Mild to Moderate Equine Asthma in Horses: Signs, BAL Diagnosis and Treatment

By Dr Duncan Houston

A horse that coughs occasionally, runs out of stamina, or feels “flat” under saddle may not be lazy, unfit, or just having an off day.

They may be struggling with mild to moderate equine asthma, previously often called inflammatory airway disease, small airway inflammatory disease, or small airway disease.

This condition affects the smaller airways deep in the lungs. Many horses look normal at rest, but when they are asked to work, the problem appears. Less stamina, delayed recovery, coughing during exercise, or poor performance can all be early clues.

The tricky part? These horses often do not look obviously sick.

Quick Answer: What Is Mild to Moderate Equine Asthma?

Mild to moderate equine asthma is a non-infectious inflammatory lung disease caused by airway irritation and hypersensitivity to dust, mould, pollen, bedding particles, ammonia, poor ventilation, and other airborne irritants.

It is part of the equine asthma syndrome, which includes:

• Mild to moderate equine asthma, previously called inflammatory airway disease

• Severe equine asthma, previously called heaves or recurrent airway obstruction

Mild to moderate equine asthma usually causes poor performance, coughing, increased airway mucus, and abnormal inflammatory cells on bronchoalveolar lavage, or BAL. Unlike severe asthma, horses with mild to moderate asthma usually do not have obvious increased breathing effort at rest. (Merck Veterinary Manual)

Is Equine Asthma an Emergency?

Mild to moderate equine asthma is usually not an immediate emergency, but it should not be ignored.

Call your vet urgently if your horse has:

• Laboured breathing at rest

• Flared nostrils

• Abdominal effort when breathing

• Blue, grey, or muddy gums

• Fever

• Depression or loss of appetite

• Thick nasal discharge

• Sudden worsening cough

• Breathing difficulty after travel

• Suspected pneumonia

• Poor performance with collapse, weakness, or severe distress

Mild asthma should be investigated, but breathing difficulty at rest is a much bigger red flag and may suggest severe asthma, pneumonia, pleuropneumonia, heart disease, airway obstruction, or another serious condition.

What Causes Small Airway Inflammation in Horses?

Equine asthma is usually driven by inhaled irritants.

Common triggers include:

• Dusty hay

• Mouldy hay

• Round bales

• Straw bedding

• Poor barn ventilation

• Ammonia from urine-soaked bedding

• Dust from arenas

• Hay stored above stalls

• Pollen

• Smoke

• Air pollution

• Fine particles from bedding and feed

Even good-looking hay can contain respirable dust and microscopic particles that irritate the lower airways. Merck Veterinary Manual notes that environmental allergens in hay and bedding, including moulds, endotoxin, microorganisms, pollen, gases, and pollutants, are key contributors to equine asthma. (Merck Veterinary Manual)

That means a horse can be beautifully rugged, perfectly shod, on the best supplements in the feed room, and still breathing in an invisible cloud of lung nonsense every day. Horses do enjoy making management complicated. It’s part of the brand.

Signs of Mild to Moderate Equine Asthma

The early signs can be subtle.

You may notice:

• Reduced stamina

• Poor performance

• Slower recovery after exercise

• Coughing at the start of work

• Coughing during exercise

• Occasional cough at rest

• Mild nasal discharge

• More mucus after exercise

• Slightly increased respiratory rate

• Trouble performing at higher intensity

• “Just not right” under saddle

Many affected horses look normal in the paddock or stable. This is why owners, trainers, and riders often notice it first as a performance problem rather than a “sick horse” problem.

Tufts notes that even small amounts of respiratory disease can show up as decreased performance in athletic horses, sometimes without obvious cough, nasal discharge, or visible abnormal breathing effort. (Cummings School of Veterinary Medicine)

Does Equine Asthma Cause Fever?

Usually, no.

Horses with equine asthma are not typically febrile unless there is a secondary bacterial pneumonia or another infectious disease present. (Merck Veterinary Manual)

This is an important difference.

A horse with mild to moderate equine asthma may cough and perform poorly, but they usually:

• Do not have a fever

• Do not act severely unwell

• Do not suddenly crash overnight

• Do not usually have thick, nasty nasal discharge

• Do not usually go off food completely

If fever, depression, thick nasal discharge, or sudden illness is present, your vet needs to rule out infectious respiratory disease.

Mild Asthma vs Severe Asthma in Horses

Mild to Moderate Equine Asthma

Usually causes:

• Poor performance

• Exercise intolerance

• Intermittent cough

• Increased airway mucus

• Abnormal BAL cytology

• No obvious breathing difficulty at rest

Severe Equine Asthma

May cause:

• Laboured breathing at rest

• Flared nostrils

• Heave line

• Persistent cough

• Exercise intolerance

• Weight loss

• Recurrent flare-ups

• More obvious respiratory distress

Severe asthma is often easier to recognise because the horse looks visibly affected. Mild to moderate asthma is sneakier. It hides in performance, stamina, and recovery.

How Is Mild to Moderate Equine Asthma Diagnosed?

Diagnosis is based on history, clinical signs, physical exam, and lower airway testing.

Your vet may recommend:

• Full clinical exam

• Temperature check

• Lung auscultation, often with a rebreathing exam

• Endoscopy to assess mucus and airway irritation

• Bronchoalveolar lavage

• Tracheal wash if infection needs to be ruled out

• Blood tests if systemic illness is suspected

• Thoracic ultrasound or radiographs in selected cases

The goal is not just to say, “Yes, the horse is coughing.”

The goal is to answer:

Is this asthma, infection, bleeding, allergy, poor fitness, cardiac disease, upper airway disease, or something else?

Bronchoalveolar Lavage: The Key Test

A bronchoalveolar lavage, or BAL, is one of the most useful tests for diagnosing equine asthma.

During a BAL:

1. The horse is sedated

2. A sterile tube or endoscope is passed into the lower airway

3. Sterile fluid is flushed into a small region of the lung

4. The fluid is recovered

5. The sample is examined under a microscope

6. The inflammatory cell pattern is assessed

BAL helps identify the type and severity of airway inflammation. Merck Veterinary Manual describes BAL as the test of choice for identifying and characterising lower airway inflammation, and BAL cytology can help distinguish mild to moderate asthma from severe asthma. (Merck Veterinary Manual)

What Do BAL Results Show?

In mild to moderate equine asthma, BAL may show increased:

• Neutrophils

• Mast cells

• Eosinophils

• Or a mixed inflammatory pattern

Merck lists normal BAL cytology as neutrophils below 5%, mast cells below 2%, and eosinophils below 1%. Mild to moderate asthma may show neutrophils above 10% but below 25%, mast cells above 5%, or eosinophils above 5%, depending on the inflammatory pattern. (Merck Veterinary Manual)

That cell pattern matters because not every asthma horse is the same. Some horses are more neutrophilic, some are more mast-cell driven, some are eosinophilic, and some are mixed.

Translation: the lungs are angry, but they may be angry in slightly different flavours.

Why a Tracheal Wash May Also Be Needed

A BAL is excellent for assessing deep airway inflammation, but it is not always the best test for infection.

If your vet is worried about bacterial pneumonia or lower airway infection, they may recommend a tracheal wash for culture and sensitivity.

The University of Saskatchewan notes that a tracheal wash may be needed to rule out pneumonia. (vmc)

This matters because asthma and infection are treated differently. Unnecessary antibiotics do not fix asthma, and missing pneumonia is obviously not ideal.

Differentials: What Else Can Look Like Equine Asthma?

Poor performance and coughing are not always asthma.

Important differentials include:

• Viral respiratory infection

• Bacterial pneumonia

• Pleuropneumonia

• Exercise-induced pulmonary haemorrhage

• Upper airway obstruction

• Laryngeal dysfunction

• Dorsal displacement of the soft palate

• Cardiac disease

• Poor fitness

• Pain or lameness affecting performance

• Lungworm in at-risk regions or donkeys/mixed grazing situations

• Parasitic lung disease

• Hypersensitivity pneumonitis

• Fungal or inflammatory lung disease

This is why BAL, endoscopy, and a proper work-up can be so useful, especially in performance horses where “not quite right” can have ten different meanings and all of them are expensive.

Treatment: The Environment Comes First

The most important treatment for mild to moderate equine asthma is reducing inhaled irritants.

Medication can help, but if the horse keeps breathing in dust and mould every day, the problem usually comes back.

Merck Veterinary Manual states that environmental management is the single most important component of controlling equine asthma, and that medication may improve signs temporarily, but signs often return if the horse remains in an allergen-rich environment. (Merck Veterinary Manual)

The University of Saskatchewan says it even more bluntly: without environmental management, drug therapies will not work. (vmc)

Best Environmental Changes for Equine Asthma

1. Improve Hay Quality and Reduce Dust

Options include:

• Feeding steamed hay

• Feeding soaked hay

• Feeding haylage where appropriate

• Using hay cubes

• Using complete pelleted feeds

• Avoiding dusty or mouldy hay

• Avoiding round bales in sensitive horses

Round bales can be a major issue because horses bury their noses into them and inhale dust and mould particles while eating. Merck identifies round bale hay as particularly allergenic and a common cause of treatment failure in pasture-managed horses. (Merck Veterinary Manual)

2. Change Bedding

Low-dust bedding options may include:

• Wood shavings

• Wood pellets

• Cardboard bedding

• Paper bedding

Avoid dusty straw in sensitive horses. The University of Saskatchewan recommends shavings rather than straw because straw bales tend to accumulate dust and mould. (vmc)

3. Increase Ventilation

Good ventilation matters.

Practical steps include:

• Opening barn doors and windows where safe

• Avoiding sealed, stale-air stables

• Keeping horses away during sweeping or blowing

• Avoiding hay storage above stalls

• Reducing ammonia build-up

• Cleaning wet bedding regularly

A stable that smells strongly of ammonia is not just unpleasant. It is a lung insult in a building.

4. Maximise Turnout Where Appropriate

Many horses improve with more time outside, especially if pasture air quality is good and pollen or summer pasture triggers are not the main issue.

The catch is that not all turnout is equal. A horse standing over a dusty round bale in a dry paddock may still be inhaling plenty of irritants.

5. Manage Arena Dust

For performance horses, the arena can be a major exposure source.

Helpful changes include:

• Watering dusty arenas

• Avoiding work during arena dragging

• Improving footing management

• Avoiding indoor arenas with poor ventilation

• Reducing exposure to dust clouds during warm-up

A horse may have a beautiful stable environment but still cough every time it works in a dusty indoor arena. Annoying, but very horse.

Medications for Equine Asthma

Medication may be needed when clinical signs are affecting performance, comfort, or airway health.

Common treatment categories include:

• Corticosteroids

• Bronchodilators

• Inhaled therapy

• Occasionally adjunctive treatments depending on the case

Medication should be prescribed by a vet, especially in performance horses, horses at risk of laminitis, pregnant mares, horses with infection risk, and horses competing under medication rules.

Corticosteroids

Corticosteroids reduce airway inflammation.

They may be given as:

• Oral medication

• Injectable medication

• Inhaled medication

Inhaled corticosteroids can deliver medication more directly to the lungs and may reduce systemic exposure compared with systemic therapy. The University of Saskatchewan notes that inhaled formulations act directly at the site of inflammation and may have fewer side effects than drugs distributed through the whole body. (vmc)

However, corticosteroids still need care. Prolonged or high-dose steroid use can suppress the immune system and may increase laminitis risk in predisposed horses. (vmc)

Bronchodilators

Bronchodilators help open the airways.

They can be useful short-term, especially when airflow obstruction is contributing to clinical signs.

However, bronchodilators should not be the whole plan. Merck specifically states that it is inappropriate to treat equine asthma with bronchodilators as the sole treatment. (Merck Veterinary Manual)

Why?

Because bronchodilators open the airway, but they do not fix the underlying inflammatory trigger. Using only bronchodilators for asthma is a bit like opening the window because the kitchen is on fire. It helps the smoke, but someone still needs to deal with the flaming toaster.

Can Equine Asthma Be Cured?

Sometimes mild cases improve dramatically, especially with early treatment and serious environmental changes.

But many horses need long-term management.

Tufts describes equine asthma as usually treatable but not necessarily curable, with many horses able to return to athletic function when diagnosis, treatment, and environmental improvements are done properly. (Cummings School of Veterinary Medicine)

That is the owner-friendly message:

This is manageable, but the lungs remember.

Monitoring Progress

Track the horse’s response over time.

Useful things to monitor include:

• Cough frequency

• Cough during warm-up

• Recovery after exercise

• Respiratory rate at rest

• Nasal discharge

• Performance level

• Energy under saddle

• Response to environmental changes

• Response to medication

• Flare-ups after hay, stabling, travel, or dusty arena work

Recheck timing depends on severity, but many horses benefit from reassessment after treatment and environmental changes. Repeat BAL may be useful if signs persist, recur, or if the original diagnosis was unclear.

Management Plan for Owners

If your horse may have mild to moderate equine asthma, a practical plan is:

1. Record the signs
   Note cough frequency, when it happens, and whether it is linked to hay, stabling, exercise, or weather.

2. Check temperature
   Fever suggests infection or another systemic problem, not simple asthma.

3. Call your vet
   Especially if poor performance is ongoing or worsening.

4. Reduce dust immediately
   Remove dusty hay, improve ventilation, and avoid dusty bedding.

5. Discuss BAL or airway testing
   Especially in performance horses or recurring cases.

6. Use medication only under veterinary guidance
   Steroids and bronchodilators are useful, but not harmless.

7. Review the environment honestly
   The stable, hay, arena, bedding, and turnout all matter.

8. Monitor response
   A horse that does not improve needs reassessment.

Common Mistakes Horse Owners Make

Mistake 1: Calling it “just a cough”

A recurring cough in a horse is not normal. It is a clinical sign.

Mistake 2: Treating with medication but changing nothing else

Medication without environmental change often leads to relapse.

Mistake 3: Ignoring poor performance

Some horses do not cough much. They simply lose stamina or feel flat under saddle.

Mistake 4: Assuming good-looking hay is low-dust

Hay can look fine but still contain respirable particles that irritate the lower airways.

Mistake 5: Using bronchodilators alone

Bronchodilators may help breathing temporarily, but they do not remove the underlying inflammation or environmental trigger.

Mistake 6: Forgetting about the arena

Dust exposure does not only happen in the stable. Indoor arenas can be brutal on sensitive lungs.

FAQs About Mild to Moderate Equine Asthma

What is small airway inflammatory disease in horses?

Small airway inflammatory disease is an older or less commonly used term for inflammatory disease affecting the lower airways. Today, many vets classify this under mild to moderate equine asthma.

Is inflammatory airway disease the same as equine asthma?

Inflammatory airway disease, or IAD, is now generally considered part of the equine asthma spectrum and is commonly referred to as mild to moderate equine asthma. (vet.purdue.edu)

Is equine asthma infectious?

Equine asthma is usually non-infectious. It is typically related to airway inflammation from dust, mould, allergens, and irritants. However, infection can look similar, so your vet may need to rule out pneumonia or viral respiratory disease.

Can a horse with asthma have a fever?

Not usually. Fever suggests infection, inflammation elsewhere, or another disease process. Horses with asthma are not typically febrile unless secondary bacterial pneumonia has developed. (Merck Veterinary Manual)

What is the best test for equine asthma?

Bronchoalveolar lavage is one of the best tests for confirming lower airway inflammation and characterising the type of inflammation present. (Merck Veterinary Manual)

Does my horse need antibiotics?

Not unless infection is suspected or confirmed. Asthma itself is not treated with antibiotics.

Should I soak or steam hay?

Both can reduce dust exposure, but steaming is often more effective and less messy when done properly. Soaked hay can help but may leach nutrients and needs careful handling, especially in warm weather. For highly sensitive horses, hay cubes, haylage, or complete pelleted feeds may be needed.

Can round bales worsen asthma?

Yes. Round bales can expose horses to high levels of dust and mould, especially when horses bury their faces into them. Merck describes round bale hay as particularly allergenic and a common cause of treatment failure in horses on pasture. (Merck Veterinary Manual)

Can horses with mild asthma still compete?

Many can, once properly diagnosed and managed. Medication rules must be considered for competition horses, so always discuss treatment timing and withdrawal requirements with your vet and relevant sporting authority.

Can equine asthma progress to heaves?

Mild to moderate asthma may progress in some horses, especially if exposure continues, but not every horse will develop severe asthma. Early environmental management may help reduce ongoing airway damage.

How long does treatment take to work?

Some horses improve within days to weeks, especially with medication and strong environmental changes. Deeper airway inflammation may take longer, and relapse is common if the horse returns to dusty conditions.

Final Thoughts

Mild to moderate equine asthma is one of the most overlooked causes of poor performance, intermittent coughing, and slow recovery in horses.

These horses often look normal at rest. That is what makes it sneaky.

The key is to recognise the early clues, rule out infection and other causes, and take the environment seriously. BAL testing can confirm the diagnosis and show what type of inflammation is present. Medications can help, but dust control, hay management, ventilation, bedding changes, and arena management are what keep the lungs happy long-term.

A coughing horse is not being dramatic.

Well, they might be. They are horses. But the cough still counts.

---

Need help working out whether your horse’s cough or poor performance could be asthma, infection, fitness, or something else?
ASK A VET™ can help you review the signs, organise the right questions for your local vet, and understand when airway testing like BAL may be needed. Visit AskAVet.com or use the ASK A VET™ App.