How to Prevent Dewormer Resistance in Horses

How to Prevent Dewormer Resistance in Horses

By Dr Duncan Houston

Deworming horses used to feel simple: rotate products, dose every few weeks, and assume the parasites were handled.

That approach is now outdated.

The problem is that parasites do not politely disappear just because we keep using the same drugs. When dewormers are used too often, used at the wrong time, underdosed, or rotated blindly, the susceptible worms are killed while the tougher, more resistant worms survive and reproduce. Over time, the farm is left with a parasite population that is harder and harder to control.

The goal of modern parasite control is not to make a horse completely worm-free. That is unrealistic. The goal is to keep horses healthy, reduce pasture contamination, and preserve the dewormers we still have.

Quick Answer

Dewormer resistance in horses happens when internal parasites survive treatment with drugs that should normally kill them. The best way to reduce resistance is to stop blanket deworming every 6 to 8 weeks, use faecal egg counts to identify high-shedding horses, perform faecal egg count reduction tests to check whether products still work on your farm, and use pasture management to reduce parasite exposure. AAEP’s updated parasite control guidance recommends faecal egg count reduction testing annually, faecal egg counts once or twice yearly, baseline deworming once or twice yearly, and stopping fixed-interval year-round deworming. (AAEP)

What Is Dewormer Resistance?

Dewormer resistance, also called anthelmintic resistance, means parasites are no longer reliably killed by a deworming product that previously worked.

This does not usually happen overnight. It builds over time.

Every time a dewormer is used, it creates selection pressure. Sensitive worms die. Worms with resistance traits are more likely to survive. If those surviving worms reproduce, their resistant genes become more common in the next generation.

In simple terms: the more casually we use dewormers, the more we train parasites to survive them.

Merck Veterinary Manual states that anthelmintic resistance is widespread globally in parasites of managed horses, with small strongyles widely resistant to benzimidazoles and pyrimidines, and resistance to macrocyclic lactones also spreading. (Merck Veterinary Manual)

Why Does This Matter?

Dewormer resistance matters because horses have a limited number of useful drug classes.

The major dewormer groups used in horses include:

Drug class	Examples	Main concern
Macrocyclic lactones	Ivermectin, moxidectin	Still important, but resistance concerns are increasing
Benzimidazoles	Fenbendazole, oxibendazole	Widespread small strongyle resistance in many regions
Pyrimidines	Pyrantel	Resistance concerns, especially with small strongyles
Isoquinolines	Praziquantel	Used for tapeworms, often in combination products

The worrying part is that no single product should be assumed to work broadly against every important equine parasite. Merck notes that equine parasites have developed resistance to all anthelmintic classes available for horses and that routine efficacy testing is needed to confirm which products still work. (Merck Veterinary Manual)

In practice, the real danger is not one horse with a few worms. The real danger is a whole property using ineffective products for years while parasite pressure quietly builds.

Which Parasites Are We Most Worried About?

The main internal parasite targets in horses are:

• Small strongyles, also called cyathostomins

• Large strongyles, especially Strongylus vulgaris

• Tapeworms

• Ascarids, especially in foals and young horses

• Pinworms

• Bots

• Threadworms and other less common parasites

For adult grazing horses, small strongyles and tapeworms are usually the major focus. For foals and young horses, ascarids become much more important. Merck identifies tapeworms, strongyles, and ascarids as the main intestinal parasite targets in equine parasite control programs. (Merck Veterinary Manual)

This is one reason blanket advice is risky. A mature gelding on a dry, low-density property is not the same parasite-control patient as a 5-month-old foal on a breeding farm.

Why the Old Deworming Rotation Failed

The old approach was usually something like this:

• Deworm every 6 to 8 weeks

• Rotate between product classes

• Treat every horse on the same day

• Repeat all year

• Assume the job is done

That sounded tidy. Unfortunately, it created huge selection pressure.

AAEP now recommends discontinuing fixed-interval year-round deworming and stopping blind rotation of anthelmintic classes. (AAEP)

The mistake was thinking rotation automatically prevented resistance. It does not.

If you rotate between drugs without testing, you may simply rotate between ineffective products. That is not strategic parasite control. That is giving the worms a buffet menu and asking them which one they would like to become resistant to next.

What Is a Faecal Egg Count?

A faecal egg count, or FEC, measures the number of parasite eggs in a horse’s manure, usually reported as eggs per gram, or EPG.

It helps answer one key question:

How much is this horse contributing to pasture contamination right now?

It does not perfectly measure the total number of worms inside the horse. It does not detect every parasite stage. It does not prove whether parasites are causing disease. But it is extremely useful for identifying low, moderate, and high egg-shedding horses.

AAEP recommends using faecal egg counts once or twice yearly to stratify horses into low, medium, and high shedders. (AAEP)

How Are Faecal Egg Counts Interpreted?

A common adult horse interpretation is:

Result	Category	What it usually means
Less than 200 EPG	Low shedder	Often needs fewer targeted treatments
200 to 500 EPG	Moderate shedder	Treatment depends on season, farm history, and vet advice
More than 500 EPG	High shedder	More likely to need targeted treatment

UC Davis describes low shedders as horses with less than 200 EPG, moderate shedders as 200 to 500 EPG, and high shedders as over 500 EPG. It also notes that low shedders make up around 50 to 75 percent of adult horses, while high shedders make up around 10 to 30 percent. (Center for Equine Health)

This is why testing matters. On many farms, a small number of horses produce most of the parasite egg contamination. Treating every horse constantly is often less intelligent than identifying the main shedders and building a plan around them.

The Important Limitation: A Low Egg Count Does Not Mean No Worms

This is where owners can get caught out.

A low faecal egg count does not mean the horse has no parasites. It means the horse was not shedding many detectable eggs in that sample at that time.

FECs do not reliably detect encysted small strongyle larvae, and they do not directly measure the full internal parasite burden. Merck states that egg counts do not correlate with adult worm burdens, and that pathogenic larval stages do not produce eggs. (Merck Veterinary Manual)

That is why faecal egg counts are a management tool, not a complete diagnostic crystal ball.

In practice, I use FECs to guide herd-level parasite control, not to dismiss a sick horse that has diarrhoea, colic, weight loss, or poor condition.

What Is a Faecal Egg Count Reduction Test?

A faecal egg count reduction test, or FECRT, checks whether a dewormer actually worked on your property.

The basic process is:

1. Collect a fresh manure sample before treatment.

2. Deworm with the selected product at the correct dose.

3. Collect another fresh manure sample about 14 days later.

4. Compare the egg count reduction.

If the egg count does not drop enough, that suggests reduced efficacy or resistance on that farm.

Merck describes FECRT as an important way to evaluate treatment efficacy, and notes that persistently high egg counts after deworming indicate resistance and a need to change the control program. (Merck Veterinary Manual)

This is the part many horse owners skip. They buy the tube, give the paste, and assume it worked. But without checking, you do not know whether you treated parasites or just donated money to the worm population.

What Is Refugia and Why Does It Matter?

Refugia refers to the portion of the parasite population not exposed to the dewormer at the time of treatment.

That includes:

• Parasites in untreated horses

• Eggs and larvae on pasture

• Parasite stages inside horses that are not affected by that product

This matters because refugia helps maintain a population of susceptible parasites. Those susceptible parasites dilute resistant genes in the wider parasite population.

The old goal was “kill everything.” The modern goal is more intelligent: reduce disease risk while avoiding unnecessary selection for resistance.

AAEP’s guidance specifically notes that no anthelmintic eliminates all parasitic stages from a horse, and that the goal is not eradication. (AAEP)

This feels counterintuitive to owners. But leaving low-shedding adult horses untreated at certain times can be part of a smart resistance-reduction plan when your vet has assessed the herd.

Should Every Horse Still Be Dewormed?

Usually, yes, but not constantly and not blindly.

For many adult horses, the modern approach is a two-tiered plan:

1. Baseline treatment once or twice yearly for all horses, depending on region and risk.

2. Additional targeted treatment for selected horses based on faecal egg counts and farm history.

Merck describes current guidelines as using a two-tiered approach, with baseline annual treatments for all horses and extra treatments based on surveillance findings. (Merck Veterinary Manual)

That means low shedders may still receive strategic baseline treatment, especially to cover parasites that are not well managed by FEC alone. But they may not need repeated strongyle-targeted treatments throughout the year.

Why Foals and Young Horses Need a Different Plan

Foals are not just small adult horses.

Young horses are more vulnerable to ascarids, also known as roundworms. Heavy ascarid burdens can cause poor growth, coughing during larval migration, pot belly, rough coat, and in severe cases, small intestinal impaction.

Merck notes that all foals are exposed to ascarid infection, with patent infection detected in approximately 50 percent of foals aged 3 to 7 months in a given foal crop. (Merck Veterinary Manual)

This matters because the dewormer plan for adult strongyles is not automatically the correct plan for foals. Youngstock need a veterinarian-guided program that considers ascarids, strongyles, age, weight, farm history, and known resistance patterns.

The mistake I see most often is taking an adult horse deworming schedule and applying it to foals. That is not good medicine.

What About Tapeworms?

Tapeworms are a separate problem.

They are not always reliably detected by a standard faecal egg count. Tapeworms can be associated with colic, especially around the ileocecal region, where the small intestine meets the large intestine.

Merck notes that tapeworms are common in grazing horses worldwide and can cause ileal impactions and intussusception, both of which can lead to colic and may require surgery. (Merck Veterinary Manual)

Depending on region and testing availability, tapeworm control may involve:

• Strategic treatment with praziquantel

• Modified faecal testing

• Blood or saliva antibody testing in some countries

• Farm-level risk assessment

Do not assume a low strongyle egg count means tapeworms are not relevant.

Risk Framework: How Serious Is the Resistance Problem?

Low Risk

A lower-risk property has:

• Low stocking density

• Regular manure removal

• Faecal egg counts once or twice yearly

• Annual FECRT or periodic efficacy testing

• Deworming based on veterinary advice

• Accurate dosing by body weight

• Good quarantine for new arrivals

Action: Keep the system going. Review results with your vet at least yearly.

Moderate Risk

A moderate-risk property has:

• Some FEC testing, but not consistently

• Occasional blanket deworming

• Unknown product efficacy

• Horses sharing pasture with mixed parasite histories

• Infrequent manure removal

• Some high shedders not identified

Action: Start by testing all horses, classifying shedders, and checking whether your main dewormers are still effective.

High Risk

A high-risk property has:

• Deworming every 6 to 8 weeks by habit

• No faecal egg counts

• No FECRT

• Blind rotation between products

• Underdosing

• Crowded pasture

• New horses joining without quarantine testing

• Young horses managed the same as adults

Action: Stop guessing. Work with your vet to build a property-specific parasite control plan.

Critical Risk

A critical-risk situation is not just “resistance exists.” It is when horses are getting sick and the farm has no idea whether the parasite control program is working.

Critical warning signs include:

• Multiple horses losing weight

• Persistent diarrhoea

• Recurrent colic

• Poor growth in foals

• Heavy parasite burdens despite treatment

• High egg counts after deworming

• Sudden severe diarrhoea, weakness, or dehydration

• Suspected larval cyathostominosis

• Suspected ascarid impaction in a foal or weanling

Action: Contact your veterinarian promptly. Sick horses need clinical assessment, not just another tube of paste.

What Else Could Look Like Parasites?

Not every thin, dull, or diarrhoeic horse has a simple worm problem.

Important rule-outs include:

• Dental disease

• Poor-quality forage

• Inadequate calorie intake

• Sand accumulation

• Gastric ulcers

• Inflammatory bowel disease

• Liver disease

• Kidney disease

• Chronic infection

• Equine metabolic syndrome

• PPID in older horses

• Cancer

• Poor social access to feed

• Chronic pain

• Stress or transport-related illness

• Salmonella or other infectious diarrhoea causes

• Potomac horse fever in relevant regions

• Lawsonia intracellularis in young horses

This is why faecal egg counts are not diagnostic tests for disease. Merck specifically notes that FECs do not prove parasitic involvement in clinical disease and do not predict disease risk. (Merck Veterinary Manual)

In practice, if a horse is clinically unwell, I do not want owners thinking, “The egg count was low, so it can’t be serious.” That is the wrong conclusion.

When Is This an Emergency?

Parasite-related disease can become serious, especially in young horses or horses with diarrhoea, colic, dehydration, or weight loss.

Call a vet urgently if your horse has:

• Colic signs

• Severe or watery diarrhoea

• Depression or weakness

• Fever

• Rapid weight loss

• Not eating

• Dehydration

• Swelling under the belly or legs

• Pale or dark gums

• Blood in manure

• Repeated rolling, pawing, or flank watching

• Poor growth or pot belly in a foal

• Suspected ascarid impaction after deworming

• Severe diarrhoea after recent deworming

• Multiple horses affected on the same property

Larval cyathostominosis is uncommon, but it can be severe. Merck describes it as a serious condition associated with sudden watery diarrhoea, dehydration, shock, oedema, colic signs, and weight loss, with a reported mortality rate of approximately 50 percent. (Merck Veterinary Manual)

Do not try to fix a severely unwell horse by giving more dewormer. That can delay treatment and may worsen the situation.

What Should Horse Owners Do Next?

1. Stop automatic 6 to 8 week deworming

If your current program is “everyone gets a tube every couple of months,” it needs updating.

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

2. Test before treating where appropriate

Use faecal egg counts to classify adult horses as low, moderate, or high shedders.

This helps decide who needs additional treatment and who may not.

3. Check whether the product actually works

Use faecal egg count reduction testing to assess dewormer efficacy on your property.

A treatment that does not reduce egg counts properly should not remain the backbone of your parasite plan.

4. Dose accurately

Underdosing is a major mistake.

Use an accurate weight tape or scale where possible. Guessing low exposes worms to a sublethal drug dose, which is exactly the sort of nonsense parasites enjoy.

5. Build a farm-specific plan

Your plan should account for:

• Age groups

• Pasture type

• Climate

• Stocking density

• Manure management

• Seasonality

• New arrivals

• Previous FEC results

• Previous FECRT results

• Pregnancy and breeding status

• Youngstock

• Seniors

• Local parasite risks

Merck notes that parasite control should be handled at the population level, with veterinarians developing a program specific to each operation. (Merck Veterinary Manual)

6. Quarantine and test new arrivals

New horses can bring resistant parasites onto a property.

A sensible arrival protocol may include:

• Quarantine

• Fresh faecal egg count

• Targeted treatment if indicated

• Follow-up testing

• Vet-guided timing before pasture mixing

Do not let a new horse arrive and immediately become the wormy welcome committee.

Pasture Management That Reduces Parasite Pressure

Medication is only one part of parasite control.

Good management reduces how many infective larvae horses ingest.

Useful measures include:

• Remove manure from paddocks regularly

• Avoid overstocking

• Avoid overgrazing

• Rotate resting areas where climate makes this useful

• Keep feed off contaminated ground

• Avoid spreading fresh manure on grazing pasture

• Separate youngstock from adult high-shedding horses where practical

• Manage wet, high-traffic areas

• Maintain quarantine procedures for new arrivals

Merck notes that parasite decisions should consider climate, season, and pasture management measures such as dragging, mowing, rotating, and hygiene. It also notes that benefits vary depending on temperature and moisture conditions. (Merck Veterinary Manual)

The practical point is simple: if the paddock is full of manure, dewormers are being asked to do a job that management should be helping with.

Common Mistakes Horse Owners Make

Deworming by calendar only

Calendar-only deworming is outdated for most adult horses. It increases selection pressure and may waste treatments.

Rotating products blindly

Rotation does not help if you do not know whether the product works.

Never doing a faecal egg count reduction test

A faecal egg count tells you shedding status. A reduction test tells you whether the dewormer worked.

Treating low shedders too often

This can reduce refugia and increase selection pressure without much benefit.

Ignoring high shedders

High shedders can contribute a lot of pasture contamination and usually need more attention.

Using adult horse plans for foals

Foals need youngstock-specific parasite control because ascarids are a major concern.

Underdosing

Underdosing is one of the easiest ways to encourage resistance.

Treating a sick horse without a diagnosis

Diarrhoea, colic, weight loss, or weakness needs veterinary assessment. Do not assume worms are the only explanation.

How to Prevent Dewormer Resistance

The best prevention plan combines testing, targeted treatment, and management.

A good parasite control program should include:

• Faecal egg counts once or twice yearly in adult horses

• Faecal egg count reduction testing to check product efficacy

• Baseline treatment once or twice yearly where appropriate

• Extra treatment targeted to high shedders

• Youngstock-specific protocols

• Tapeworm risk assessment

• Accurate dosing by weight

• Quarantine testing for new arrivals

• Regular manure removal

• Lower stocking density where possible

• Avoiding unnecessary deworming

• Veterinary review of the plan at least yearly

AAEP’s updated guidance includes annual FECRT, once or twice yearly FECs, baseline deworming once or twice yearly, targeted treatment for selected horses, and no fixed-interval year-round deworming. (AAEP)

That is the modern mindset: test, target, treat intelligently, and preserve the drugs we still have.

FAQs

Should I still deworm my horse if the faecal egg count is zero?

Sometimes, yes. A zero or low faecal egg count does not prove the horse has no parasites, and it does not assess every parasite type or life stage. Many adult horses still need baseline strategic treatment, depending on region, season, tapeworm risk, and vet advice.

How often should horses have faecal egg counts?

Many adult horses should have faecal egg counts once or twice yearly, especially while establishing their shedding category or monitoring a property. Higher-risk farms, youngstock, new arrivals, and horses with changing management may need more frequent testing.

Is ivermectin still effective in horses?

Ivermectin is still an important equine dewormer, but it should not be used blindly. Product efficacy should be monitored on the farm with faecal egg count reduction testing, because resistance and reduced performance are increasing concerns.

Are natural dewormers effective for horses?

Natural or herbal deworming products should not be relied on as a replacement for evidence-based parasite control. UC Davis states that natural and organic remedies have not been shown to be effective for managing equine intestinal parasites. (Center for Equine Health)

Can parasites cause colic?

Yes. Parasites can contribute to colic, especially tapeworms, ascarids in young horses, and large strongyles in certain settings. However, colic has many causes, so a colicking horse needs veterinary assessment rather than guesswork.

Final Thoughts

Dewormer resistance is not a future problem. It is already part of modern horse care.

The answer is not panic. It is better strategy.

Stop treating every horse the same. Stop rotating products blindly. Stop assuming a tube of paste worked just because it was given. Start testing. Start tracking. Start managing the whole property, not just the individual horse standing in front of you.

Good parasite control is not about killing every worm. It is about keeping horses healthy while preserving the drugs we still have.

That takes a little more planning, but it is far better than waiting until the dewormers stop working.

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If you are unsure whether your horse needs deworming, whether your faecal egg count result is concerning, or how to build a safer parasite control plan for your property, ASK A VET™ can help you work out what needs testing, what needs treatment, and what should be discussed directly with your equine veterinarian.