• Gov. Gavin Newsom of California declared a "proactive" state of emergency over the H5N1 bird flu.
• The virus has spread rapidly through US dairy cattle herds, with 16 states affected.
• The CDC reports low public risk with no human-to-human spread, but 61 human cases have been detected.

Gov. Gavin Newsom of California declared a state of emergency over the H5N1 avian influenza virus on Wednesday.

The bird flu has been spreading rapidly through US dairy cattle herds since March, with infections confirmed in 16 states. Its jump from birds to cows surprised many virologists and raised concerns about the possibility that it could mutate enough to sustain human-to-human transmission.

For now, the US Centers for Disease Control and Prevention has not detected humans spreading the virus to each other and says the risk to the public remains low.

Still, 61 human cases have been confirmed across the country, with 34 of them in California. Many of these cases have been linked to infected cows or birds.

Newsom's declaration, which his office called a "proactive action," followed the detection of new cattle infections on dairy farms in Southern California, according to the office's statement.

"This proclamation is a targeted action to ensure government agencies have the resources and flexibility they need to respond quickly to this outbreak," Newsom said in a statement.

The FDA has said that grocery-shelf beef and dairy continue to be safe to consume. However, the FDA and unaffiliated virus experts have advised against drinking raw milk, which is not pasteurized and can contain harmful microorganisms.

"While the risk to the public remains low, we will continue to take all necessary steps to prevent the spread of this virus," Newsom said.

Also on Wednesday, the CDC confirmed the first case of severe symptoms in a human H5N1 infection, in Louisiana.

Slowing bird flu's spread

The H5N1 virus was first detected in a California cow on August 30. Since then, the governor's office reported, the state has distributed millions of pieces of protective equipment to dairy-farm workers and run a public education campaign.

Infectious-disease experts have previously told BI that limiting the virus's spread through cows can help reduce the odds of sustained human transmission.

That's because the more the virus replicates itself, the more opportunities it has to mutate, and the more new mutations can take hold and spread to new animals. As H5N1 spreads in cattle, a mammal population that lives close to humans, it gets more chances to adapt to humans.

"There's such a vast amount of virus at the moment. And clearly it is changing, and it's doing new and unexpected things," Christopher Dye, an epidemiologist and senior research fellow at the University of Oxford, told BI in June.

In a paper in the medical journal BMJ, Dye and his colleague Wendy Barclay argued that the risk of a major human outbreak was "large, plausible, and imminent" — but not inevitable.

When that paper was published in early June, there had only been three confirmed human cases in the US.

"Influenza has always been a concern for decades and decades, and this particular form of influenza for at least two decades," Dye said. Bird flu, he added, has "risen to a level of concern, I think, which is greater than ever before."