Dairy industry experts say the virus entered California after local cows were shipped to another state and then returned to California.

However, there is urgent concern over dairy cows. In California’s Central Valley, the carcasses of dead cattle have been piling up, a phenomenon documented in videos taken by a local veterinarian. According to the CDC, there have been confirmed infections in more than 300 herds of cattle in the U.S. in 14 states since March.

Seven cases of H5N1 bird flu were detected in California this week. No confirmed cases have occurred in people or livestock in Arizona.

Cows in California are dying at much higher rates from bird flu than in other affected states, industry and veterinary experts said, and some carcasses have been left rotting in the sun as rendering plants struggle to process all the dead animals.

Federal and state health officials said they have confirmed a third human case of bird flu in California and identified an additional two new possible cases.  The case was identified in a Central

California is investigating five possible human cases of bird flu among dairy farm workers, in addition to the six cases previously confirmed in the state, the state health department said on Monday.

All of the state's cases have been linked to contact with infected dairy cows, officials said. No human-to-human transmission has been documented.