Preliminary laboratory results indicate avian cholera caused a recent waterfowl die-off at the Butte Sink Wildlife Management Area (WMA) in northern California. While avian cholera is lethal to waterfowl and other water birds, it does not affect humans.

"All indications from the history of the area, the weather, the time of year, and the concentrations of ducks was that we had an avian cholera outbreak," said Dr. Pam Swift, Department of Fish and Game (DFG) wildlife veterinarian. "Even so, the Department and the U.S. Fish and Wildlife Service (Service) collected samples from birds involved in the die-off and submitted them to both to state and national laboratories to test for a variety of diseases - including avian influenza - to confirm our assessment."

Avian cholera (not related to human cholera) is a relatively common disease of North American waterfowl. It is caused by a bacterium and spreads rapidly from bird-to-bird and can kill thousands of birds in a single outbreak. A bird infected with avian cholera dies quickly. Avian cholera die-offs in waterfowl commonly occur during the winter months in California, especially during cold spells and fog.

DFG received a report of ruddy ducks, wigeons, coots, and other waterfowl dying at the Butte Sink WMA (part of the Sacramento National Wildlife Refuge Complex) from Service personnel on Jan. 9. More than 500 birds have been collected to date by Service personnel. DFG personnel sampled birds and submitted swabs and carcasses to the California Animal Health and Food Safety Lab for necropsy and avian influenza testing - which is standard protocol for any wild bird die-off. The Service separately submitted carcasses to the National Wildlife Health Center in Madison, WI for diagnostics.

Samples submitted to the California laboratory were negative for the highly pathogenic avian influenza. Service sample results are still pending.

"Our field biologists routinely deal with avian cholera in wild birds every winter, generally around the time that the temperature drops below freezing," said Kevin Foerster, Manager of the Sacramento Refuge Complex. "We are undertaking standard disease control operations and following all established safety protocols to limit the spread of cholera within the wild bird population."

Over the last 30 years in California, documented bird losses due to diseases ranged from a low of 10,500 in 1977-78 to a high of 169,300 in 1991-92. The majority of the bird losses in 1991-92 consisted of 150,000 eared grebes that died due to avian cholera at the Salton Sea. Average annual loss of migratory birds to disease in California is about 25,000 birds. These figures are for birds actually picked up and disposed of, and the actual losses are greater. In 2005, the last full year of available data from the National Wildlife Health Center, of the nearly 12,000 birds picked up in California, most diagnosed causes of mortality were: petroleum spills (5,000); salmonellosis (2400); botulism (1800) and starvation (1500).

DFG and the Service are working closely and quickly in responding to this die-off, and are poised to rapidly investigate and collect samples for necropsy and avian influenza testing if other die-offs should occur in the Central Valley or elsewhere in California.