Pasture-associated severe equine asthma (pasture asthma) is a respiratory disease that affects adult horses housed on pasture in hot, humid climates. The disease is relatively common in the southeastern United States, but it has been reported in other hot, humid locations. Its prevalence is unknown.

Potentially life-threatening asthma exacerbations characterized by difficulty breathing and occasional coughing generally begin in summer (July). Fewer horses experience this form of asthma in the spring. Affected horses might have a history of seasonal cough and/or exercise intolerance. Despite continued grazing on the inciting pasture, asthma remission occurs when high temperatures and humidity decrease (October/November).

Veterinarians have previously referred to pasture asthma as chronic obstructive pulmonary disease, heaves, pasture-­associated obstructive pulmonary disease, and pasture-associated recurrent airway obstruction. The name “asthma” is most appropriate because affected horses exhibit clinical features of severe forms of human asthma. Researchers have not yet identified pasture asthma’s causative agent, but the seasonality of signs coupled with evidence of improvement when caretakers isolate affected horses from inciting pasture implicates inhaled particulates from the pasture ­environment. 

The environmental factors that give rise to pasture asthma differ from those known to elicit severe equine asthma, as originally described in horses housed in dusty indoor environments in cooler climates and/or fed moldy hay. Management recommendations for this latter group of asthmatic horses include pasture turnout and no hay. In contrast, horses with pasture asthma improve within hours to days of being brought into stalls, even in the absence of therapeutic interventions such as bronchodilators and corticosteroids. If owners and veterinarians don’t recognize the factors that differentiate pasture asthma from barn-associated asthma, they might follow environmental management recommendations intended for the latter, aggravating horses’ pasture asthma rather than alleviating it. 

If a horse is having difficulty breathing while housed on pasture during hot, humid conditions, move him to an indoor environment as distant from the pasture as possible, confirm he does not have a fever, and hose him off. This condition warrants a prompt visit from your veterinarian, who will check that the horse does not have other conditions that cause respiratory distress. Your vet will then document whether respiratory distress improves after administering a bronchodilator and will likely administer ­corticosteroids.

A horse’s improvement after being isolated from pasture particulates in a stall environment is an important aspect of confirming a pasture asthma diagnosis and identifying the best course of management. A history of asthma exacerbations during hot, humid seasons, followed by remission during the winter can also confirm the condition. Some horses react to inhaled seasonal particulates so severely as to necessitate drastic isolation measures, such as in an air-conditioned stall. These horses might continue to show signs while stalled but are worse on pasture and require intensive environmental and therapeutic management.

Indeed, the lynchpin of pasture asthma management is isolation from aero­allergens in the pasture. Owners of affected horses house them and commonly feed good-quality hay (not dusty or moldy) that has been dunked briefly in water—simply spraying with water does not reduce the dust horses inhale when eating hay. Because affected horses’ airways are overly responsive to inhaled particulates, take additional steps to minimize dust in the indoor environment, such as using low-dust shavings or pelleted or paper bedding and minimizing dust from sweeping (and never using a leaf blower). Avoid using straw bedding and round bales, even during seasonal asthma remission, due to their high mold and dust content.

Recognizing mild respiratory compromise and removing affected horses from pasture prior to severe asthma episodes can minimize the need for therapeutic intervention. Again, therapeutic interventions include inhaled and/or systemic corticosteroids and bronchodilators. However, these therapeutics tend to be effective for just a short, transient period in the absence of vigilant environmental management.

Horses with pasture asthma generally cannot be ridden during warm seasons due to the severity of their respiratory compromise. Without aggressive environmental management, these animals worsen each year and are eventually euthanized. Vets have noted very rare cases of resolution when horses remain in the same environment. However, moving affected horses to more temperate and continental climates early in the course of the disease gives them the best quality of life, potentially allowing them to return to their prior levels of athletic performance.