Adults with asthma struggle to breathe when they are exposed to dust and allergens. They can exhale without too much difficulty, but their inflamed lungs with narrowed airways make it hard to inhale enough oxygen, and the mucus in their airways leads to coughing.

When horses have the same clinical signs, the condition is often referred to as “heaves” (though the official name was recently changed to, simply, equine asthma). In Ontario, Canada, where many horses are stabled indoors regularly, it’s a very common condition.

“Any barn owner with 10 or more horses will know about heaves,” says Dorothee Bienzle, DVM, PhD, Dipl. ACVP, a pathobiology professor at the University of Guelph’s Ontario Veterinary College.

It’s unknown why some horses develop the condition and others don’t. A susceptible horse will become sick if it is kept in a stall with dusty air, or if it is fed hay that is dusty or moldy, while other horses that are similar in age and condition remain healthy