• 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.