Washington is investigating other possible exposures and has tested 25 people, the department said Monday.

The new cases fuel growing concern among public health experts, as the virus' jump to dairy cattle this year and infections of farm workers have worried scientists and federal officials about the risks to humans.

Officials in California and Washington have said they are seeking to administer seasonal flu vaccines to farm workers in an effort to reduce their risk of being infected with both bird flu and seasonal influenza.

Infections with both types of virus simultaneously could increase the risk of changes that could make bird flu spread more easily in people and potentially cause a pandemic, virologists say.

"We don't have evidence yet of transmission between people," said Roberto Bonaccorso, spokesperson for Washington's health department.

Washington sent samples to the U.S. Centers for Disease Control and Prevention for final confirmation and analysis, the department reported on Sunday. The CDC had no immediate comment on Monday.

The four tested presumptively positive after working at a commercial egg farm infected by the virus, Washington's health department said. They experienced mild symptoms and were given antiviral medication, the department said, adding that their use of protective gear was inconsistent.

About 800,000 chickens were culled at the egg farm after birds tested positive in Washington's first outbreak this year, state officials said. The workers were removing carcasses and litter and cleaning the facilities where chickens were culled, according to the health department.

Nationwide, 27 people had tested positive in 2024 before the cases in Washington, as the virus has spread to poultry farms and, for the first time, to dairy herds across the country. All but one of the people had known exposure to infected poultry or dairy cattle.

More than 100 million U.S. chickens, turkeys and other birds have been eliminated by the virus since the nation's outbreak began in 2022.