Many names and acronyms have been used over the years to describe airway conditions characterized by inflammation and airway hyperreactivity. Among them: broken wind, heaves, COPD (chronic obstructive pulmonary disease) and IAD (inflammatory air disease).
In hope of bringing some continuity to discussions, the American College of Veterinary Internal Medicine recently suggested that all the labels be lumped under the term “equine asthma syndrome.”
Inflammatory Airway Disease (IAD) and Recurrent Airway Obstruction (RAO)
It’s been common practice to refer to equine airway problems as distinct diseases. As such, IAD referred to a problem affecting young horses, whereas RAO was more commonly used to describe the noninfectious respiratory problems of older horses. The term COPD once was in common use, but the disease in horses is quite different from human COPD, so it fell out of favor.
Younger horses with IAD typically were normal at rest but coughed and displayed exercise intolerance when active. On the other hand, RAO horses usually were not normal at rest; even at rest they coughed and had an increased respiratory rate. IAD horses could completely recover, even without treatment, whereas RAO horses usually required long-term management and could develop permanent sequela.
In older horses, RAO typically was associated with narrowing and spasms of the air passages (bronchoconstriction and bronchospasm) and excessive mucus production. They commonly had a chronic cough and nasal discharge, and they could have difficulty exercising or breathing.
Over time, due to muscular hypertrophy from the effort required for breathing, a heave line could develop along the bottom edge of the ribs. Severe, chronically affected horses lost weight or appetite. Fever was rare unless a secondary infection was involved. RAO was seen primarily in horses kept in barns and fed hay, and in those that lived in pasture. The latter condition was known as summer pasture- associated obstructive pulmonary disease, or SPAOPD.
Under current guidelines, these horses are properly described as having equine asthma syndrome.
Most evidence suggests that equine asthma results from hypersensitivity to inhaled antigens in the horse’s lungs—similar to what occurs in human asthmatics. The disease has both allergic and inflammatory components, and it is set off by dust, mold and endotoxin, a substance found in hay and straw.
Equine asthma has worldwide distribution, but in the United States it’s most commonly seen in the Northeast and Midwest, affects both sexes and usually begins from ages 9 to 12. Stabled horses are most commonly affected in the winter and spring, whereas horses whose clinical signs are exacerbated by pasture are seen mostly in the summer or early fall.
A genetic component to equine asthma appears to be present as well.
Typical clinical history and signs lead to a diagnosis of equine asthma in most horses. Myriad diagnostic tests may be employed to confirm the presence of the disease and refine the diagnosis, but they may not be needed in many cases.
Endoscopy of the upper airway and trachea usually reveal signs of inflammation and mucous accumulation. Bronchoalveolar lavage (BAL) usually uncovers excessive accumulations of neutrophils. Radiographs may be useful to evaluate the lungs, especially in horses that fail to respond to therapy.
For horses with equine asthma, lifelong environmental and dietary management is critical. Management goals are to reduce the horse’s exposure to dusts and mold, and to improve ventilation. Environmental and dietary changes may be the only treatment needed for horses with mild to moderate disease.
Hay and straw are particular culprits. In an attempt to avoid dusty barns, full-time pasture is a common recommendation for equine asthma horses, but round bale hay, which can be high in endotoxin and organic dust content, can be a cause of treatment failure. On the other hand, horses that develop asthma in the pasture usually should be kept in a well- ventilated barn during the summer and fall.
Hay should not be stored above stalls, and floors should be cleaned when horses are not in the stall. Straw bedding is a poor choice for horses with equine asthma; low-dust bedding is preferred. Soaking hay in water can help some individuals but worsen signs in others. Removing hay and substituting with a pelleted complete feed may be a good option.
Without good environmental management, medical management can be futile.
The two most commonly used medications for equine asthma patients are corticosteroids and bronchodilators. Corticosteroids, administered either systemically or by aerosol, can rapidly improve lung function, but some horses require chronic administration in order to keep clinical signs of disease at bay.
Dexamethasone often is used in therapy, and it can be given orally or by intramuscular injection. For less severe cases, oral prednisolone may be considered. Prednisolone usually is considered less potent, but perhaps less likely to cause side effects. Oral prednisone is poorly bioavailable in horses and usually is not recommended.
Aerosolized corticosteroids include beclomethasone diproprionate or fluticasone proprionate. They directly target inflammation in the lungs but require a significant investment in medication and in devices used for administration, such as the Equine Haler or Aerohippus.
Because aerosolized corticosteroids have a much lower total dose, they may be an option in horses for which side effects of systemic corticosteroid administration are a concern. In severe cases, aerosolized medication may be used concurrently with systemic therapy.
Aerosolized bronchodilators such as albuterol provide immediate relief of bronchospasm in some horses and can aid in mucous clearance. Oral bronchodilators such as clenbuterol are commonly employed but may lose effectiveness if given chronically. However, since bronchodilators have no anti-inflammatory activity, they should not be a standalone treatment for equine asthma.
Depending on the clinical signs and severity of equine asthma, affected horses may be managed successfully, not only for trail or pleasure riding, but also for competition. Successful management requires dedication on the part of horse owners, and control of equine asthma requires a lifelong commitment to the horse’s health.
Dr. David W. Ramey is a Southern California equine practitioner who specializes in the care and treatment of pleasure horses. His website is www.doctorramey.com.
Originally published in the February 2017 issue of Veterinary Practice News. Did you enjoy this article? Then subscribe today!