A fast-growing outbreak of avian flu has upended California’s dairy industry, the nation’s largest producer of milk, infecting most of the state’s herds and putting thousands of farmworkers at risk for contracting the virus.

In just about four months, cows in 645 dairies in California have tested positive for H5N1, even as many ranchers have taken strict precautions to stop the virus from spreading. Gov. Gavin Newsom was concerned enough Wednesday that he declared a state of emergency over the outbreak in California.

The virus is spreading so quickly that dairy farmers are calling it “Covid for cows,” and scientists are racing to figure out how to stop the contagion.

“We’re trying to do everything we possibly can, and this has just been the worst crisis we’ve ever dealt with in the dairy industry in California,” said Anja Raudabaugh, the chief executive of Western United Dairies, a trade organization that represents most of the state’s dairy farms.

Avian flu primarily affects birds, but it can also infect mammals, including humans. There have been 61 human cases reported in the United States so far this year, and most of the individuals have had mild symptoms, including pink eye, fever and muscle ache.

But officials reported on Wednesday that the nation’s first severe human case of infection had been identified in an individual in Louisiana who had been hospitalized with bird flu.

There has been no evidence that the virus can spread easily between humans, though disease experts warn that viruses can evolve as more infections occur.

Consuming eggs and pasteurized milk won’t make people sick, according to the Food and Drug Administration. (Raw milk from infected cows, however, has been deemed unsafe, and California recently recalled raw milk products after the virus was detected in samples.)

The most common way humans have contracted bird flu has appeared to be through close contact with infected cattle and poultry. The virus was first detected in cows early this year in Texas, but has since reached herds in 15 other states, including California.

Milk from infected cows has very high levels of the virus, and experts believe that contaminated vehicles, equipment and workers play a role in spreading the virus from farm to farm. Those who milk cows can face high risks because the virus is highly concentrated in infected milk, which can splash into workers’ eyes, said Michael Payne, a veterinary medicine expert at the University of California, Davis.

Farmers took precautions by cutting off contact with other dairy farms, regularly testing their milk for the virus, disinfecting new equipment and preventing workers from other farms from visiting, said Dr. Payne, who studies biosecurity on farms. This fall, cattle ranchers in California also scrambled to isolate their herds because it has been believed that avian flu spreads through close contact between cows.

Yet those measures haven’t always worked.

“Some of them have just done everything right, and they still got infected,” Payne said. “It’s enormously frustrating. You’ve got producers that upend their entire life and system of management — it’s enough to make you want to throw up your hands.”

Federal and state scientists are scrambling to identify other ways the virus may be spreading among cattle, such as whether wild birds, rodents or other animals like skunks may be transmitting the virus between farms.

Last week, dairy cows in Southern California tested positive for avian flu, hundreds of miles from infected herds in the Central Valley, the state’s agricultural hub. Shipments of cows between the two regions have been shut down for weeks, Ms. Raudabaugh said.

That the virus had reached cows beyond the Central Valley, Newsom said on Wednesday, was a sign that the outbreak had become a statewide crisis that requires more monitoring and resources. His emergency declaration waived certain labor restrictions to allow for more staffing and suspended requirements for equipment purchases.

In October, a severe heat wave in the Central Valley compounded problems.

“Cows were just falling down dead. I’d never been so traumatized,” Ms. Raudabaugh said.

And cows that recover from the virus only produce two-thirds as much milk when they return, Ms. Raudabaugh said. She said that milk production in November in California was 4 percent lower than at the same time last year.

“That’s the long-term damning impact,” she said.

In California, 34 people have tested positive for bird flu, and almost all of them had been directly exposed to infected cattle, according to state officials.

The actual number of infected farmworkers is likely higher than what has been reported because many tend to avoid testing so they don’t have to miss work, said Elizabeth Strater, a national vice president of the labor union United Farm Workers. Farmworkers who are undocumented may also be reluctant to report that they’re sick, she said, because they are worried about potentially having to provide their personal information to a government agency.

“These are people who have a very thin social safety net,” Strater said. “These are people that are living at or below the poverty line, and these are the people that we are counting on to keep the rest of us safe from things like avian flu.”

California’s poultry farms have also suffered from the virus, but they tend to be better protected. Unlike at dairy operations, where cows move between farms, bird flocks stay together on one farm, and large poultry operations are often indoors, where they are more protected from other animals.

Still, when the virus does reach a flock, the impacts are far more extreme. The virus is fatal in chickens and spreads much faster among them than cows, so poultry farmers must euthanize an entire flock — potentially more than a million birds — if one gets infected. Since early November, 6.5 million egg-laying hens have died nationwide, including 2.5 million in California, according to the California Department of Food and Agriculture.

That has dented the state’s egg supply, and many California grocery stores have been running low on cartons right before the holidays. At some stores, shelves are mostly bare, and customers have been restricted from purchasing more than one carton at a time.

Katya Rosales, 43, turned up at a Food 4 Less grocery store in Los Angeles with her two young daughters on Wednesday, only to find empty shelves.

Rosales said that she was worried about how she would find the ingredients for the cupcakes and flan she typically makes for her four children for Christmas.

“We need to figure out where we’re going to get eggs,” she said.

Soumya Karlamangla  Orlando Mayorquín and Jesus Jiménez are reporters with The New York Times. Copyright 2024, The New York Times.