Causes and Susceptible Species
Aspergillosis is a disease affecting the respiratory tract of birds and mammals. In regards to wildlife, aspergillosis is almost entirely confined to birds. It is caused by fungi in the genus Aspergillus, most commonly A. fumigatus. These fungi are ubiquitous in the environment and are especially common in soil and decaying plant matter. Birds are constantly exposed to the spores of Aspergillus. Under normal circumstances a bird's immune system will prevent infections from developing. If the immune system has been compromised, or if the bird has been exposed to an overwhelming number of spores, chronic or acute forms of the disease may develop.
The chronic form of the aspergillosis generally occurs in birds that have been weakened and stressed by malnutrition, injury, other disease, or exposure to toxicants. In the wild in New York, chronic aspergillosis is most often seen in gulls, common loons and raptors. It is a fairly common complication in wild birds that are held captive for rehabilitation.
Symptoms and Diagnosis
Chronic aspergillosis typically starts as small plaques on air sac walls. Plaques may grow, coalesce and completely cover the interior lining of air sacs. They may also form large rubbery masses that envelope blood vessels, particularly in the vicinity of the heart. Mature lesions often include sites of spore production, manifested by a dusty-looking, grey-blue/green surface; i.e. looks like fruiting mold on spoiled food. Despite, the fairly long-term growth in parts of the respiratory tract, actual lung involvement seems to occur only in the terminal phase of disease progression. Clinically, birds with chronic aspergillosis are thin (especially breast muscle), and are often reluctant to fly or are incapable of sustained flight.
The acute form of the disease is triggered by inhalation exposure to massive numbers of spores. In New York outbreaks are occasionally seen in potentially granivorous waterfowl (mallard ducks, Canada geese) feeding on moldy silage. Acute aspergillosis directly affects the lungs and is characterized by the development of small (1-3 mm diameter), yellow-white nodules throughout the lungs. The disease progresses rapidly over a period of several days and the birds typically die without noticeable weight loss.
Preventing the Spread of Aspergillosis
People who feed birds have been traditionally warned about feeding birds moldy birdseed or bakery products. This directive has a sound basis and may in part account for the fact that we rarely see aspergillosis in songbirds, including those found sick or dead near bird feeders (see Salmonellosis and House Finch Conjunctivitis). Live or dead birds with aspergillosis pose no significant threat to human health.