Gross Findings: Both birds were in excellent body condition with abundant fat stores. In both birds, extending from the jejunum to the colon and affecting approximately 80% of the intestinal tract, there was moderate dilation with dark red serosal and mucosal surfaces, and dark red luminal content which was variably fluid, pasty, gelatinous, or fibrinous (Fig. 1A, B). The contents and mucosa contained many <1 mm diameter white structures (trematodes) (Fig. 1B). In one bird (002), on the intestinal serosa were many approximately 2 to 5 mm diameter discrete, oval to irregular foci composed of a tan rim with a central red area (ulcers) (Fig. 1C); frequently on the corresponding mucosal surface were approximately 2 to 3 mm diameter foci of soft white material (necrotic debris) (Fig. 1D). There were no other significant gross findings.

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Histopathological Findings: In the small intestine, there are multifocal transmural areas of ulceration, necrosis, and mixed inflammation, often associated with abundant necrotic debris and hemorrhage in the lumen (Fig. 2A, B, C, D). In one area, an adult trematode is within the ulcerated lamina propria adjacent to a transmural area of necrosis, and luminal debris contains trematode eggs (Fig. 2A, B, C, D).

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Morphologic Diagnosis/es:

1. Enteritis, necrohemorrhagic, ulcerative, multifocal, marked, with intralesional trematodes

Etiology: Sphaeridiotrema globulus (Trematode, Digenea)

Seasonality: In the United States., mortality events are most common in the late summer or early fall, but are also reported in the winter and early spring as seasonal migrations occur.

Host range: Known to cause disease and mortality in waterfowl, including swans, diving ducks, and coot.

Transmission: Birds become infected by consuming the infected snail intermediate host. The invasive faucet snail (Bithynia tentaculate) is often implicated as an intermediate host, although other snail species can also be infected. In mute swans (Cygnus olor), as few as 20 metacercariae may cause mortality.

Clinical signs: Weakness, lethargy, and blood-stained vents; acute death is common.

Pathology: Ulcerative hemorrhagic enteritis, particularly within the jejunum and ileum; small (less than 1 mm diameter) white trematodes may be evident within the hemorrhagic intestinal content. Trematode adults feed on blood and cause additional hemorrhage due to mucosal erosion and migration into submucosa and muscular tunic. Birds typically die acutely from blood loss or shock.

Diagnosis: Characteristic gross or microscopic lesions with intralesional trematodes; Sphaeridiotrema species can be confirmed by PCR or gross morphology.

Public health concerns: None

Wildlife population impacts: May cause repeated localized mortality of susceptible species in areas where the intermediate snail host is plentiful.

Management: If undertaken, management efforts focus on control of the intermediate snail host

References:

• Huffman JE and Roscoe DE. 1989. Experimental infections of waterfowl with Sphaeridiotrema globulus (Digenea). J Wildl Dis. 25(1) pp. 143–146. https://doi.org/10.7589/0090-3558-25.1.143
• Roscoe DE and Huffman JE. 1982. Trematode (Sphaeridiotrema globulus)-induced ulcerative hemorrhagic enteritis in wild mute swans (Cygnus olor). Avian Dis. 26(1) pp. 214–224.
• United States Geological Survey. 1999. Field manual of wildlife diseases: general field procedures and diseases of birds, https://pubs.usgs.gov/itr/1999/field_manual_of_wildlife_diseases.pdf. Accessed May 2022.
• United States Geological Survey. 2020. Pathology Case of the Month - Mallard Ducks, https://www.usgs.gov/news/pathology-case-month-mallard-ducks. Accessed May 2022.