• A bird flu outbreak has ravaged the world's birds since 2020 and infected cattle earlier this year.
• California Gov. Gavin Newsom declared a state of emergency over the virus this week.
• Health officials also confirmed the first "severe" case of and hospitalization for the H5N1 virus.

The burgeoning global bird flu outbreak continued its flight path across the country this week, with two major developments that point to the virus's increasingly concerning spread.

California Gov. Gavin Newsom declared a state of emergency over the virus on Wednesday, citing a worrying number of infected herds throughout the state in recent months and a need for more resources.

Since the state first identified the H5N1 avian influenza virus in cattle in late August, California's agriculture department has confirmed 645 infected dairy herds.

Newsom's announcement, meanwhile, came just hours after health officials confirmed the first severe case of bird flu in Louisiana, saying a person was hospitalized with an infection after being exposed to sick birds in his backyard.

In recent months, infectious disease experts have grown more and more nervous about the possibility of a human pandemic linked to the virus, even as the Centers for Disease Control and Prevention has maintained that the public health risk for humans is low.

Here's where things stand.

Bird flu outbreak

The H5N1 virus first reemerged in Europe in 2020 and has since become widespread in birds around the world. The outbreak has killed tens of millions of birds and tens of thousands of sea lions and seals in recent years.

Birds carry the disease while migrating and can expose domestic poultry to the virus while never showing signs themselves, according to the CDC.

The virus jumped to cattle herds for the first time ever earlier this year in a major escalation. Then, in October, a pig in Oregon tested positive for the virus, an especially concerning case as swine can host both bird and human flu viruses.

There has been no known human-to-human transmission yet. Still, the growing pattern of mammal-to-mammal transmission has infectious disease experts on guard against the possibility that H5N1 could eventually become a human pandemic.

"If it keeps spreading in animals, then it is eventually going to cause problems for humans, either because we don't have food because they've got to start exterminating flocks, or because it starts to make a jump in humans," Dr. Jerome Adams, a former surgeon general and the director of health equity at Purdue University, told Business Insider in April.

"The more it replicates, the more chances it has to mutate," he added.

The ongoing multi-state dairy cattle outbreak, which is believed to have started in Texas, has infected 865 herds across 16 states, according to the CDC, and has led to a growing number of human cases among US dairy and poultry workers.

The CDC has thus far confirmed 61 reported human cases and seven probable cases across the US, though some scientists estimate that the real number of infections is higher.

More than half of the human cases are tied to interaction with sick cattle. The remaining infections have been traced to exposure to sick poultry or have an unknown origin, the CDC said.

State of emergency

California's Wednesday announcement will give state and local authorities increased resources to study and contain the outbreak, Newsom said.

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

Earlier this month, the Agriculture Department said it would start testing the nation's milk supply for traces of the virus, requiring dairy farmers to provide raw milk samples upon request. Up until then, cattle testing for potential infections had been almost entirely voluntary.

Dr. Monica Gandhi, a professor of medicine and associate chief of the Division of HIV, Infectious Diseases, and Global Medicine at the University of California, San Francisco, said the declaration will likely give California a greater ability to surveil dairy farms for signs of the virus.

But declaring a state of emergency could be a double-edged sword.

Phrases "like 'state of emergency,' given that we've just been through a pandemic, can induce panic," Gandhi said.

And it's not time to panic yet, she said.

Gandhi praised the CDC's "very measured" messaging around the virus thus far and said health officials are closely monitoring the spread.

• 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."