Wellfleet Bay Virus

NWDC is in the process of updating this fact sheet. 


After investigating several recent severe mortality events among common eiders in the Cape Cod Bay town of Wellfleet, Massachusetts, scientists have isolated a new virus and have tentatively named it the Wellfleet Bay Virus (WFBV). Wellfleet Bay Virus is considered to be novel Orthomyxovirus virus from the Quaranjavirus genus.


Wellfleet Bay Virus is a new disease that is affecting common eider populations in the northeast. Since 1998, eider mortality events have occurred every year on Cape Cod, Massachusetts. Death counts have ranged from 30 to 3,000 birds each year, with most deaths occurring in the fall. Since the virus is newly discovered, many aspects of the disease remain unclear at this time. 

Species Affected

To date, common eiders are the only species with documented disease caused by Wellfleet Bay Virus. However, antibodies against WFBV have been found in the white-winged scoter, the ring-billed gull, and the herring gull, indicating that they have at least been exposed to the virus. WFBV is not known to affect people. 


Though it is not known how widespread WFBV is at this time, it is likely that the virus is present wherever common eiders are found. There are four subspecies of common eider in North America. The American subspecies that is currently being heavily affected breed as far north as Labrador, Canada and winter in Newfoundland, Canada to as far south as Long Island, New York. Thousands of birds overwinter at Cape Cod and Nantucket Sound, Massachusetts. To date, all eiders found dead from WFBV have been collected from Jeremy Point in Wellfleet Bay, Massachusetts. However, testing of live birds has detected WFBV antibodies in eiders from Quebec, Canada, Nova Scotia, Canada, and Maine, and Rhode Island.


Currently, transmission of WFBV is not fully understood. Based on the genetic make-up of the virus, scientists believe the disease may be transmitted through an avian soft-bodied tick that dwells in the eider’s nests, though other nest-dwelling parasites are not being ruled out. Studied have found that the virus is shed orally and from the cloaca, though it is not known at this time if the virus is transmitted directly between birds. 

Clinical Signs

Clinical signs are mostly non-specific and include lethargy, impaired coordination, respiratory distress, diarrhea, seizures, and emaciation. At necropsy, birds have been found with systemic viral infections, lymphoid depletion, and multi-organ inflammation or death, including in the liver, pancreas, spleen, and intestines.


Wellfleet Bay Virus is diagnosed by isolating the virus from the liver. 


There is currently no treatment or vaccine for Wellfleet Bay Virus. 


Since so little is known about this new disease, beach surveillance and monitoring to learn as much as possible about WFBV is the key focus at this time. Scientists are actively collecting and testing any recovered dead birds, as well as blood samples from live birds as a point of comparison and to detect changes in WFBV prevalence. Scientists are also using satellite telemetry and aerial surveys to learn as much as possible about common eider movement. Any dead birds should be reported to local US Fish and Wildlife Service Regional Avian Health and Disease Program Control Coordinators, the USGS National Wildlife Health Center, or the Northeast Wildlife Disease Cooperative. 

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