Case History: Mortality of captive exotic ducks was reported by the owner of an aviary (species and location are withheld from this report to protect privacy). An adult female duck was submitted for cause-of-death determination.
Pathology Case of the Month - Exotic Duck
Gross Findings: The bird was in emaciated body condition with pectoral muscle atrophy and no discernable fat stores. The spleen was enlarged (~1-cm diameter), firm, and yellow, with only a small amount of normal tissue remaining at the periphery (Fig. 1A). The liver contained widely disseminated multifocal to coalescing 1- to 3-mm diameter firm yellow nodules (Fig. 1A). The right lung and intestinal serosa contained similar 1- to 2-mm diameter firm yellow nodules.
Histopathological Findings: Within the liver, there were many granulomas characterized by a rim of epithelioid macrophages surrounding a central accumulation of eosinophilic and pyknotic (necrotic) debris (Fig. 1B, C). On Fite’s acid fast stain, macrophages contained numerous acid-fast positive bacilli (Fig. 1D). Similar granulomas were within the spleen, lung, kidney, pectoral muscle, and intestinal serosa.
Liver, lung, spleen, kidney, intestine: Granulomas, multifocal, chronic, marked, with intralesional acid-fast bacteria
Disease: Avian Tuberculosis
Etiology: Mycobacterium avium subsp. avium (M. genavense is another possible cause)
Host range: All avian species are susceptible; infection is more common in birds in captivity than in free-ranging wild birds. Among wild birds, cranes, pigeons, crows, gulls, and raptors are considered most susceptible. Mammals are also susceptible, including wild mammals such as mink, rabbits, and deer; and livestock such as pigs and sheep. Human infection is possible.
Transmission: Transmission typically occurs via ingestion of contaminated food or water, inhalation, or through skin wounds. Susceptibility and transmission are increased in captive settings under crowded and poorly ventilated conditions. The bacterium can persist for months in the environment, leading to prolonged potential for transmission at contaminated sites.
Clinical signs: Clinical signs are nonspecific and include weakness, lethargy, and emaciation.
Pathology: Gross lesions include emaciation with variably sized yellow to white nodules (granulomas) throughout the viscera, particularly within the liver, spleen, lung, and intestines. Nodules may also be seen within the skin. Microscopically, acid-fast positive bacilli can be found within granulomas. In cases of primary intestinal infection, there may be thickening of the intestinal wall with infiltration of the lamina propria with macrophages containing acid-fast bacteria.
Diagnosis: A definitive diagnosis can be made based on the presence of acid-fast bacilli within granulomas and detection of M. avium subsp. avium by PCR or culture. Mycobacterium avium is slow-growing in culture and most readily isolated from a non-contaminated sample.
Public health concerns: While humans can contract M. avium, they are considered relatively resistant, and human-to-human transmission is not reported. Infection is more common in people with immunosuppression or other underlying conditions.
Wildlife population impacts: Avian tuberculosis is typically a disease of individual wild birds and rarely causes large-scale mortality events in wild populations. However, captive facilities such as zoos or breeding facilities housing threatened or endangered wild species may experience significant mortality.
Management: Because treatment is not effective and the bacterium is long-lived in the environment, it is typically recommended to destroy infected captive flocks and avoid the use of the site to house additional birds for at least two years.
OIE terrestrial manual: Avian tuberculosis. 2018. In: Manual of Diagnostic Tests and Vaccines for Terrestrial Animals, 8th edition. Chapter 3.3.6, p: 860-870. https://www.oie.int/fileadmin/Home/fr/Health_standards/tahm/3.03.06_AVIAN_TB.pdf. Accessed March 2021.
Schmidt, RE, Reavill, DR, Phalen, DN. 2003. Pathology of pet and aviary birds. Iowa State Press, Ames, Iowa. 234 pp.
U.S. Geological Survey. 1999. Field manual of wildlife diseases: general field procedures and diseases of birds, https://pubs.er.usgs.gov/publication/itr19990001. Accessed March 2021.