Equine asthma is a term now used to include several respiratory diseases.  Inflammatory airway disease is a common condition in performance horses, and it is now called mild to moderate equine asthma, whereas recurrent airway obstruction is now called severe equine asthma.  Recurrent airway obstruction was also once called COPD or heaves. These name changes make everything confusing.  There is also a summer pasture-associated airway obstruction syndrome in grazing horses that is now part of the equine asthma syndrome.  I think that placing all of these diseases under asthma is more confusing as they are all different to some degree and this makes it more difficult to describe them. 

Regardless, all of these diseases cause inflammation in the lungs. All except the pasture-associated disease is related to barn confinement and the higher amount of hay dust and other particles inhaled, as well as higher concentrations ammonia found in a stall.  To determine the amount of particles ingested, a study was performed in which horses were fitted with instruments around the nostrils to sample the air that they breathe. Results indicated that inflammatory airway disease is due to hypersensitivity to inhaled allergens.  Many times hay nets are used in stalls to assist feeding and to slow the eating process, which helps alleviate boredom.  However, the researchers found a four-fold higher dust exposure in horses eating from hay nets inside the stall.  Horses eating hay from nets essentially keep their noses in the hay the entire time, which they do not do when fed off the ground.  So if you have a horse with respiratory disease, feeding hay off the ground may be a better option than feeding from a hay net, especially if the horse is fed in a stall.