More avian flu outbreaks have been reported in Northern California, including a backyard flock of chickens in Sacramento County and a large outbreak among 1,500 privately owned birds in Butte County.

The Butte County discovery prompted a declaration of a “local health emergency” Wednesday. It appears to be the largest outbreak in California, according to records compiled by the U.S. Department of Agriculture. Until now, only 40 birds were found to have been infected with the disease in California, according to the USDA’s tally, although those records appear to be incomplete.

In addition to the discoveries in Butte and Sacramento counties, the California Animal Health and Food Safety Laboratory has also found two cases of the Eurasian strain H5N1 strain in Stanislaus County and one each in Santa Clara and Mendocino counties, according to a memo from the California Department of Food and Agriculture.

The disease, which is rare in humans and typically caused by direct contact with birds or infected surfaces, can cause symptoms similar to those of human influenza. There is no treatment for the disease. The U.S. Centers for Disease Control and Prevention reported one case of a human contracting the disease in May. Officials said the person was an inmate at a prison in Colorado who was working with chickens as part of a pre-release program.

All told, about 40 million birds have been infected across the country, the USDA says. The disease has been most prevalent in Iowa, where more than 13.3 million birds have gotten such a diagnosis.

Last month, two infected Canada geese and an American white pelican — discovered in Glenn and Colusa counties — were put down at the Sacramento National Wildlife Refuge complex.

State health officials said the virus also was previously detected in other wild birds in Placer, Plumas, Siskiyou, Solano and Sonoma counties.

“Although avian influenza viruses naturally circulate among waterbirds, the strain of H5N1 currently in circulation in the U.S. and Canada has been causing illness and death in a higher diversity of wild bird species than during previous avian influenza outbreaks,” fish and wildlife officials said at the time.

There is potential for the virus to infect other mammals as bird flu viruses evolve.

Wildlife officials have urged bird owners to not allow wild birds into enclosures with domestic birds nor to allow wild and domestic birds to share food or water.