Confined, stabled horses have to contend with a lack of fresh air and the inability to continually nibble on fresh grass.
In addition, they're exposed to potential contamination in their environment from dust; more than 50 types of molds; toxins
associated with dried forage, bedding and stale air; and a mixture of noxious gas (ammonia, hydrogen sulfide, methane, carbon
dioxide) and airborne dust—both organic and inorganic.
All these pollutants can negatively impact horses' respiratory systems. Indeed, such horses have an increased risk of developing
a chronic, recurrent and debilitating respiratory syndrome, says Jean-Pierre Lavoie, DMV, Dipl. ACVIM, professor of equine
internal medicine at Université de Montréal, Canada.
This respiratory syndrome, called recurrent airway obstruction (RAO), or heaves, occurs in up to 20 percent of adult horses
in the cold northern hemisphere and in temperate climes. Current thought, according to Lavoie, is that RAO "likely results
from complex interactions between innate and acquired immune responses, environment and genetic susceptibility." Horses with
heaves exhibit a chronic disorder of the airway and present with variable and recurring airflow obstruction, bronchial hyperresponsiveness
and airway inflammation. They also show increased respiratory effort at rest, cough and decreased exercise performance.
"What we know for certain about heaves is that in many horses it's precipitated by stabling," says Ed Robinson, BVetMed, PhD,
Matilda R. Wilson Chair in Large Animal Clinical Sciences at Michigan State University's College of Veterinary Medicine. "Our
current understanding is that it's exacerbated from inhaling the particles from hay, such as stable dust. Heaves is defined
as a recurrent airway obstruction caused by inhalation of inorganic dust."
A horse with labored breathing at rest, without infection as a cause of these signs, is suffering from heaves, says Renaud
Léguillette, DMV, MSc, PhD, Dipl. ACVIM, associate professor of equine internal medicine, Veterinary Clinical and Diagnostic
Sciences, University of Calgary Veterinary Medicine Department. Horses have such a large lung capacity that when their lungs
aren't functioning well enough to properly respire at rest, the problem of inflammation can become extensive.
"Once you see a horse with heaves, it's really too late, as the damage is severe enough to disrupt normal breathing," says
Léguillette. All of the lung tissue is severely damaged, affected by a lot of inflammation and lung tissue remodeling. The
damage is a triad of
- Inflammation—inflammatory cell accumulation in the airway affects the small bronchi and alveoli
- Mucous plugs, or overproduction of mucous in the small airways.
"The three insults combined are really terrible for proper lung function," Léguillette says. "The bronchoconstriction narrows
the airways, plus increased mucous accumulation causes the airway to be constricted and partially blocked, which severely
limits normal breathing."