Equine asthma (EA) is the 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.

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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

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