The coughing had started three weeks earlier. It was a relatively quiet but exceptionally persistent cough that the owners heard whenever they were near the mare. Then she began to have very obvious difficulty breathing. The broodmare actually had to strain to push the air out of her lungs before she could pull in another breath, and the effort showed plainly in a line of tensed muscle stretching along her abdomen. From the flared nostrils and heaving sides, you’d think she had just finished a mile gallop at racing speeds when, in fact, all she could manage was a laborious walk across the pasture each evening to be put up for the night.

There was no accounting for the mare’s respiratory difficulty. Other than the fact that she had some residual lameness from an episode of laminitis several years before, she had always been perfectly healthy. She didn’t have any fever or other indication that she was suffering from a respiratory infection. Her appetite was good. She was alert and content, so long as she didn’t have to move about very much. But that nagging cough, the wheezes and the flared nostrils were unmistakable evidence of respiratory distress.

Suspecting that their mare had developed an allergy to something in her stable environment, the owners scrupulously dealt with the usual suspects: They changed her bedding from straw to less dusty wood shavings; they presoaked her hay to douse the dust or sometimes fed her cut grass in her stall; they faithfully administered the daily oral doses of the bronchodilating drug clenbuterol that the veterinarian had prescribed to help relax the constricted airways. And they kept the mare turned out more than ever, expecting her to flourish in the fresh, clean air of early summer pasture. The mare had all the signs of chronic obstructive pulmonary disease (COPD), better known as heaves, and these measures to reduce her exposure to the airborne irritants of a closed, somewhat stuffy stable were bound to improve her breathing.

But they didn’t. The cough became explosive, insistent; the breathing grew even more tortured. Finally, in the extreme of respiratory distress, the mare was shipped to a university clinic where more sophisticated diagnostics could be performed. Upon her arrival, she was breathing twice as quickly as normal, her heart was racing at three times average speed, and her gums were deep blue, the color of asphyxiation.

Asphyxiation was precisely the problem, but not because the mare was lacking in fresh air. In fact, the longer she was out on pasture, the worse she got. Something in the great outdoors-perhaps a pollen, a mold, a plant or agricultural chemical-was triggering an allergic reaction in the mare’s airways. The bronchioles, the smallest branches of the respiratory tree leading to the air sacs (alveoli) were squeezing down, swollen by inflammation and clogged with mucus. No wonder the horse had to fight so desperately to move air in and out of her lungs: The passageways were practically plugged.

The diagnosis was an exceptionally severe case of summer pasture-associated obstructive pulmonary disease (SPAOPD), a respiratory condition that can be a dead ringer for heaves, except for the timing and location. In effect, both SPAOPD and COPD are equine asthma, a recurring but reversible narrowing of the airways. Like human asthma, which showed a 42 percent upswing in case numbers and a 32 per- cent rise in related deaths between 1982 and 1992, the incidence of the equine pasture-associated version appears to be increasing. But that may be as much a function of greater awareness of a little- studied disease as of an outright rise in case numbers.

SPAOPD occurs in the southeastern United States, Britain and possibly other areas where heat and humidity activate the yet-to-be-identified outdoor irritants that make life so miserable for hypersensitive horses. There’s no cure, but once a horse has been correctly diagnosed with the condition, the debilitating signs can be controlled with management and medications until the cause or causes fade away with the season.

This article first appeared in EQUUS magazine, issue 273.