Equine asthma (EA) is a relatively new collective term for chronic respiratory signs in horses that range in severity from mild to severe.

These conditions were previously known as inflammatory airway disease (IAD) or recurrent airway obstruction (RAO), respectively.

Equine asthma is characterized by inflammation and mucous production in the lungs, which leads to lower airway obstruction. The horse may cough, and breathing may or may not be laboured.

Asthma is most commonly observed in horses stabled over winter months that have become hypersensitive to dust, airborne allergens, mold spores, or mites in stable bedding or stored hay. [1][2]

Unstabled horses may also react to pollens when at pasture in summer, with the condition known as Equine Pasture Asthma (EPA). [3]

The formerly used terms Chronic Obstructive Pulmonary Disease (COPD) and Summer Pasture Associated Obstructive Pulmonary Disease (SPAOPD) are now obsolete. [2][3][4]

Other terms that have been used interchangeably in the past include equine emphysema and chronic bronchiolitis. [5] These conditions are also now grouped under equine asthma. [4][5]

Equine Asthma

Like some forms of human asthma, equine asthma has an immunological basis. The disease is strongly linked to environmental dust, pollen, and in some cases to preceding bacterial and viral infections.

The exact mechanisms of this immunological response are not yet fully understood, nor is it known why only some horses are affected. [4] Individuals of all ages, breeds and both sexes are affected, although it does appears more frequently in older horses. [6][7]

In horses over 20 years, a declining immune function may be a factor, with a tendency towards the exaggerated inflammatory responses that underpin this disease. [7]

Younger horses may have been affected by early immune events, with the memory component of the adaptive immune system being repeatedly triggered by the same antigen. [7] As clinical cases are usually more advanced or late stage in older animals, it is difficult to identify the earlier disease processes. [3]

Additionally, a genetic factor has been identified in asthmatic horses. The presence of a major gene in families demonstrates that a predisposition to EA can be inherited. [6]

As yet, there is no cure for equine asthma, but in most cases, a combination of treatment and environmental management can bring about reversal through clinical improvements.

Types of Equine Asthma

Asthma in horses is usually classified in one of two ways, based on the degree of hypersensitivity to allergens affecting the lungs:

  • Mild to moderate EA: mainly subclinical, making diagnosis challenging

  • Severe EA: has clear clinical signs and is often chronic, recurring seasonally and worsening if inadequately treated

Environmental Conditions

The main environmental factors triggering the condition further determine the categorization of mild, moderate or severe asthma.

Severe equine asthma is commonly found in stabled horses exposed to dust and molds from poor-quality hay and bedding for long periods of time in winter.

Over 50 different bacterial endotoxins mold species, microbial toxins, storage mites, plant particles, other microbes, and inorganic dust have been identified in stable dust. [2] Affected horses are often aged over 7 years. [4]

Horses at pasture in summer may present mild to moderate signs of equine pasture asthma in response to pollen, tropical grasses, fungal spores, dust from harvesting, or crop burning. [4]

Both mild and severe asthma are strongly linked to environmental dust, and in some cases to earlier bacterial and viral infections such as: [4]

  • Equine influenza virus (EIV)
  • Equine herpesviruses (EHV)
  • Equine rhinitis viruses (ERVs)
  • Streptococcus equisubspecies zooepidemicus
  • Actinobacillus
  • Pasteurella spp

Occasionally, horses may present severe forms of both EA and EPA. [4]

Pathophysiology

As a condition of the lower respiratory airway, equine asthma has a similar pathophysiological progression to its human counterpart. The exact processes involved are unclear, although it is thought to be a multifactorial process.

In non-asthmatic horses, dust particles are expelled from the lungs towards the nasopharynx. If dust particles remain in the lung, they are effectively dealt with by the alveolar macrophages.

These are white blood cells, also known as ‘dust cells’, which form the first line of defence against pathogens in the lung. [8]

Allergic Hypersensitivity

In asthmatic horses, an elevated inflammatory response occurs, and immunoglobulin E (IgE) antibodies are produced by plasma cells. T-cells (white blood cells) are also recruited into the lungs. [3]

With this allergic response, several things happen.

  • The tissues become inflamed. This is due to an increase in neutrophils (white blood cells), which fight infection.

  • The presence of neutrophils also drives mucus production. This is thick and contributes to airway restriction and blockage. [4]

  • In severe cases, inflammation and mucus production cause the airway tissues to swell, resulting in partial airway obstruction.

  • The smooth muscles that line the airways constrict and tighten (bronchospasm), causing an increase in muscle mass.

  • With ongoing bronchospasm, the muscles become hyperplasic, ie. spasmed, hard and enlarged. Some horses have 300% more peripheral airway smooth muscle than before, and 50% more in central airways. [3]

  • Fibrosis (hardening of muscle tissue fibres) restricts the peripheral airways even more. Over time, permanent tissue remodelling may occur. [4]

  • Damage to the alveoli (air sacs in the lungs) can occur, causing the inner walls of the sacs to rupture. This leads to emphysema, with permanent shortness of breath. [4]

Signs & Symptoms

Equine asthma may have a sudden or gradual onset. It often coincides with a management change, when the horse is brought in from the pasture to the stable. Sometimes more than one horse in a stable may be affected.

Equine pasture asthma can commence with the summer months and a rise in pollen. [9]

Mild Asthma

Mild asthma in horses can be subclinical and hard to detect, with possible signs including:

  • Exercise intolerance at higher gaits
  • Reduced performance at higher levels

Moderate Asthma

Moderate equine asthma presents additional signs to the above, including:

  • Mild nasal discharge
  • Labored breathing after exercise
  • Coughing during or after exercise, (not all horses present this symptom)

The condition can be progressive if untreated and the horse is continually exposed to the allergens.

Severe Asthma

Severe asthma in horses is characterized by the following commonly reported signs:

  • Labored breathing at rest (dyspnea) with abdominal lifting (previously known as heaves)
  • Mucoid nasal discharge

Clinical signs of severe EA detectable through veterinary diagnostics include: [4]

  • Increased blood pressure
  • Abnormal respiration sounds, ie. crackles, wheezes, etc., on either one side or both, audible through auscultation (listening with a stethoscope)
  • Increased diameter of the pulmonary artery, visible on ultrasound
  • Increased right ventricular wall thickness, visible on cardiac ultrasound

Diagnosis

Differential Diagnosis

Although equine asthma is an umbrella term, a differential diagnosis is made against several other conditions affecting the lungs, including: [9]

Case History

A tentative diagnosis can be made based on the owner-provided history. This may immediately reveal that severe equine asthma episodes correlate to seasonal triggers.

This is confirmed if the horse improves within hours or days of being isolated from the triggering stable or pasture.

Case history may also reveal other risk factors, including whether the horse’s sire or dam is affected by equine asthma, what forage the horse is fed, and how the feed is delivered. [4]

A history of febrile (fever-causing) illness is also relevant, as well as any recent viral infection. [9]

While case history can be a compelling element for diagnosing severe EA, more diagnostics are needed for mild or subclinical EA. [4]

Auscultation

Listening to the lungs following light exercise may reveal crackling lung sounds in mild EA cases. A simple rebreathing bag may yield the same results. [9] Further tests can then take place.

Tracheal Endoscopy & Tracheal Wash

Practicing veterinarians commonly prefer to use the combination of tracheal endoscopy and tracheal wash. [4]

Tracheal endoscopy enables the visual detection of mucus in the lower airways. It offers a reliable diagnosis for mild to moderate asthma of fewer than 3 weeks duration, as well as severe asthma, and is acceptable to owners and trainers. [4]

A tracheal wash involves collecting a fluid sample from the trachea for cytological examination. As well as enabling pathological confirmation in the laboratory, it allows sequential sampling to monitor disease progression and remission. [4]

Endobronchial Biopsy

Endobronchial biopsies are easy to obtain, and when processed for standard histology, they allow the assessment of airway epithelium, lamina propria and smooth muscle.

However, they do not permit a precise quantification of the deepest bronchial structures (smooth muscle, cartilage and peribronchial tissues). [5]

A limitation is that biopsies show no clear difference between severe EA cases in remission and unaffected horses. [4]

Endobronchial Ultrasound (EBUS)

This is a technique for imaging the bronchial wall using a probe to evaluate the inflammation and remodelling affecting all the airway structures. [5]

Lung Function Testing

Some specialist equine clinics offer lung function testing, which is the ultimate tool for assessing and tracking asthma-affected respiratory function in humans.

Lung function tests can detect severe EA, but are less useful for mild cases in which the baseline lung function is often unaffected. [4]

Bronchoalveolar Lavage (BAL)

This method of examining airway secretions is effective for diagnosing mild, moderate and severe EA.

Saline is infused into the lungs and collected to measure neutrophils, mast cells and eosinophils. Affected horses will have an increase in neutrophil and lymphocyte percentages. [3]

However, there is a lack of established criteria for diagnosing the disease and no set testing protocols for bronchoalveolar lavage. [10]

Other Tests

Several commercial serological tests for identifying asthma in horses involve measuring antibody levels, such as serum-free immunoglobu