Avian tuberculosis is caused by the bacterium Myobacterium avium, and is transmitted mainly through direct contract with infected birds, ingestion of contaminated feed and water, or contact with a contaminated environment. Inhalation can also cause respiratory tract infections. All birds, humans, livestock, and other mammals are susceptible to avian tuberculosis, although humans are considered highly resistant. The disease occurs at all times of the year in wild, captive, and domestic birds throughout the North Temperate Zone.
Tuberculosis rarely causes major die-offs, and is difficult to detect in wild birds. Infected birds are often weak, emaciated, lethargic, and exhibit wasting of the muscles. These signs are similar to those of lead poisoning and other conditions. There are no clinical signs specifically for identifying avian tuberculosis. Abscesses and nodular growths are commonly found on the face, legs, wing joints, and deeply embedded tissue, though external lesions are not always present.