Some horses are mildly affected during certain parts of the year. Clinical signs include periodic coughing spells, an accentuated expiratory effort, and wheezing. More severely affected horses can have a deep cough with mucopurulent, copious discharge that either is expelled from the nostrils or contained within the lower airways.
Some horses develop a "heave line" from prolonged respiratory dyspnea and become exercise intolerant. The horse may have flared nostrils and act nervous or anxious. Auscultation may reveal wheezes, crackles and increased lung sounds. Horses with secondary bacterial infections may develop a fever. Horses that are stabled indoors can suffer from sudden "asthmatic attacks" from dust and mold in the environment or feed, or from certain changes in the weather.