Equine Asthma and Performance in Horses
The Whole-Airway Problem Hiding in Plain Sight
Breathing is one of the most important yet overlooked factors affecting equine performance.
Even mild airway inflammation can increase the work of breathing, reduce oxygen delivery, and alter how a horse performs under saddle. Understanding how the entire airway system functions—from the nostrils to the lungs—helps explain why respiratory health plays such a critical role in stamina, recovery, and athletic performance.
What owners, trainers, competitors, students, and veterinarians should know
Many horses with early or subclinical respiratory disease do not show obvious signs of illness. Instead, they simply fail to perform to their full potential.
They may require longer warm-ups than they used to. They may feel flat or inconsistent under saddle. Some tire earlier in the work or recover more slowly afterward. Some cough occasionally—but many do not cough at all.
Often the first change isn't physical—it's behavioral. The horse may seem reluctant, inconsistent, or "behind the leg."
This is rarely laziness. More often it reflects the increased effort required to breathe.
Equine asthma is now recognized as one of the most common and under-recognized causes of poor performance in horses. Modern research also shows that it is not simply a lung disease. It affects the entire airway system.
From the nostrils to the alveoli, the horse breathes through one continuous airway. When any part of that system becomes compromised, the entire performance engine is affected.
What Is Equine Asthma?
Equine asthma is a chronic inflammatory disease of the lower airways that interferes with airflow and oxygen exchange.
Historically, several different terms were used for these conditions, including:
- Inflammatory Airway Disease (IAD)
- Recurrent Airway Obstruction (RAO)
- Chronic Obstructive Pulmonary Disease (COPD)
- Heaves or "broken wind"
Modern veterinary medicine recognizes these conditions as a spectrum of the same disease, now referred to as equine asthma (EA).
Equine asthma is generally classified into two forms:
Mild-to-Moderate Equine Asthma (mEA)
Previously known as inflammatory airway disease.
Severe Equine Asthma (sEA)
Previously known as recurrent airway obstruction or heaves.
How Common Is Equine Asthma in Horses?
Equine asthma is extremely common in performance horses.
Research across disciplines shows:
60–80%
of horses show airway inflammation consistent with mild asthma
~80%
of racehorses meet cytologic criteria for mild asthma
14–20%
of adult horses develop severe asthma
Additional Signs of Equine Asthma
- Tracheal mucus is frequently seen in racehorses, jumpers, dressage horses, Standardbreds, and event horses
- Cough is present in only about 30% of cases
Many horses with asthma appear healthy. They simply do not perform to their potential.
Environmental Triggers of Equine Asthma
Equine asthma is largely driven by environmental exposure to inhaled particles.
These triggers commonly include:
- dust from hay and bedding
- mold spores
- pollen
- organic debris
- endotoxins from bacteria
- barn and arena dust
These particles are typically smaller than 4 microns, allowing them to reach the lower airways.
Viruses such as equine herpesvirus and equine influenza can worsen airway inflammation but are rarely the primary cause of asthma.
Common Environmental Patterns
Stable-Associated Equine Asthma
Often occurs during winter when horses spend more time indoors.
Typical triggers include:
- hay dust
- bedding particles
- poor ventilation
- dusty arenas
Many horses improve when moved outdoors where air quality is better.
Summer Pasture-Associated Equine Asthma
Some horses develop respiratory signs primarily during warm, humid seasons.
Triggers often include:
- grass pollen
- fungal spores
- high humidity
This pattern is frequently seen in the southeastern United States.
Affected horses may actually improve when brought indoors.
The Equine Airway: One Continuous Breathing Tube from Nostrils to Lungs
Why Asthma Increases the Work of Breathing
When airflow becomes restricted anywhere in the airway, a chain of physiological changes occurs that directly affects athletic performance.
Airway Remodeling
Chronic inflammation causes airway walls to swell. Over time this leads to airway remodeling, increased smooth muscle mass, and mucus accumulation. Even small reductions in airway diameter dramatically increase resistance to airflow.
Increased Inspiratory Effort
As resistance rises, the horse must generate greater negative pressure to inhale. This draws more energy away from locomotor muscles and increases respiratory muscle fatigue.
Upper Airway Collapse
The stronger suction force is transmitted throughout the entire airway. Structures including the nostrils, pharyngeal walls, soft palate, and vocal folds become susceptible to dynamic narrowing during exercise.
Reduced Oxygen Delivery
Inefficient ventilation and air trapping limit the volume of oxygenated blood reaching working muscles, directly reducing aerobic capacity and accelerating fatigue onset.
In practical terms, the horse must work harder simply to move air.
Upper and Lower Airway Interaction
When lower airway resistance increases, the horse must generate stronger negative pressure during inspiration to draw air into the lungs.
That suction force is transmitted throughout the entire airway—from the lungs back through the trachea, larynx, pharynx, nasal passages, and nostrils.
Certain regions of the upper airway are particularly susceptible to dynamic narrowing when inspiratory pressures become high. These include:
- the nostrils (external nares)
- the rostral nasal passages near the naso-incisive notch where bony support is limited
- the pharyngeal walls
- the soft palate
- the vocal folds and aryepiglottic folds
When inspiratory pressures become sufficiently negative, these tissues may partially collapse or lose coordination during exercise. This helps explain why horses with lower airway inflammation may also develop dynamic upper airway dysfunction, particularly during periods of increased respiratory demand.
How Equine Asthma Affects Performance
Airflow is the currency of athletic performance.
When airways become inflamed or obstructed:
- oxygen delivery decreases
- work of breathing increases
- fatigue occurs earlier
- recovery becomes slower
Common performance clues include:
- longer warm-up periods
- loss of finish late in work
- reduced impulsion
- inconsistent performance
- subtle attitude changes
Many horses with mild asthma appear healthy but consistently perform below their potential.
Equine Asthma and EIPH
Lower airway inflammation has also been associated with exercise-induced pulmonary hemorrhage (EIPH).
When airway resistance increases:
- intrathoracic pressure becomes more negative
- pulmonary capillary pressures rise
- stress on the alveolar-capillary interface increases
These changes can increase the likelihood of pulmonary capillary rupture during intense exercise.
Diagnosing Equine Asthma
Diagnosis typically requires combining several pieces of information.
Key tools include:
- clinical history
- physical examination
- airway endoscopy
- bronchoalveolar lavage (BAL) cytology
These tools allow veterinarians to evaluate airway inflammation, mucus accumulation, and airway structure.
Managing Equine Asthma
Effective management usually requires a multimodal approach.
Environmental Management
Environmental control is the most important intervention. Strategies include:
- reducing dust exposure
- improving barn ventilation
- feeding steamed hay or haylage
- using low-dust bedding
- controlling arena dust
Medical Therapy
Medications may include:
Corticosteroids
- dexamethasone
- fluticasone
- beclomethasone
Bronchodilators
- clenbuterol
- albuterol
Bronchodilators improve airflow but do not treat inflammation, which is why they are typically used with corticosteroids.
Key Take Home Points
- Equine asthma is common and frequently under-recognized
- Many affected horses appear healthy but perform below their potential
- Airway inflammation increases the effort required to breathe
- The upper and lower airways function as a single system
- Effective management requires attention to the entire airway—from nostrils to lungs
