Pathology Case of the Month - Common Eider

History: Approximately 200 other Common Eider carcasses were present, scattered up and down the beach, as well as lesser numbers of sick eiders. Sick eiders appeared weak and lethargic, with some stumbling when attempting to walk. A recent migration of 300,000 to 500,000 Common Eiders occurred in the vicinity.

Gross Findings:

External: The conjunctiva and oral mucous membranes are pale. The keel is prominent.

Internal: There is no subcutaneous, visceral, or epicardial fat. The pectoral muscles are thin and pale, and the blood within the heart is thin and watery. Scattered, irregular, 1-3 mm pale foci are present in the liver (Fig. 1A). Petechial and ecchymotic hemorrhages are present in the pancreas (Fig 1B).

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Histopathological Findings:

Throughout the liver, there are moderately sized foci of coagulative hepatocellular necrosis (Fig. 2A). Affected hepatocytes are eosinophilic with nuclear pyknosis, karyorrhexis, or loss of nuclei. Foci of necrosis are randomly distributed.  In the pancreas, there are multiple areas of acinar cell disorganization and necrosis, randomly distributed within the section (Fig. 2B). Hemorrhages are present in some areas of necrosis.  There are random foci of acute splenic necrosis. Germinal centers are markedly depleted.  In the thymus, there is apoptosis of most cortical lymphocytes.

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Morphologic Diagnoses:

1. Emaciation
2. Acute, moderate, multifocal to coalescing, random hepatic necrosis.
3. Acute, marked, multifocal pancreatic necrosis.
4. Acute, moderate, multifocal splenic necrosis.
5. Acute, marked, diffuse thymic lymphoid necrosis

Disease: Wellfleet Bay virus disease

Etiology: Wellfleet Bay virus (WFBV) is a Quaranjavirus that was first isolated from the liver of an affected Common Eider in 2006, during a mortality event. Quaranjavirus is a small genus within the family Orthomyxoviridae.

Distribution: This disease has only been documented in Common Eiders at Wellfleet Bay, Massachusetts, USA.

Seasonality: Most die-offs occur at the time of Spring or Fall migration.

Host Range: This virus has only been shown to cause disease in Common Eiders. However, antibodies to WFBV have been found in White-winged Scoters (Melanitta deglandi), Ring-billed Gulls (Larus delawarensis), and Herring Gulls (Larus argentatus). 

Transmission: The route of transmission is currently unknown.

Clinical Signs: Lethargy, weakness, respiratory distress, and emaciation.

Pathology: Hepatic necrosis, pancreatic necrosis, splenic necrosis, intestinal necrosis, esophageal ulceration, lymphoid depletion, emaciation.

Diagnosis: The presence of pathologic abnormalities typical of WFBV disease in a Common Eider from Massachusetts is highly suggestive of the disease. Confirmation is by isolation of the virus from tissues of an affected bird.

Public Health Concerns: WFBV is not known to infect humans.

Wildlife Population Impacts: WFBV disease has been responsible for mass mortality events with numbers of dead eiders as high as several thousands.

Management: Management is hindered by the current lack of knowledge regarding this disease.

References:

Allison et. al. 2015. Cyclic avian mass mortality in the Northeastern United States is associated with a novel orthomyxovirus. J Virol 89(2):1389–1403.

Ballard et. al. 2017. Detection of Wellfleet Bay virus antibodies in sea birds of the Northeastern USA. J Wildl Dis 53(4):875–879.

Shearn-Bochsler V, Ip HS, Ballmann A, et al. 2017. Experimental infection of Common Eider ducklings with Wellfleet Bay Virus, a newly characterized Orthomyxovirus. Emerg Infect Dis 23(12):1958–1965.