The young falcon, taken from a wild nest, had the best of care. She dined only on the finest of quail, had immaculate housing, and was treated like royalty. When she reached the age of four months she began her training as a falconry bird. She progressed quickly, learning to come to a swinging lure and take a high pitch. Suddenly, the beautiful bird began to lose weight, with a weight drop of 20% in only 5 days. She became lethargic and began to have trouble breathing. Soon she stopped eating altogether. The antifungals that had been prescribed had little effect to reverse her decline. She died the next afternoon, felled by the fungus growing inside her. Such is the face of avian aspergillosis, the stealthy killer.
The unlikely pairing of fungus and falconry has ancient roots. The “Sport of Kings,” falconry has long been plagued by the lethal fungal disease aspergillosis. Since the early 1800s we have known of this disease in wild birds, and it was probably the reason for the historical difficulty in obtaining and keeping the elegant white gyrfalcons of royalty.1 In raptors, Aspergillus fumigatus is the culprit in 95% of cases.2 Unfortunately, this fungus is a relatively common environmental contaminant because of its usual role as a saprobic decomposer2. In any given year, a large percentage of wild hawks may test positive for Aspergillus spores in a throat swab1, but few will develop disease. Some species of raptors are more susceptible than others, with goshawks, gyrfalcons and redtailed hawks having a high risk of infection.
Aspergillus fumigatus is an opportunistic fungus. It does not usually cause disease in a healthy hawk. However, hawks which are at a low weight, injured or sick are all under stress, decreasing their resistance to infection. Young first-year raptors are naturally under a lot of stress as they attempt to survive their first winter, with up to 80% mortality. The young raptors appear to be most prone to infection around 2 to 4 months of age.2 Unfortunately, this is exactly the age at which most raptors begin their training for falconry, resulting in this fungus’ status as the most common and most lethal disease of captive birds of prey.4
Young birds used in falconry face relatively few threats from predators, injury or illness compared to their wild cousins. Most are kept only one year and are then released to contribute to the wild breeding populations. The problem is that keeping raptors in captivity creates a few challenges in housing in order to prevent aspergillosis. Perhaps one of the most important aspects of prevention is proper housing design.
A captive hawk’s home is called a mews. Generally this is a building with both an inside and outside portion and many perches — there are countless variations. A large part of the problem with aspergillosis in falconry birds used to be caused by the substrate placed in their housing. In conditions which are either too moist or too dry and dusty, high levels of spores can be carried into the air. Problematic substrates include sand, sawdust and peat, all of which do not drain well when wet and can become dusty when dry.3 Today most raptors are kept on pea gravel or other well-drained surfaces to prevent Aspergillus growth. In addition, buildings are designed with airflow and hygiene in mind in order to prevent the buildup of mutes, or feces, and stale air.
Raptors most often develop one of two different forms of aspergillosis. In the acute form, a bird inhales overwhelming doses of spores from a single source, which rapidly results in large numbers of granulomas, or inflammatory tumors, in the lung.3 Affected birds usually die within a week, although cases of less than 48 hours have been recorded! The other form can be far more insidious, in that affected birds may not show obvious symptoms of infection in early stages. Gradually, large granulomas form in the lungs while furry mats of sporulating mold develop in the air sacs and airways. Despite sporulation inside the live bird, no direct transmission has been recorded.3 Some unusual, chronic cutaneous and ophthalmic (eye) forms are also found in raptors. The chronic forms can be treated if caught in time, but treatment is very difficult and misdiagnosis is common.4
The best treatment for aspergillosis in infected birds is generally through high doses of antifungal drugs: azoles or amphotericin B.3 Generally birds with acute infections never recover, while those harboring chronic infections must be carefully monitored to make sure that the infection is fully cleared before discontinuing drug treatment. In any diagnosis of aspergillosis in a raptor, prognosis is guarded due to the number of pneumatic bones. These are hollow and connect with the respiratory system, providing deep loci for Aspergillus invasion, while at the same time sheltering the fungus from antifungal drugs.
Due to the difficulty in treating aspergillosis, falconers have now turned to preventative medicine in new birds. Treatment usually lasts for two weeks with oral itraconazole or nebulized clotrimazole.2,3 The potential problem with preventative treatment is the possibility of fungal resistance if the bird already had a low-grade chronic Aspergillus infection which was not cleared in two weeks. Some falconers have begun to hybridize more susceptible raptor species with much hardier species.2 The fully imprinted, captive-bred offspring ideally have the disease resistance of one parent combined with the size or strength of the other parent, as is seen in the many gyrfalcon hybrids.
While there are ways to preventatively treat for aspergillosis today, this disease still claims the lives of many raptors each year. It can suddenly overtake a bird in perfect health with alarming rapidity, or slowly eat away at the lungs of a young falcon with hardly a sign of the disease externally.
Author Abby Duvall took these photos of healthy falcons at a New York State Falconry Association meet. The first is an anatum peregrine falcon (Falco peregrinus anatum); the second is a northern goshawk (Accipiter gentilis).
- Redig, P.T., M.R. Fuller, and D.L. Evans. 1980. Prevalence of Aspergillus fumigatus in Free-Living Goshawks (Accipiter gentilis atricapillus). Journal of Wildlife Diseases 16: 169-174.
- Abundis-Santamaria, E. Aspergillosis in Birds of Prey.
- Joseph, V. 2000. Aspergillosis in Raptors. Seminars in Avian and Exotic Pet Medicine 9: 66-74.
- Ward, F.P., D.G. Fairchild and J.V. Vuicich. 1969. Pulmonary Aspergillosis in Prairie Falcon Nest Mates. Journal of Wildlife Diseases 6: 80-83.