A virus that is nearly 100% fatal to birds has been sweeping through the Bay Area with the fall migration and is reaching record-breaking levels throughout the U.S., according to the U.S. Centers for Disease Control and Prevention (CDC).

More than 49 million birds in the U.S., most domestic poultry, have died from bird flu or have been culled due to exposure to infected birds since early 2022. The largest prior outbreak, in 2015, killed 50.5 million birds in 21 states. The current outbreak spans 46 states, according to the CDC.

Known as Highly Pathogenic Avian Influenza H5N1 virus, it has been confirmed in wild bird populations in Palo Alto and in various parts of Santa Clara and San Mateo counties. Since August, the California Department of Fish and Wildlife Health Lab has received six wild birds from San Mateo County and 32 from Santa Clara County for avian influenza testing, of which three from San Mateo County and 17 from Santa Clara County were confirmed. One bird was an American white pelican, said Krysta Rogers, California Department of Fish and Wildlife senior environmental scientist, in an email on Wednesday.

Statewide, the virus has been confirmed in 138 wild birds in 33 counties since early August.

Domestic and commercial birds have also been infected. As of Nov. 3, the current outbreak in California's backyard and commercial flocks has infected 684,940 birds, according to the U.S. Department of Agriculture Animal and Plant Health Inspection Service.

Rogers said the strain of the virus currently in circulation has not been previously detected in North America. It had been on the rise across parts of Europe prior to last December, however, Rogers said.

"Many different species of wild birds may be susceptible to infection, and many birds may die of infection, unlike previous outbreaks with different HPAI viruses," she said.

Avian influenza, also called bird flu, has two forms: a mild version that can cause ruffled feathers and less egg-laying in domestic poultry but in which most wild birds show few signs. The current HPAI form, however, is nearly always fatal, according to the CDC.

Infected birds typically exhibit neurological symptoms such as tremors and an inability to stand, lethargy, sneezing, difficulty breathing, head swelling and purplish discolored legs, said Buffy Martin-Tarbox, spokesperson for the Peninsula Humane Society/SPCA in San Mateo County.

Most of the suspected cases in San Mateo County have been among geese. In one case, Peninsula Humane Society officers responded to a call of "a goose in a pond that was just circling," Martin-Tarbox said.

Ashley Kinney, Wildlife Center of Silicon Valley hospital manager, said the center began receiving suspected cases of avian influenza in August.

"We had 128 suspected cases and 18 were confirmed positive," she said, including Canada geese, several pelicans, hawks, owls and corvids such as American crows.

Cody Macartney, supervisor of Palo Alto Animal Control Services, said he received calls regarding at least two pelicans and five or six Canada geese — five were found dead in Adobe Creek in one day — in early September.

Other animal control officers on patrol in Mountain View have reportedly found other deceased waterfowl: an egret, a goose and a cormorant, he said.

The good news: In the past few weeks they haven't found any more, he said.

Kinney also said in the past few weeks the cases seem to have diminished, Martin-Tarbox said no new birds have entered the Peninsula Humane Society's wildlife facility with bird flu symptoms for the past week and a half.

People encountering wildlife or backyard flocks that appear to be sick or have died should still exercise caution, Macartney said. The virus is highly contagious, and it's important that people who come across dead wildlife don't try to handle it. Those who are trained to handle the animals wear full personal protective equipment when handling the sick or dead birds.

"If you find suspected or sick wildlife, just call us," Martin-Tarbox said.

If that's not possible, people can bring in the dead animal, but they should wear gloves and change their clothing when they return home, she added.

While rare, there's another reason to beware of handling sick or dead wild birds. Although rare, some HPAI strains, particularly H7 and H5, can infect people and domestic pets. Human infections range from no symptoms and mild disease to severe illness and death. Lineages H7N9 and H5N1 that started in Asia have been responsible for most of the severe human illnesses and deaths, according to the CDC's website. Since 2003, there have been more than 880 cases of human infection caused by earlier strains of H5N1 virus.

The virus enters a person's eyes, nose or mouth or is inhaled, often from touching contaminated surfaces or birds. Person-to-person spread is extremely rare and has only shown to affect a few people, the CDC said.

"However, because of the possibility that bird flu viruses could change and gain the ability to spread easily between people, monitoring for human infection and person-to-person spread is extremely important for public health," the CDC said.

The current outbreak infected one person in Colorado in late April. The person was in direct contact with a flock of poultry being culled due to presumed infection with the H5N1 virus. The risk to people is still deemed low, however, the CDC said.

The CDC has more information on the virus at cdc.gov/flu/avianflu/prevention.htm.