Dr. N. Ed Robinson, is the Matilda Wilson Professor of Large Animal Clinical Sciences and directs the pulmonary laboratory at Michigan State University College of Veterinary Medicine. His research has focused on pulmonary disease in horses.

Dr. Robinson began with a review of the anatomy and physiology of the equine pulmonary system. Did you know that the horse breathes 375 gallons of air per minute? The majority of the speech was dedicated to what is known as "heaves", or chronic obstructive pulmonary disease (COPD). The now accepted terminology for this condition is Recurrent Airway Disease (RAD). Horses that are stabled and fed indoors are subjected to organic dust; composed of molds, fungal parts, rodent feces, insect parts, mite parts, and endotoxin (bacteria parts). The equine respiratory system responds to the foreign particles in three major ways: cough, bronchospasm and increased production of mucus. When these protective mechanisms overreact to the environmental stimuli, the horse is said to have "heaves". Clinical signs of heaves are a persistent cough and difficulty expiring air, often resulting in exercise intolerance.

It is estimated that 27 percent of Thoroughbreds in training in the United States have heaves. Even higher estimates of 33 percent have been reported in the United Kingdom and 54 percent in Sweden. Research has also shown that there may be a genetic component to the disease. When the condition persists, irreversible thickening of the bronchioles occurs. Dr. Robinson’s research has demonstrated that these irreversible changes may occur earlier than previously thought, within days of the initial episode of airway bronchospasm. Studies conducted at Michigan State University demonstrated that short exposures (7 hours) to an environment high in organic dust, such as a barn, may initiate the clinical signs of heaves. After a single exposure, 6-7 weeks of pasture rest is typically required for the heavey horse to recover.

Changing the environment in which the horse lives is the best way to treat heaves. Housing horses outdoors, with an open-sided shed for protection from the wind and rain is ideal. Horses that are particularly sensitive should not be fed hay (high in organic dust), but instead fed a complete feed or alfalfa pellets. If horses must be housed indoors, they should be bedded in wood shavings or shredded paper and fed a complete feed or alfalfa pellets. Dr. Robinson also believes that shipping in a trailer exposes the horse to high levels of organic dust. Studies have demonstrated that if long trailer rides (>6 hours) are planned, it is best to allow the horse to rest off the trailer for at least 6-8 hours to clear its airways of organic dust and debris. A horse will do this naturally by lowering its head or lying down.

Many times it is not feasible to house horses outdoors or feed a complete feed, and in addition, a few horses may not respond to even these measures. Under these circumstances it becomes necessary for pharmacologic management of horses with heaves. There are several classes of therapeutic pharmaceutics which are important in the treatment of heaves. The first-line treatment is steroids, but more attention is now being placed on the quaternary ammonium products (ipratropium bromide) and the b -2 adrenergics (clenbuterol, albuterol, salbuterol, etc.). The steroid that is most commonly used first in the treatment of an episode of heaves is dexamethasone, a very potent steroid that acts quickly to reduce airway inflammation and suppress the allergic immune response.

According to Dr. Robinson, human asthmatics are now treated earlier in the course of their disease with steroids to prevent the irreversible thickening of the airways. Unfortunately, horses cannot be maintained on dexamethasone for more than 7-10 days because of the potential for causing laminitis, a very serious condition of vascular compromise in the horses’ hooves causing chronic lameness and even loss of structural support of the foot. The quaternary ammonium compounds have not yet been developed for use in the horse, but research efforts are focused on these compounds to explore their potential bronchodilatory effects in the horse.

The b -2 adrenergics, in addition to causing bronchodilation, also are slightly anti-inflammatory and improve mucociliary clearance. Boehringer Ingelheim is the sponsor for Ventipulmin (clenbuterol) syrup, the first b -2 adrenergic approved by the FDA for the treatment of heaves in horses. It is administered twice-a-day at the dose of 0.5 mL per 100 pounds body weight for 3 days. If there is no improvement, the dose is increased incrementally up to 2.0 mL per 100 pounds. If there is still no improvement, the horse is considered a non-responder, and clenbuterol should be discontinued. Ventipulmin can be administered safely up to 30 days.

Boehringer Ingelheim is investigating a variety of bronchodilators and antiinflammatories to be administered using a specially designed face mask, which allows for the administration of aerosolized inhalant products. A disposable rubberized administration device that fits inside the horse's nostril is also being developed by 3M. With the invention of the face mask and the nasal delivery system, inhaled steroids such as beclomethasone and fluticasone will also be possible. Using steroids and b -2 adrenergics (bronchodilators) together has a synergistic effect in alleviating the clinical signs of heaves.

Another drug used in human asthmatics is furosemide, more commonly known as Lasix. It works by causing the release of prostaglandin E2 (a protective prostaglandin) and affecting the non-cholinergic, non-adrenergic receptors in the airways resulting in bronchodilation. The overall message is that effective pharmacologic agents are being developed for treating the horse with heaves and that the inhaled treatments are becoming more practical for use in the horse.

 Credits: US Food and Drug Administration