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

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.

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.

The Centers for Disease Control and Prevention said Thursday California has confirmed a third human case of H5 bird flu. All three confirmed cases had occupational exposure to infected dairy cows.

The California Department of Public Health (CDPH) reported Friday that 13 total human cases of bird flu have been confirmed in the state. The CDPH said the cases have been identified from Central Valley residents who had direct contact with infected dairy cattle.

The state now has 11 cases of people catching the virus, as health officials monitor any further spread from poultry and livestock.

Another five suspected cases of bird flu were reported today in California, where six cases have already been confirmed, according to the California Department of Public Health. The state, which is the country’s largest dairy supplier,

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.