Field Notes
U.S. Fish & Wildlife Service
Field Notes Entry   
SAN LUIS NWR: Avian Cholera Outbreak in the North Grasslandsof Merced County
California-Nevada Offices , February 15, 2012
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Refuge irrigator Brandon Jordan, San Luis NWR, picks up birds afflicted with cholera at the Refuge's Kesterson Unit.
Refuge irrigator Brandon Jordan, San Luis NWR, picks up birds afflicted with cholera at the Refuge's Kesterson Unit. - Photo Credit: USFWS

By Madeline Yancey, San Luis NWRC

An avian cholera outbreak and die-off struck waterbirds in the Grasslands this winter for about a month from mid-January to mid-February. The areas impacted by the disease were primarily four wetland basins on the Kesterson Unit of the San Luis National Wildlife Refuge, a private waterfowl hunt club to the south of Kesterson, and a small area of the China Island Unit of the California Department of Fish and Game’s North Grasslands Wildlife Area to the northwest.

Refuge personnel spent more than 120 man-hours searching several hundred acres of wetland and adjacent habitat for dead and diseased birds. They collected nearly 300 dead birds from Kesterson and the private hunt club, and State personnel collected more than 100 dead birds from China Island. Dead birds were sent to wildlife disease labs for analysis and the diagnosis of avian cholera was confirmed. More than 90% of the dead birds were American coots; but the disease also killed northern shoveler, northern pintail, green-winged teal, bufflehead, snow geese, white-faced ibis, gulls, cormorant, plover, phalarope, black-necked stilt, and yellowlegs.

The water manager for the duck club reported that a very large flock of snow geese was present on the club until the first of February and he had started noticing dead coots about mid-January.

According to the National Wildlife Health Center (NWHC), disease carriers exist in wild snow goose breeding colonies and one should always suspect avian cholera when large numbers of dead waterfowl are found in a short period of time. Avian cholera infections among waterfowl are common and can result in death in only six to 12 hours after exposure, though 24 to 48 hours is more common. Because of the acute nature of the disease, one will rarely see sick birds unless the die-off has persisted for several weeks. When dead birds are found, they appear to be otherwise healthy.

Whenever dead waterbirds are found, whether on private or public lands and disease is suspected as the cause of death, State and Federal agency personnel should be contacted immediately.

The effective control of avian cholera requires the prompt daily removal of dead birds from the environment, both the wetland basins and adjacent uplands, and the proper disposal of the carcasses, either by burial or incineration. Where avian cholera has been confirmed, after the removal of dead birds flushing the wetlands with fresh water is recommended to further reduce the impact of the disease on waterbird populations.

The recent outbreak of avian cholera in the North Grasslands had a positive outcome. Working together, duck club owners along with State and Federal wildlife personnel were able to successfully contain the outbreak in a small area, most likely preventing a large-scale die-off among the waterfowl population.

Contact Info: Jack Sparks, 209-826-3508, jack_sparks@fws.gov
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