Horses and Asthma: An Overview

“Equine asthma syndrome” is the new term for the spectrum of disorders resulting from chronic airway inflammation. This syndrome includes everything from mild asthma, also called inflammatory airway disease (IAD) in young horses, to severe asthma, widely referred to as heaves (chronic obstructive pulmonary disease, COPD) and pasture- or summer-associated airway disease.

Horses with asthma, regardless of severity, show similar clinical signs: chronic coughing, excess mucus production, and poor performance.

“These clinical signs primarily develop following exposure to aerosolized particles in the horse’s environment, especially in stabled horses,” explained Laura Petroski, B.V.M.S., staff veterinarian for Kentucky Equine Research (KER) in Versailles.

Specific examples of aerosolized particles include fungi, molds, endotoxin (toxins produced by certain types of bacteria), beta-D-glucan (a type of fiber), ultra-fine particles (<100 nm in diameter), microorganisms, mite debris, vegetative material, inorganic dusts, and noxious gases such as ammonia from urine.

Following repeated inhalation of such particles, inflammation develops in the airways. Various cells of the immune system can be found in the fluid collected from an affected horse’s trachea or deeper airways (via tracheal or lung wash—bronchoalveolar wash) such as neutrophils or eosinophils. Inflammation and mucous in the airways impedes airflow and the ability for oxygen to diffuse across the lining of the lung tissues, the alveoli, to enter the bloodstream.

“In addition to the influx of mucous and various types of white blood cells, the composition of the actual walls of the airway also change,” said Petroski.

Specifically, the wall of the airway becomes thicker, and the muscles lining the walls that control the diameter of the airways also thicken. Like the presence of white blood cells and mucous, these changes make it more difficult for oxygen to diffuse from the airways into the bloodstream.

As summarized by one research group*, equine asthma “is a naturally occurring chronic lung inflammatory disease of horses exposed to airborne molds and particulate material.”

“Scientists also suspect that a genetic component to equine asthma exists,” Petroski added.

While we may not be able to control genetics, we can control inflammation and smooth muscle relaxation—the mainstays of asthma management. Indeed, the most up-to-date consensus statement created by a group of equine internal medicine specialists** recommend the following:

• Anti-inflammatory corticosteroids to reduce inflammation
• Bronchodilators to increase the diameter of the small airways; and
• Omega-3 fatty acids (fish oil) for additional anti-inflammatory properties.

KER offers EO-3, a potent marine-derived oil rich in both DHA and EPA that can be top-dressed on a horse’s feed.

*Tessier, L., O. Côté, M.E. Clark, et al. 2017. Impaired response of the bronchial epithelium to inflammation characterizes severe equine asthma. BMC Genomics. 18(1):708.

**Couëtil, L.L., J.M. Cardwell, V. Gerber, et al. 2016. Inflammatory airway disease of horses—Revised consensus statement. Journal of Veterinary Internal Medicine. 30(2):503-515.