In severe cases, heavy breathing and coughing up of bloody mucus are seen. Sometimes blood is apparent on the walls where affected birds have been coughing. Birds may recover from the illness within 2 weeks but can remain carriers of the virus for long periods of time afterwards. These carrier birds become a threat for other poultry owners. A bird that appears to be healthy may be introduced into a new flock and two weeks later all of the other birds are sick.
Stress also helps to bring out the disease as it makes carrier birds shed the virus.
Mixing new birds together, poor ventilation, inadequate space or food, or changes in temperature may all help in causing the disease. Hobby flocks are more often identified with this disease than commercial flocks. The commercial poultry industry has developed practices of extreme isolation of their flocks from the point of hatching through the various phases of production. Biosecurity is the expression used for the combined practices of physical isolation, routine disinfection, restricted human and animal access, and environmental control. Most hobby flocks don't follow these practices and the resulting movement of people and birds from one backyard to another spreads disease through these flocks.