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Case History: A mortality event involving four swans was reported in December in Indiana, U.S.A. Two Trumpeter Swans (Cygnus buccinator) were collected and submitted for diagnostic necropsy.

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.

Photographs of intestine and lumen from a Trumpeter Swan (Cygnus buccinator) found dead
Figure 1. Photographs from a Trumpeter Swan (Cygnus buccinator) found dead in Indiana, U.S.A.  (A) The majority of the intestinal serosa is dark red. (B) The lumen contains gelatinous dark red material and many <1 mm diameter white structures (trematodes) (arrows). (C) On the serosal surface are two 2 to 5 mm discrete ulcers (arrow). (D) On the corresponding mucosal surface is an accumulation of soft tan material (necrotic debris) (arrow).

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).

Photomicrographs from a Trumpeter Swan intestine and trematodes.
Figure 2. Photomicrographs from a Trumpeter Swan (Cygnus buccinator) found dead in Indiana, U.S.A. (A) Within the small intestine is an area of transmural necrosis (arrow) with abundant luminal necrotic debris and hemorrhage containing few trematode eggs (arrowhead), and an adjacent trematode (*).  H&E stain.  (B) Detail of A. The ulcer contains abundant necrotic debris (arrow). An adult trematode is adjacent to the ulcer (arrowhead). H&E stain.  (C) Detail of B. The trematode has a smooth tegument (arrow) with prominent ventral acetabulum (*). H&E stain.  (D) Detail of A. Luminal trematode eggs exhibit a characteristic yellow refractile shell (arrows) H&E stain. 

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


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