outdated.

AAEP’s updated parasite control guidance recommends annual fecal egg count reduction testing, fecal egg counts once or twice per year, baseline deworming once or twice per year where appropriate, targeting additional treatment to high strongyle shedders, and stopping fixed-interval year-round deworming and blind rotation.

The problem with old rotational programs is that they often treated horses that did not need treatment, at times when treatment was not useful, with products that may not have been effective.

That creates selection pressure.

Selection pressure means you are repeatedly killing susceptible worms while leaving resistant worms behind. Over time, you can accidentally train the parasite population to survive your program. Rude, but effective.

The Main Parasites That Matter in Horses

A good parasite plan starts with the parasite, not the product.

The main internal parasites of concern include:

Parasite group	Most important in	Why it matters
Small strongyles, also called cyathostomins	Adult horses and young horses	Very common, resistance is a major issue
Large strongyles	Managed horse populations, especially if control decreases too much	Historically dangerous, now less common in many managed groups
Ascarids, also called Parascaris species	Foals, weanlings and yearlings	Can cause poor growth, coughing, colic and intestinal impaction
Tapeworms	Grazing horses	Associated with ileocecal colic and impaction risk
Pinworms	Young and adult horses	Tail rubbing, irritation, difficult treatment response
Bots	Many horses	Usually mild pathogens, often managed seasonally

Merck lists tapeworms, strongyles and ascarids as the primary intestinal parasite targets in horses, and notes that parasite control should be handled at the population level rather than as random treatment of individual horses. (Merck Veterinary Manual)

This matters because adult horses and foals are not the same parasite problem.

A mature gelding with low strongyle egg shedding should not be managed like a 5-month-old foal at risk of ascarid impaction.

Which Dewormers Are Affected by Resistance?

The short answer: all major classes deserve monitoring.

AAEP’s updated guideline summarises documented resistance patterns across managed horse herds. Cyathostomins show widespread resistance to benzimidazoles and pyrimidines. Parascaris species show widespread resistance to macrocyclic lactones. Pinworm resistance to macrocyclic lactones has also been described, and early indications of tapeworm treatment failure have been reported for praziquantel and pyrantel pamoate.

Drug class	Common examples	Resistance concern
Benzimidazoles	Fenbendazole, oxibendazole	Widespread small strongyle resistance
Pyrimidines	Pyrantel pamoate, pyrantel tartrate	Widespread small strongyle resistance
Macrocyclic lactones	Ivermectin, moxidectin	Still important for adult strongyle control, but egg reappearance periods have shortened; ascarid resistance is widespread
Isoquinolines	Praziquantel	Tapeworm-specific activity, with early treatment failure concerns reported in some areas

This is why “just use ivermectin” or “just use moxidectin” is too simplistic.

Macrocyclic lactones remain very important for many adult strongyle programs, but ascarids in foals are often resistant to them. On the other hand, benzimidazoles may be useful in foal ascarid programs, while being unreliable for adult small strongyles unless testing proves otherwise. AAEP specifically states that benzimidazole and pyrantel resistance for strongyles should be assumed until proven otherwise, while macrocyclic lactone resistance should be assumed for ascarids until proven otherwise.

Why Ivermectin and Moxidectin Are Not a Free Pass

Ivermectin and moxidectin are still valuable drugs in adult horse parasite control, but their performance needs monitoring.

AAEP notes that the strongyle egg reappearance period, or ERP, has decreased dramatically for ivermectin and moxidectin over the past three decades. In the 1990s, ivermectin commonly suppressed strongyle egg shedding for around 8 to 10 weeks and moxidectin for around 12 to 16 weeks. Since 2017, several studies have reported egg reappearance periods around 4 to 5 weeks for both drugs.

That does not always mean classic resistance in the same way as a failed fecal egg count reduction test. But it does mean egg shedding can return faster, pasture contamination can increase sooner, and old timing assumptions may no longer hold.

The practical takeaway:

Do not panic.
Do not throw away the drugs.
Do not use them blindly.
Test whether your program is working.

Why Fecal Egg Counts Matter

A fecal egg count, or FEC, measures parasite eggs in manure.

In adult horses, FECs are mainly used to identify whether a horse is a low, moderate or high strongyle egg shedder. This helps decide which horses are contributing most to pasture contamination and which horses may need extra treatment.

AAEP recommends using fecal egg counts once or twice yearly to stratify horses into low, medium and high shedders, and Merck notes that FECs help identify persistent strongyle shedders and monitor ascarid infections in young horses.

In practical terms:

• Low shedders may need fewer additional treatments
• High shedders may need targeted additional treatments
• Foals and weanlings need FECs interpreted differently from adults
• A herd-level pattern matters more than one isolated number

FECs are one of the strongest tools we have, but they are not magic.

What Fecal Egg Counts Cannot Tell You

This is where many owners get caught.

A fecal egg count does not tell you the total number of worms inside the horse. It does not reliably detect encysted small strongyle larvae. It does not reliably detect tapeworms with standard methods. It does not detect pinworms well. It does not prove that parasites are causing a sick horse’s signs.

AAEP is very clear that fecal egg count magnitude does not correlate with parasite burden, cannot be used to evaluate whether parasites are causing clinical disease, and does not indicate risk of disease or adverse reaction to treatment.

That means:

A high FEC does not automatically mean the horse is sick because of worms.
A low FEC does not mean the horse has no parasite risk.
A normal-looking horse can still be a high shedder.
A sick horse needs diagnosis, not just a wormer.

In practice, FECs are best used for surveillance and treatment planning, not as the sole explanation for weight loss, diarrhea, colic or poor performance.

What Is a Fecal Egg Count Reduction Test?

A fecal egg count reduction test, or FECRT, checks whether a dewormer actually worked.

The basic process is:

1. Collect fecal samples before treatment

2. Perform fecal egg counts

3. Treat with the selected dewormer at the labelled dose

4. Repeat fecal egg counts about 14 days later

5. Compare the reduction in egg counts across the tested group

AAEP describes FECRT as the method used to evaluate treatment efficacy, with fecal samples collected before treatment and again 14 days after treatment. Merck also states that parasite egg counts remaining high after use of a dewormer indicate possible resistance and a need to change the control program.

A simple way to explain it:

FEC tells you who is shedding eggs.
FECRT tells you whether the drug worked.

That second part is essential. A horse being “dewormed” does not mean the parasites were actually controlled.

Dewormer Resistance Risk Framework

Risk level	What it looks like	What it may mean	What to do
Low risk	Adult horse with low FEC, no clinical signs, good weight, consistent parasite program	Routine surveillance is working	Continue FEC monitoring and baseline treatments with your vet
Moderate risk	Adult horse with repeated moderate to high FECs, no FECRT history, frequent pasture exposure	High shedding or unknown drug efficacy	Run FECRT and adjust treatment timing
High risk	Horses treated repeatedly but egg counts remain high, multiple high shedders, youngstock on same property	Resistance or poor program design possible	Vet-led herd parasite plan and drug efficacy testing
Critical	Foal with colic and suspected ascarid burden, horse with severe diarrhea, weight loss, ventral edema, dehydration or colic	Parasite-associated disease or another serious illness possible	Urgent veterinary assessment, not blind deworming

The key decision point is this: resistance management is usually a herd-level problem, but a sick horse is an individual medical case.

Do not confuse the two.

When Are Parasites an Emergency?

Most parasite control is planned, not urgent. But some situations require immediate veterinary attention.

Call your veterinarian urgently if your horse has:

• Moderate to severe colic
• Repeated rolling, pawing or flank watching
• No manure or very reduced manure
• Severe diarrhea
• Fever with diarrhea
• Depression or weakness
• Rapid weight loss
• Dehydration
• Swelling under the belly
• Poor growth in a foal
• Pot belly, coughing or colic in a foal
• Colic shortly after deworming
• A young horse suspected of heavy ascarid burden

AAEP lists larval cyathostominosis as a pattern involving diarrhea or loose feces, hypoalbuminemia, neutrophilia, weight loss, dehydration, ventral edema and thickened large intestinal walls on ultrasound. It also lists ascarid impaction as a colic presentation in foals, weanlings and yearlings with evidence of a large ascarid burden.

That is the important clinical difference.

A high egg count in a bright adult horse is a parasite control problem.

A foal with colic and suspected ascarid burden is a medical emergency.

The Modern Adult Horse Deworming Strategy

For mature adult horses, modern parasite control is built around two tiers.

AAEP describes a two-tiered strategy: baseline treatments considered for all horses, plus surveillance-based treatments based on fecal egg counts.

For mature horses, AAEP recommends:

• Annual FECRT to check product efficacy
• One or two baseline macrocyclic lactone treatments per year where appropriate
• Praziquantel included at least once yearly for tapeworms if horses have access to green pasture
• Additional treatment targeted to high strongyle shedders over 500 EPG
• Timing treatments during the active strongyle transmission season where pasture contamination matters most

That is very different from “every horse gets a tube every 8 weeks.”

The modern strategy asks:

Which horses are high shedders?
Which drugs still work here?
When is parasite transmission actually happening?
Which parasites are we targeting with each treatment?

That is proper parasite control.

What About Foals, Weanlings and Yearlings?

Young horses need a different plan.

AAEP states that the primary parasite targets in foals, weanlings, yearlings and youngsters appear in order as ascarids, strongyles and tapeworms. Targeted selective treatment based on FEC is not recommended in this age group, although FECs are still important for monitoring which parasites are present.

For foals, AAEP recommends treating twice for ascarids: first around 2 to 3 months of age and again around 5 months of age. Benzimidazoles are recommended for these treatments, although pyrantel pamoate may also be effective, and fenbendazole or oxibendazole are labelled at 10 mg/kg orally once for ascarids.

This is where generic deworming advice becomes risky.

Foals can develop heavy ascarid burdens. If a large number of worms die or move at once, intestinal impaction can occur. AAEP notes that deworming a heavily parasitized foal with an effective anthelmintic may trigger impaction of dead or dying worms in the intestine.

A thin, pot-bellied, coughing or colicky foal should not be casually dosed from the feed store. That foal needs a veterinarian.

What About Senior Horses?

Senior horses generally follow mature horse recommendations, but they deserve closer monitoring.

AAEP notes that some senior horses can revert to being high strongyle shedders, and PPID has been associated with higher egg counts.

This matters because many senior horses already have competing risks:

• Dental disease
• Weight loss
• PPID
• Reduced immunity
• Poor body condition
• Reduced pasture competition
• Chronic illness

Do not assume a senior horse’s weight loss is worms. Also do not assume seniors are low risk because they have been on the same farm for years.

Check. Interpret. Then treat.

What Else Can Look Like a Parasite Problem?

This is important because owners often blame worms when a horse is thin, dull, rough-coated or colicky.

Parasites can cause disease, but they are not the only possibility.

Important rule-outs include:

• Dental disease
• Poor quality forage
• Inadequate calories or protein
• Gastric ulcers
• Sand accumulation
• Chronic colitis
• Liver disease
• Kidney disease
• PPID in older horses
• Equine metabolic disease
• Inflammatory bowel disease
• Cancer in older horses
• Chronic pain
• Social bullying around feed
• Lawsonia in young horses
• Salmonella or other infectious diarrhea
• Tapeworm-associated colic
• Right dorsal colitis from NSAIDs

Merck specifically warns that fecal egg counts are not clinical diagnostic tools and that a positive FEC does not prove parasites are causing observed disease signs. (Merck Veterinary Manual)

The clinical rule is simple:

Use fecal testing to guide parasite control.
Use a veterinary workup to diagnose a sick horse.

Those are not the same job.

What Should You Do Right Now?

1. Stop deworming blindly

If the current plan is “every horse gets wormed every 8 weeks,” it needs updating.

AAEP recommends discontinuing fixed-interval year-round deworming and stopping blind rotation.

2. Run fecal egg counts

For adult horses, run fecal egg counts once or twice yearly to identify shedding category.

Useful information includes:

• Horse age
• Pasture group
• Date of last deworming
• Product used
• Dose given
• Weight estimate
• FEC result in eggs per gram
• Clinical signs, if any

3. Perform FECRT annually

A farm can only know whether products still work by testing. AAEP recommends annual FECRT to ensure effective dewormers are being used in every herd or barn.

4. Build a parasite plan by age group

Do not put foals, yearlings, mature adults and seniors on the same program.

A 4-month-old foal is not a 12-year-old gelding in a smaller rug.

5. Treat high shedders strategically

High adult strongyle shedders over 500 EPG are usually the horses that need additional targeted treatment beyond baseline programs. AAEP’s mature horse recommendations focus additional treatments on high strongyle shedders over 500 EPG.

6. Dose accurately

Underdosing can contribute to treatment failure.

Use a scale where possible or a weight tape if a scale is not available. Then dose for the correct weight using the correct product and route.

Do not dose by optimism. Horses are professional weight deceivers.

7. Manage manure and pasture

Drug use is only one part of parasite control.

AAEP explains that strongyle eggs begin in manure piles and must develop into infective larvae before being eaten by horses. Prompt feces removal could prevent infection, and proper composting can kill strongyle larvae when manure reaches over 104°F or 40°C for at least one week. Non-composted horse manure should not be spread on horse pastures because it increases parasite contamination.

Practical steps include:

• Remove manure from small paddocks regularly
• Avoid overstocking
• Avoid feeding directly on heavily contaminated ground
• Compost manure properly before pasture use
• Rest or rotate pastures where climate and land allow
• Separate youngstock management from adult pasture plans
• Avoid creating muddy, manure-heavy sacrifice areas without management

Common Mistakes With Dewormer Resistance

Mistake 1: Rotating products blindly

Rotation sounds clever, but blind rotation is outdated. If a product class is ineffective on your property, rotating back to it just gives resistant worms a free lap.

Mistake 2: Treating low shedders too often

Many adult horses shed low numbers of strongyle eggs. Treating them too frequently increases selection pressure without much benefit to herd parasite control.

Mistake 3: Assuming all horses need the same plan

Foals, yearlings, adults and seniors need different parasite strategies.

Mistake 4: Using FECs to diagnose disease

FECs are surveillance tools. They do not prove that worms are causing weight loss, diarrhea, colic or poor performance. (Merck Veterinary Manual)

Mistake 5: Not checking whether treatment worked

If you never do FECRT, you do not know if your dewormer is working.

Mistake 6: Using herbal or “natural” dewormers instead of diagnostics

AAEP notes that the efficacy of organic or herbal dewormers has not been demonstrated in formal controlled evaluations, and that non-drug products generally have little oversight for label claims, safety and efficacy compared with approved drugs.

Mistake 7: Reaching for combination products without a reason

Combination deworming has more evidence in ruminants than horses. AAEP states there is currently insufficient evidence to endorse combination deworming as a sustainable strongyle control strategy in horses, although veterinarians may consider extra-label combinations case by case.

Myth vs Reality

Myth	Reality
“All horses should be dewormed every 6 to 8 weeks.”	Fixed-interval year-round deworming is no longer recommended.
“Rotating products prevents resistance.”	Blind rotation is outdated and can keep using ineffective drugs.
“A fecal egg count tells me how many worms are inside the horse.”	FEC measures egg shedding, not total worm burden.
“A low FEC means the horse has no parasite risk.”	Encysted larvae, tapeworms and pinworms can be missed or underestimated.
“Ivermectin and moxidectin always work.”	They remain important, but egg reappearance periods have shortened and ascarid resistance is widespread.
“Natural dewormers are safer.”	Efficacy is not proven in controlled equine studies, and relying on them can delay real parasite control.

Prevention: How To Slow Resistance on Your Farm

The goal is to reduce parasite disease without using dewormers so aggressively that resistance accelerates.

A smart prevention plan includes:

• Fecal egg counts once or twice yearly in adult horses
• Annual FECRT for each operation
• Separate plans for foals, yearlings, adults and seniors
• Baseline treatments where appropriate
• Additional treatment only for high adult strongyle shedders
• Treatment timing based on local transmission season
• Accurate weight-based dosing
• Fresh fecal samples handled properly
• Prompt manure removal in high-use areas
• Proper composting before manure is returned to pasture
• Avoiding overstocking
• Veterinary review of products used
• Keeping detailed records

Good records should include:

• Horse name
• Age
• Weight estimate
• Pasture group
• Date of fecal sample
• FEC result
• Product used
• Active ingredient
• Dose
• Date treated
• Follow-up FEC result
• Any signs before or after treatment

The best parasite programs are not the ones with the most medication.

They are the ones with the best information.

Frequently Asked Questions

Is rotational deworming still recommended for horses?

No. Blind rotational deworming is no longer recommended. AAEP recommends stopping fixed-interval year-round deworming and abandoning blind rotation of dewormer classes.

How often should adult horses have fecal egg counts?

Most adult horses should have fecal egg counts once or twice yearly to identify low, moderate and high strongyle egg shedders. Your vet may recommend different timing depending on climate, pasture system, treatment history and herd risk.

What is the difference between FEC and FECRT?

A fecal egg count measures parasite egg shedding in manure. A fecal egg count reduction test compares egg counts before and after treatment to check whether a dewormer worked. FECRT is the tool used to evaluate treatment efficacy and detect likely resistance.

Should I only use ivermectin or moxidectin now?

No. Drug choice depends on the parasite, horse age, farm resistance pattern and treatment goal. Macrocyclic lactones are important for adult strongyle control in many areas, but Parascaris resistance to this class is widespread in foals, so ivermectin or moxidectin is not automatically the right choice for every horse.

Can herbal dewormers replace regular parasite control?

No reliable evidence supports herbal or organic dewormers as replacements for approved parasite control programs. AAEP states that efficacy of these products has not been demonstrated in formal controlled evaluations.

The Bottom Line

Dewormer resistance is not coming. It is already here.

That does not mean horse owners should stop deworming. It means deworming needs to become smarter.

The old plan of treating every horse every 6 to 8 weeks created exactly the pressure that helped resistance build. The modern plan is more thoughtful: test, treat strategically, check whether treatment worked, manage manure and pasture, and use different protocols for foals, adults and seniors.

If you remember one thing, make it this:

The right deworming plan is not based on the calendar. It is based on the horse, the parasite, the property and the evidence.

A tube of dewormer is easy to buy.

A sustainable parasite control plan is what protects the horse long term.

---

If you are unsure whether your horse needs deworming, fecal testing, a fecal egg count reduction test, or a full parasite control plan, ASK A VET™ can help you organise the history, track results and decide what should happen next.