U.S. and California health officials confirmed two new cases of H5N1 bird flu in dairy farm workers in the state on Friday, bringing the total of infected dairy workers in that state to six and the total of human cases nationwide this year to 20.

Ten of the U.S. cases were tied to exposure to infected poultry, nine were tied to sick or infected dairy cows, and one case in Missouri occurred in a person who had no known contact with infected animals, the U.S. Centers for Disease Control and Prevention said in a statement on Friday.

The CDC said all six of the cases in California appeared to have been caused by animal-to-human spread. So far, the infected workers have had mild symptoms, which included eye redness or conjunctivitis. None have been hospitalized.

The agency said it thought the immediate risk to the general public from bird flu remained low, but people with exposure to infected animals are at higher risk of infection.

Genetic sequencing of three virus samples from the California cases showed they were closely related to those detected in dairy cattle, the CDC said.

Sequencing of the first two cases showed no changes in the virus to suggest it could spread more easily among people or that it was becoming resistant to antiviral medications, the agency added.

The California Department of Public Health said in a statement that it was working closely with local public health laboratories and departments to provide health checks for exposed individuals and ensure testing and treatment are available as needed.

Of the six California cases, all occurred in workers exposed to the virus. California health officials said they expected additional cases to be identified among individuals who have regular contact with infected dairy cattle.

The state said it has received 5,000 additional doses of seasonal flu vaccine for farmworkers from the CDC with the aim of reducing the risk of becoming 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.

According to the U.S. Department of Agriculture, 298 dairy herds in 14 U.S. states have confirmed cases of H5 bird flu virus infections in dairy cows.