Pathology Case of the Month - Mallard Ducks

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Case History: In December 2020, a waterfowl mortality event involving approximately 125 mallard ducks (Anas platyrhynchos) was reported in Idaho, USA.

Sick birds were observed to be weak, unwilling to fly when approached, or crashing when attempting to fly away. Labored breathing was observed in one live mallard that died a few hours later. The area where birds were found was undeveloped, and there were grassy fields containing cattle and horses nearby. Six dead mallard ducks were submitted for necropsy examination and diagnostic testing.

Gross Findings: Three of the six mallard ducks were examined grossly. All ducks were in good body condition with adequate fat stores. Gross findings in all birds were similar and included dark red lungs containing disseminated white to tan nodules up to 3 mm diameter (Figs. 1A and 1B); similar nodules were occasionally present on the caudal air sacs. One bird also had a few firm nodules up to ~1 x 1 x 2 cm within the wall of the intestinal tract. 

Photographs of mallard duck lungs with arrows pointing to white to tan nodules.

Figure 1. Photographs from a mallard duck (Anas platyrhynchos) found dead in Idaho, USA.  (A) The lungs are dark red and have multifocal 1-3 mm diameter white to tan nodules disseminated throughout (arrows).  (B) Cut section of the lung showing the nodules within the parenchyma (arrows). (Credit: Jaimie Miller, National Wildlife Health Center. Public domain.)

Histopathological Findings: Tissues from two of the three necropsied mallard ducks were examined microscopically. The ducks had similar histologic findings. Multiple granulomas were present throughout the lung. Granulomas were composed of heterophils, macrophages and multinucleated giant cells surrounding central areas of necrosis. Necrotic areas contained degenerate granulocytes and many fungal hyphae (Fig. 2A, 2B, and 2C). Fungal culture of the lungs from these two ducks resulted in growth of Aspergillus fumigatus from both samples.  The small intestines contained similar fungal granulomas within the walls (Fig. 2D).

Photomicrographs of lung from a mallard duck showing fungal hyphae.

Figure 2. Photomicrographs from a mallard duck (Anas platyrhynchos) found dead in Idaho, USA.  (A) Multiple granulomas (*) with eosinophilic necrotic centers efface the lung. H&E stain.  (B) Granulomas contain many degenerate heterophils and a few multinucleated giant cells (arrows). H&E stain.  (C) Granulomas (*) in the lung contain numerous fungal hyphae. Inset: Fungal hyphae have 3-8 µm-wide, thin parallel walls with regular septations and acute angle branching. GMS stain.  (D) Multiple granulomas (*) similar to those in the lung efface the wall of the intestine. H&E stain. (Credit: Jaimie Miller, National Wildlife Health Center. Public domain.)

Morphologic Diagnosis/es:

  1. Lung: Marked, multifocal, necrotizing, and heterophilic pneumonia with intralesional fungal hyphae
  2. Small intestine: Marked, multifocal, necrotizing, and heterophilic enteritis with intralesional fungal hyphae

Disease: Aspergillosis

Etiology: Aspergillus sp., most commonly A. fumigatus, a saprophytic mold that grows in damp soils, decaying vegetation, organic debris, and feed grains.

Distribution: Worldwide

Seasonality: Most aspergillosis outbreaks in waterfowl occur in the fall to early winter. Individual cases can occur at any time, especially among birds that are concurrently stressed (such as by malnutrition, other disease conditions, recent captures, etc.). A specialized springtime form of aspergillosis occurs in chicks or ducklings raised on game farms or as part of captive-rearing programs; this disease is known as brooder pneumonia.

Host range: All avian species are vulnerable, with young birds appearing much more susceptible than adults. Mammals can also be infected.

Transmission: Inhalation of spores (called conidia) in a contaminated environment. 

Clinical signs: Emaciation, severe and progressive difficulty breathing (gaping or rapid opening and closing of the bill), unthriftiness, and weakness; some may have drooping wings and fail to escape when approached. In cases of epizootic aspergillosis and brooder pneumonia, sudden deaths may occur in previously healthy birds.

Pathology:

Acute: Good body condition and fat stores. Air sacs are usually thickened. Lungs are dark red, firm and often peppered with 1-2 mm yellow nodules.

Chronic:  Lungs and air sacs have variably sized, flattened, yellow plaques with a cheesy appearance and consistency. Air sacs may have extensive fungal growth in the form of a velvety, blue-green or grey fungal mat (appears similar to bread mold).

Diagnosis: Whole carcasses should be submitted for examination. Diagnosis is based on finding typical gross lesions and isolating the fungus from tissues. Microscopic examination can identify Aspergillus sp. but cannot determine the specific species.

Public health concerns: Aspergillosis is not contagious from person-to-person. However, immunocompromised humans can develop a rapid, acute infection following environmental exposure. It is unlikely that intact infected bird carcasses would provide sufficient exposure to result in human infection.

Wildlife population impacts: Acute epizootics of aspergillosis can involve hundreds of wild birds, whereas chronic aspergillosis affects individual birds. Aspergillosis can also be a serious problem in wild birds that are placed in temporary or permanent captivity, such as marine birds placed in rehabilitation centers.

Management: Avoid disposing of moldy waste grain or otherwise spreading moldy or dusty straw, sileage, or feed in areas where waterfowl and other birds congregate. Restrict access of birds to fields where moldy agriculture waste products are used or have accumulated. For game farm operations, practice regular sanitation to eliminate decomposing organic matter in warm, dark, and moist environments.

References:

Converse KA. 2007. Aspergillosis.  In: Infectious diseases of wild birds. Thomas NJ, Hunter DB, Atkinson CT, editors. Blackwell Publishing, Iowa, pp. 360–374.

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 January 2021.

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