How To Diagnose Equine Asthma in Horses

By Dr Duncan Houston

Equine asthma can be obvious, or it can be annoyingly subtle. Some horses cough, heave, flare their nostrils and struggle to breathe at rest. Others simply lose performance, take longer to recover after work, or feel flat without looking dramatically sick.

That is why diagnosis matters. A horse with asthma may need dust control and anti-inflammatory treatment. A horse with bacterial pneumonia, viral infection, exercise-induced pulmonary hemorrhage, upper airway disease, heart disease or pleuropneumonia needs a different plan entirely.

This article explains how equine asthma is diagnosed, why bronchoalveolar lavage remains the key test for lower airway inflammation, where blood biomarkers fit, and when coughing or breathing changes should be treated as urgent.

Quick Answer

Equine asthma is diagnosed using a combination of history, clinical signs, airway examination and lower airway cytology, most commonly bronchoalveolar lavage. BAL fluid cytology helps confirm lower airway inflammation and distinguish mild-moderate asthma from severe asthma, but it is not always safe in horses that are struggling to breathe at rest. Blood biomarkers such as surfactant protein D, haptoglobin and secretoglobin are promising research tools, but they do not currently replace BAL for routine diagnosis. (Merck Veterinary Manual)

What Is Equine Asthma?

Equine asthma is an umbrella term for chronic, non-septic inflammatory airway disease in horses. It includes what used to be called inflammatory airway disease, recurrent airway obstruction, heaves and equine COPD. Mild-moderate asthma usually refers to horses with subtle lower airway inflammation, while severe asthma refers to horses with more obvious respiratory effort, airway obstruction and long-term airway changes. (Merck Veterinary Manual)

The condition is usually driven by inhaled particles such as dust, mold, endotoxin, pollen, microorganisms, bedding particles, hay dust, ammonia and other irritants. Severe asthma is often triggered by stable and hay exposure, while a pasture-associated form can occur in hot, humid climates. (Merck Veterinary Manual)

The real concern is not just the cough. The concern is what is happening inside the lower airways: inflammation, mucus, bronchospasm, airway hyperresponsiveness and, in longstanding severe cases, airway remodeling that may not fully reverse. (Merck Veterinary Manual)

Why the Name Changed From Heaves and COPD

Older terms such as heaves, COPD and recurrent airway obstruction were useful, but they made the disease sound narrower than it really is. Equine asthma better reflects a spectrum of airway inflammation, from mild poor-performance cases to severe horses with obvious breathing difficulty. Merck Veterinary Manual describes mild-moderate asthma as the category previously called inflammatory airway disease, and severe asthma as the category previously labelled recurrent airway obstruction or heaves. (Merck Veterinary Manual)

This matters because not every asthmatic horse is standing in the stable visibly heaving. Some athletic horses with mild asthma only show reduced performance, occasional cough or slow recovery after work.

Mild, Moderate and Severe Equine Asthma

Mild-moderate asthma may present with nonspecific signs such as poor performance, reduced willingness to work and intermittent coughing. Increased respiratory effort at rest is not typical of mild asthma and is one of the key differences between mild-moderate and severe disease. (Merck Veterinary Manual)

Severe asthma is more likely to cause obvious expiratory effort, frequent coughing, nasal discharge, wheezes, a prolonged expiratory phase and, in longstanding cases, a visible “heave line” from abdominal muscle effort. (Merck Veterinary Manual)

Signs of Equine Asthma

Common signs include:

• Coughing

• Poor performance

• Slow recovery after exercise

• Increased respiratory rate

• Increased breathing effort

• Nasal discharge

• Mucus in the trachea

• Wheezing or crackles on auscultation

• Flared nostrils

• Abdominal effort when breathing out

• Weight loss in severe chronic cases

The tricky part is that coughing is not always present. A major review notes that coughing is reported in only 38% of horses with mild asthma, so absence of cough does not rule out mild lower airway disease. (Frontiers)

In practice, I take poor performance seriously when it is paired with coughing, slow recovery, mucus, dust exposure, stable worsening, or a horse that suddenly seems less willing to work. Horses are excellent at making vague signs expensive.

Why Accurate Diagnosis Matters

The biggest diagnostic mistake is treating every chronic cough as either “just allergies” or “just infection.”

Asthma is primarily inflammatory and often non-infectious. Bacterial pneumonia, pleuropneumonia, viral respiratory disease and other infections are different. This matters because corticosteroids can be very helpful for asthma, but they can be risky if the real problem is an active infection. Merck notes that horses with asthma are not typically febrile unless secondary bacterial pneumonia has developed. (Merck Veterinary Manual)

So the decision is not simply “does the horse cough?” The decision is: is this lower airway inflammation, infection, bleeding, upper airway disease, heart disease or something else?

Bronchoalveolar Lavage: The Key Test

Bronchoalveolar lavage, usually called BAL, is the main test used to identify and characterize lower airway inflammation in horses. A sterile fluid is introduced into the lower airways and then recovered so the cells in the sample can be examined under a microscope. BAL fluid cytology is considered the gold standard for diagnosing equine asthma and for identifying the type and severity of lower airway inflammation. (MDPI)

BAL can show whether inflammation is mainly:

• Neutrophilic

• Mastocytic

• Eosinophilic

• Mixed

That matters because different inflammatory patterns can influence the clinical interpretation and treatment plan.

Commonly used BAL cytology patterns include increased neutrophils in severe asthma, while mild-moderate asthma may show increased neutrophils, mast cells and/or eosinophils. Merck lists normal BAL values as neutrophils below 5%, mast cells below 2% and eosinophils below 1%, with severe asthma commonly showing neutrophils around 25% to 90%. (Merck Veterinary Manual)

When BAL May Not Be Safe

BAL is very useful, but it is not the right first move in every horse.

In a horse that is dyspneic at rest, meaning visibly struggling to breathe, BAL may not be clinically appropriate because the procedure can transiently worsen respiratory compromise. Merck specifically notes that BAL is not innocuous in horses that are dyspneic at rest and may not be appropriate in severe uncontrolled asthma. (Merck Veterinary Manual)

In those cases, the vet may first stabilise the horse, reduce airway obstruction, give a fast-acting bronchodilator, improve the environment and delay sampling until the horse is safer.

The clinical rule is simple: do not chase the perfect diagnostic test while the horse is actively struggling to breathe. Stabilise first.

Tracheal Wash vs BAL

Tracheal wash and BAL are both airway sampling techniques, but they answer different questions.

A tracheal wash samples secretions from the larger airways and is often preferred when infection is suspected because it can be used for cytology and microbial culture. BAL samples the smaller lower airways and is preferred when non-infectious inflammatory disease such as equine asthma is suspected. Purdue’s equine respiratory diagnostic guidance states that tracheal wash is recommended when bacterial or fungal respiratory infection is suspected, while BAL is preferred for non-infectious disease such as asthma or exercise-induced pulmonary hemorrhage. (vet.purdue.edu)

A vet may choose one or both depending on the horse.

In practice:

• Suspected asthma: BAL is usually more useful.

• Suspected bacterial pneumonia: tracheal wash is often more useful.

• Unclear case: both may be needed.

What Does Endoscopy Show?

Endoscopy allows the vet to look inside the upper airway, trachea and larger airways. It can show mucus, inflammation, airway anatomy problems, blood, dorsal displacement of the soft palate, laryngeal dysfunction or other upper airway problems.

Endoscopy can support an asthma diagnosis when mucus and airway inflammation are seen, but it does not replace lower airway cytology. Merck notes that endoscopy may reveal mucus accumulation, edema and mucosal inflammation, while mucus scoring systems can be useful but subjective. (Merck Veterinary Manual)

Endoscopy is particularly useful when the horse has:

• Noisy breathing

• Poor performance

• Nasal discharge

• Suspected upper airway disease

• Suspected exercise-induced pulmonary hemorrhage

• Mucus in the trachea

• Unclear respiratory signs

Do X-Rays Diagnose Equine Asthma?

Thoracic radiographs can be helpful, but they are not usually the main diagnostic test for asthma.

Radiographs may show a diffuse bronchial or interstitial pattern, but Merck notes they are of little benefit in confirming equine asthma. Their greater value is helping identify important differentials such as bacterial pneumonia, interstitial pneumonia or pulmonary fibrosis. (Merck Veterinary Manual)

So chest X-rays are not useless. They are just not the asthma “answer” in most cases.

They are more likely to be useful when the horse has:

• Fever

• Weight loss

• Severe nasal discharge

• Suspected pneumonia

• Poor response to asthma treatment

• Abnormal lung sounds

• Chronic severe disease

• Possible masses, fibrosis or pleural disease

What About Lung Function Testing?

Lung function testing can measure airflow limitation and airway hyperresponsiveness, but in horses it is mostly limited to referral centers and research settings. Frontiers in Veterinary Science notes that lung function testing remains available only to a few specialised centres, and field-friendly systems are still an area needing development. (Frontiers)

This is one reason BAL remains so important in clinical practice. It is more available than advanced lung function testing and gives practical information about airway inflammation.

Blood Biomarkers: Promising, But Not a Replacement Yet

Blood biomarkers are a tempting idea because they would be easier than BAL in the field. The problem is that the current evidence is not strong enough to replace BAL.

A 2019 study from the Université de Montréal group, including Jean-Pierre Lavoie, evaluated serum surfactant protein D, haptoglobin and secretoglobin in horses with mild to moderate asthma. The combination of all three biomarkers had 100% specificity but only 45% sensitivity, meaning it was good at avoiding false positives in that study but missed many asthmatic horses. (PubMed)

That is a very important interpretation.

A highly specific test can help support a diagnosis when it is positive. A low-sensitivity test cannot safely rule out asthma when it is negative.

So the practical takeaway is:

• Blood biomarkers may become helpful diagnostic aids.

• They may support a diagnosis in selected horses.

• They are not currently a replacement for BAL.

• A negative biomarker result does not rule out equine asthma.

• BAL remains the key test when a definitive lower airway diagnosis is needed.

Merck’s current summary is blunt: several systemic biomarkers have been evaluated, including serum amyloid A, haptoglobin and surfactant proteins, but there are currently no reliable systemic biomarkers for routine diagnosis of equine asthma. (Merck Veterinary Manual)

How Worried Should You Be?

Low Concern

This is lower concern if the horse:

• Has an occasional cough

• Is bright and eating

• Has normal breathing at rest

• Has no fever

• Performs normally

• Has no nasal discharge or only mild clear discharge

• Improves after dust exposure is reduced

What to do: monitor closely, reduce dust exposure and speak with your vet if the cough persists, worsens or affects performance.

Moderate Concern

This is more concerning if the horse:

• Coughs repeatedly

• Has reduced performance

• Takes longer to recover after exercise

• Has mucus or nasal discharge

• Coughs more in the stable or when eating hay

• Has mild increased respiratory rate

• Has signs lasting more than a few days

What to do: arrange a veterinary respiratory exam. Mild-moderate asthma is often missed without BAL or airway assessment.

High Concern

This is high concern if the horse:

• Has increased breathing effort at rest

• Has frequent coughing

• Has nostril flare

• Uses the abdomen to breathe

• Has wheezing

• Is losing weight

• Has poor appetite

• Has signs that return whenever stabled or fed dry hay

What to do: call your vet. This may be severe asthma or another serious lower airway disease.

Critical

This is urgent if the horse:

• Is visibly struggling to breathe

• Has marked abdominal effort

• Has flared nostrils at rest

• Is weak, distressed or unable to settle

• Has blue or purple gums

• Has a high fever

• Has severe depression

• Has thick nasal discharge with fever

• Has sudden severe breathing difficulty

• Cannot walk normally because of respiratory distress

What to do: emergency veterinary care is needed. Do not exercise, transport unnecessarily or perform dusty stable chores around the horse.

When Is This an Emergency?

Call a vet urgently if your horse has:

• Laboured breathing at rest

• Flared nostrils

• Marked abdominal push when breathing out

• Rapid respiratory rate that does not settle

• Blue, purple or grey gums

• Severe coughing fits with distress

• Fever with cough or nasal discharge

• Depression or not eating

• Thick yellow, green or foul-smelling nasal discharge

• Sudden worsening over hours

• Collapse or weakness

• Suspected pneumonia

• Suspected pleuropneumonia

• Breathing difficulty after transport

A horse with severe asthma may improve with rapid environmental changes and bronchodilator therapy, but respiratory distress is still serious. Merck notes that severe equine asthma causes expiratory airflow limitation, increased work of breathing, air trapping and hypoxemia in affected horses. (Merck Veterinary Manual)

The simple rule: if the horse is working hard to breathe at rest, do not wait.

What Else Can Look Like Equine Asthma?

Equine asthma is common, but it is not the only cause of cough or poor performance.

Important differentials include:

• Bacterial pneumonia

• Viral respiratory infection

• Influenza

• Equine herpesvirus respiratory disease

• Strangles

• Pleuropneumonia

• Exercise-induced pulmonary hemorrhage

• Upper airway obstruction

• Laryngeal paralysis

• Dorsal displacement of the soft palate

• Guttural pouch disease

• Dental or sinus disease causing nasal discharge

• Parasitic lung disease

• Hypersensitivity pneumonitis

• Interstitial pneumonia

• Pulmonary fibrosis

• Heart disease

• Poor fitness

• Pain or lameness causing poor performance

• Heat stress

• Anemia

• Gastric ulcers or other non-respiratory performance problems

In horses with eosinophilic BAL inflammation, Merck specifically lists parasitic pulmonary disease and hypersensitivity pneumonitis as important differentials. (Merck Veterinary Manual)

This is why diagnosis matters. A cough is a sign, not a diagnosis.

How Vets Diagnose Equine Asthma

A good work-up usually starts with the basics, then escalates based on severity.

Your vet may assess:

• Duration of cough

• Stable environment

• Hay type and feeding method

• Bedding type

• Turnout routine

• Recent travel

• Fever history

• Vaccination status

• Exposure to sick horses

• Performance changes

• Recovery after exercise

• Nasal discharge

• Respiratory rate and effort

• Lung sounds at rest and after rebreathing, if safe

• Endoscopy findings

• BAL cytology

• Tracheal wash cytology and culture if infection is suspected

• Radiographs or ultrasound if pneumonia or other disease is suspected

Merck describes the diagnostic goal as identifying and characterising lower airway inflammation while excluding other causes of lower airway disease. (Merck Veterinary Manual)

That line is the whole article in one sentence. Diagnosis is not just “prove asthma.” It is “prove asthma and make sure we are not missing something more dangerous.”

What Should You Do Right Now?

If your horse has a chronic cough, poor performance or suspected asthma:

1. Check the horse’s breathing at rest.

2. Take a rectal temperature.

3. Note whether signs are worse in the stable, during hay feeding, during exercise or at pasture.

4. Remove obvious dust exposure where possible.

5. Do not work the horse hard if coughing or breathing effort is present.

6. Do not start corticosteroids without a veterinary diagnosis.

7. Call your vet if signs persist, recur or affect performance.

8. Ask whether BAL, endoscopy or tracheal wash is most appropriate.

9. If infection is possible, ask whether culture or respiratory pathogen testing is needed.

10. Treat breathing difficulty at rest as urgent.

For mild cases, small environmental changes can help, but diagnosis is still important if the horse is an athlete, signs are recurring or medication is being considered.

Treatment After Diagnosis

Treatment depends on severity and whether infection or another disease has been ruled out.

Environmental Management

Environmental control is the foundation of asthma management. Merck states that appropriate environmental management is the cornerstone of treatment, and that medication may improve signs but signs often return if the horse remains in an allergen-rich environment. (Merck Veterinary Manual)

Practical changes include:

• Feed steamed or soaked hay

• Use low-dust forage

• Avoid moldy hay

• Feed from the ground where appropriate

• Improve ventilation

• Avoid sweeping or blowing dust near the horse

• Avoid dusty bedding

• Reduce ammonia

• Increase turnout if pasture is not the trigger

• Keep the horse away from hay storage dust

• Avoid indoor arenas with heavy dust

Corticosteroids

Corticosteroids are commonly used to reduce airway inflammation in asthma. They may be given systemically or inhaled, depending on the case. Merck lists systemic options such as dexamethasone and prednisolone and inhaled options such as fluticasone, beclomethasone and ciclesonide, with ciclesonide FDA-approved for severe equine asthma. (Merck Veterinary Manual)

Corticosteroids should be used carefully if infection has not been ruled out. They also need extra caution in horses with laminitis risk, EMS or PPID.

Bronchodilators

Bronchodilators can help open constricted airways and may provide relief during flare-ups. They do not replace environmental control or anti-inflammatory treatment.

Antibiotics

Antibiotics are not routine asthma treatment. They are used when bacterial infection is suspected or confirmed.

This is one of the most common mistakes: a horse with inflammatory asthma does not automatically need antibiotics, and a horse with infection should not be treated as simple asthma.

Common Mistakes Owners Make

Assuming Every Cough Is Asthma

Coughing can come from asthma, infection, upper airway disease, bleeding, dental or sinus disease, or environmental irritation.

Assuming Every Cough Is Infection

Many asthmatic horses are not infected and do not need antibiotics.

Starting Steroids Without Diagnosis

Steroids can be very useful, but they can complicate infectious disease if the diagnosis is wrong.

Waiting Until the Horse Is Heaving

Mild asthma can show up first as poor performance, slow recovery or occasional cough.

Ignoring the Stable Environment

Medication helps, but dust control is the long-term foundation.

Thinking Clear Nasal Discharge Means It Is Harmless

Clear discharge can still occur with airway inflammation. The whole pattern matters.

Skipping BAL in Poor Performance Horses

A horse can have lower airway inflammation without dramatic coughing or obvious breathing effort.

Can Equine Asthma Be Prevented?

Not every case can be prevented, especially when there is genetic or individual susceptibility, but risk can be reduced.

Practical prevention includes:

• Use low-dust forage

• Steam or soak hay where appropriate

• Avoid moldy hay

• Choose low-dust bedding

• Improve stable ventilation

• Keep hay storage separate from stabling areas

• Avoid sweeping near horses

• Reduce ammonia build-up

• Maximise turnout when appropriate

• Avoid dusty indoor arenas

• Monitor cough frequency

• Track respiratory rate and recovery after work

• Investigate poor performance early

• Adjust management seasonally if triggers are predictable

A horse with asthma is often manageable, but the environment has to change. If the horse keeps breathing the same dust, the lungs will keep sending the same complaint.

Will My Horse Be Okay?

Many horses with equine asthma do well when the diagnosis is accurate and the environment is controlled.

The outlook is better when:

• Signs are mild

• The horse is diagnosed early

• Dust exposure can be reduced

• The horse responds to environmental changes

• BAL findings are mild to moderate

• There is no severe airway remodeling

• The owner can maintain long-term management

The outlook is more guarded when:

• The horse has severe asthma

• Breathing effort is present at rest

• The horse has chronic airway remodeling

• Dust exposure cannot be reduced

• The horse has repeated flare-ups

• There is concurrent infection or another respiratory disease

• The horse is expected to perform at a high athletic level

Merck notes that severe asthma is a lifelong disease in many horses, while mild-moderate asthma in younger horses may be less intensive to treat and may sometimes resolve spontaneously or with treatment. (Merck Veterinary Manual)

The honest answer is this: asthma can often be controlled, but it is rarely solved by medication alone.

FAQs

What is the best test for equine asthma?

Bronchoalveolar lavage cytology is the main diagnostic test for lower airway inflammation and is widely considered the gold standard for equine asthma diagnosis. It helps identify the type and severity of inflammation. (MDPI)

Can a blood test diagnose equine asthma?

Not reliably on its own. Blood biomarkers such as SP-D, haptoglobin and secretoglobin are promising, but current evidence does not support them as a replacement for BAL. One study found excellent specificity but low sensitivity, meaning many asthmatic horses would be missed. (PubMed)

Is equine asthma the same as heaves?

Heaves is an older term that usually refers to severe equine asthma, previously called recurrent airway obstruction. Equine asthma is broader and includes mild, moderate and severe lower airway inflammatory disease. (Merck Veterinary Manual)

Should a horse with asthma get antibiotics?

Not unless infection is suspected or confirmed. Asthma is usually inflammatory rather than bacterial. If bacterial pneumonia is suspected, your vet may recommend tracheal wash and culture rather than treating blindly. (vet.purdue.edu)

When is equine asthma an emergency?

It is urgent if the horse has laboured breathing at rest, flared nostrils, strong abdominal effort, blue or purple gums, severe depression, fever, collapse or sudden worsening. A horse working hard to breathe needs veterinary care promptly.

Final Thoughts

Equine asthma is common, but it is not always obvious. Severe cases may be recognised from the stable door. Mild cases may hide behind poor performance, occasional cough or slow recovery after work.

The best diagnostic approach is not guessing. It is history, clinical exam, environmental assessment, BAL cytology when appropriate, endoscopy or tracheal wash when needed, and imaging when differentials such as pneumonia or fibrosis need to be ruled out.

Blood biomarkers are exciting, but they are not ready to replace BAL. They may become useful support tools, especially in field settings, but a negative blood biomarker result should not make you ignore a horse with compatible signs.

The main takeaway is simple: diagnose before medicating, rule out infection before reaching for steroids, and remember that dust control is not an optional extra. It is the foundation of asthma care.

If your horse has a chronic cough, poor performance, breathing effort, abnormal BAL results or confusing respiratory signs, ASK A VET™ can help you understand what to monitor and what questions to ask your treating vet before the next diagnostic or treatment step.