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Avian Flu: What to Know About H5N1 Virus Risks, Beyond the Headlines

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Cows wait to be milked at the Faria Dairy Farm in Escalon, California, on June 2, 2009. (Justin Sullivan/Getty Images)

More than four years after the start of the COVID-19 pandemic, it can be anxiety-inducing to think about another virus causing a similar amount of havoc.

And you may have already seen the headlines, public health officials are now responding to the spread of H5N1: the virus that typically causes avian flu — sometimes also called “bird flu” — among birds and other animals, but that has recently spread across U.S. dairy farms.

In 2024, three human cases of H5N1 have been confirmed in the United States: two in Michigan and one in Texas. All three infections were reported among dairy workers and the CDC confirms that these workers had direct contact with infected cows. Most of the time, H5N1 only affects birds — but in 2024, the virus has mutated enough to cause infections among cattle.


Considering that there are only a handful of confirmed human H5N1 infections in the U.S. right now, public health experts told KQED that having accurate information about a virus is one way we can all better protect ourselves against future potential threats.

“H5N1 is really a metaphor for lots of emerging threats in the future where we need to use the lessons we’ve learned during COVID and apply them again,” said Peter Chin-Hong, infectious disease physician at UC San Francisco. “We are being vigilant but not afraid.”

Keep reading for the latest on what researchers know about H5N1 — and how you can think realistically about the risks.

Jump straight to:

What is avian flu?

Avian flu has existed among different bird species for centuries and is caused by the influenza A virus. Most of the time, the influenza A virus sticks to birds, but some strains can jump to other species. The seasonal flu we see each year, for example, is caused by an influenza A virus strain, while another strain of that same virus type caused the 2009 swine flu pandemic that cost tens of thousands of lives around the world.

Since 2023, another strain of the influenza A virus, H5N1, has mutated enough to infect a wide variety of mammals, including foxes in Finland, sea lions in Peru, and, most recently, dairy cows in the U.S.

The number of infected cattle herds across the country continues to grow. According to the Department of Agriculture (USDA), 92 herds in 12 states have confirmed H5N1 cases. And since the USDA does not require cattle ranchers to test their herds for H5N1, researchers do not even have a complete picture of how far H5N1 may have spread among American cattle.

“The concern about H5N1 has always been there,” said Dr. Jennifer Nuzzo, professor of epidemiology and director of the Pandemic Center at the Brown University School of Public Health. “But in the last couple of years, [the virus] started doing things that have been a bit unusual.”

“We’ve seen this virus start to infect mammals and a wide range of mammals,” she said, pointing at the different outbreaks throughout the world. “That’s a concern because humans are mammals, and so mammals are more like us than birds are.”

But what makes this recent outbreak the most concerning for Nuzzo is that the virus is now capable of infecting cows. “Cows are mammals that humans have a lot more contact with than all the other mammals that we’ve seen get infected,” she said.

So, if H5N1 spreads among cattle, how easily can it infect humans?

H5N1 has not yet evolved to efficiently spread between humans, researchers told KQED.

At the moment, the three confirmed cases among humans were most likely caused by these individuals interacting directly with a sick cow. All three infected humans worked with dairy cows in states with confirmed H5N1 cases among cattle.

But if the virus can jump from cow to human, it still needs to evolve a lot more for it to move from human to human. “Certain mutations are necessary for that to happen,” said Dr. Abraar Karan, who studies infectious diseases at Stanford University. Some of those mutations would allow for “the virus remains stable in the air and that it becomes more efficient at infecting certain types of receptors in human lungs.”

Jon Arizti Sanz, PhD, Postdoctoral Fellow, is preparing for the extraction protocol while in the lab. The Broad Institute, Sabeti Lab, is testing purchased milk at area grocery stores for the presence of bird flu. (David L. Ryan/The Boston Globe via Getty Images)

The CDC reports that the close contacts of those dairy workers who tested positive for H5N1 have not themselves developed symptoms.

However, because cattle ranchers aren’t required to test their herds for H5N1, researchers still don’t know how many cows have been exposed to the virus — and consequently, how many humans could now be interacting with infected cattle.

“We haven’t really contained the virus within dairy cows yet,” Chin-Hong said. “And the more it continues in the dairy cows, the more it’s going to probably result in contact with dairy workers.”

“The more transmission events there are, the more chances the virus will have to mutate and suddenly, by accident, be able to enter the bodies of humans more effectively.”

Even before the first confirmed human case of H5N1 back in April, the research community pushed federal officials to prioritize the H5N1 outbreak among cattle, specifically to make testing more accessible. On May 30, the USDA announced that it would transfer $824 million from other existing initiatives in order to launch the Voluntary H5N1 Dairy Herd Status Pilot Program.

How dangerous is H5N1 for humans? What are the symptoms?

The CDC continues to monitor the three individuals infected with H5N1 in 2024. The first case, a dairy worker in Texas reported eye redness as their only symptom and, according to health officials, is recovering.

The second person infected with bird flu also experienced pink eye, but the third case reported new symptoms: a sore throat and a cough.

Since 1997, the CDC has confirmed a total of 909 H5N1 human infections worldwide. 52% of those total cases have been fatal, confirming that H5N1 indeed has the potential to be very dangerous to humans.

However, there’s an important context to this. For one thing, this number is a global total spanning a time period of over 25 years. Outbreaks have been scattered throughout the world, and health officials have quickly monitored infected individuals to reduce community spread as much as possible.

It’s also very important to remember that, according to researchers, the avian flu virus cannot currently spread from human to human. Only people who work in very close proximity to infected animals, like cows or chickens, run some risk.

“If you’re a farmworker,” Karan said, “and you’re feeling symptoms — or feel like you may have the flu — it’s important that you let your doctor or healthcare provider know, ‘Hey, I work pretty closely with animals.’”

What do officials say at this stage? Could we be looking at another COVID-style pandemic?

The CDC said that the risk of H5N1 to the general public “remains low.”

To understand the risk of H5N1, researchers are working to understand the molecular structure of the virus, the way it moves in the air and how human cells respond to exposure. A big part of this work is looking at the genetic information of the virus.

More Health Stories

In April, the CDC sequenced the genome of the H5N1 virus found in the dairy worker infected in Texas. Researchers identified some mutations in the virus but not the necessary mutations that would give the virus the tools to jump from one human to another.

Researchers point out that the overwhelming majority of H5N1 cases in the past two decades show humans caught the virus from animals, not other humans.

“The thing that we’re most worried about right now is the people who work on dairy farms may be exposed to this virus in the course of doing their jobs,” Nuzzo said.

“The challenge in understanding what’s going on right now is that we don’t have very good surveillance on cows,” she said, and “we also don’t have great surveillance on the farms of the people who work there.” She attributed this in part to the fact that “there’s a lot of disincentives for having health officials show up and try to test workers on these farms, [who are] somewhat of a disenfranchised group.”

I heard bird flu was detected at a San Francisco market and in local wastewater. Does that mean there’s an H5N1 outbreak in the Bay Area?

The short answer: No, there is no H5N1 outbreak in the Bay Area.

Locally, researchers regularly test the region’s wastewater to measure the levels of certain viruses, like COVID-19, among the population. Since the start of the coronavirus pandemic, high levels of COVID-19 in Bay Area wastewater usually match up with spikes of COVID-19 infections.

At the end of May, researchers did indeed detect the H5N1 virus in San Francisco wastewater, which set off alarms across social media. And while it makes sense why a headline like that can cause panic, digging deeper into the data shows a different story.

San Francisco is one of few hundred cities in the country with a combined sewer system, which means stormwater and sewage flow through the same pipes. When researchers collect wastewater samples, that includes water coming from bathrooms, but it also comes mixed with used water from other parts of the city, including businesses and storm drains.

The timing of H5N1 in the wastewater matched up with two cases of H5N1 among chickens at a live animal market—sometimes referred to as a wet market—in San Francisco. The infected chickens—identified by California food safety regulators and showing no symptoms at the time of testing—were removed from the market, and public health officials have also monitored the market workers who were in contact with the chickens.

San Francisco’s health department does not believe that any of the wastewater hits for bird flu are related to human infection. “No one really thinks at this point that the detections are due to human cases of H5N1,” said Dr. George Han, director of the San Francisco Department of Public Health’s communicable disease prevention and control program.

If you go to a live market, will you be exposed to H5N1? The experts KQED spoke to said no. “Surveillance of poultry has been going on for a long time, even before this,” Karan said.

But he nonetheless recommends that if folks can avoid direct contact with poultry in these settings, they should. “But if you’re going to be near these kinds of birds or near their feces or any kind of secretions they may have,” he said, “wear masks, eye protection and wash your hands.”

Is there anything I should practically be doing right now to lower my risks?

Most of the recommendations from the CDC are still just for people who work closely with animals that are vulnerable to H5N1 infections, like cows and chickens. This includes recommending people wear personal protective equipment (PPE) when interacting with infected — or potentially infected — animals.

However, some early recommendations for the overall population include avoiding interacting with sick or dead animals you may find, like wild birds and chickens. One big recommendation from the CDC: That folks avoid consuming “raw milk,” or milk that has not been pasteurized.

Most milk sold in the U.S. is pasteurized, which means that during production, this milk was briefly heated to a certain temperature in order to kill harmful germs that may be present. In April, the FDA reported that 1 in 5 pasteurized milk products nationwide tested positive for the H5N1 virus — but since the pasteurization process kills the virus, any H5N1 found was already dead.

While there haven’t yet been any cases of people getting sick with H5N1 after drinking raw milk, pasteurizing milk still remains a best practice in the industry to ensure the safety of dairy products and kill not just the H5N1 virus but many other viruses and bacteria that can spread through cow milk and make humans sick.

In California, it is legal to sell raw milk, and data shows that demand for this product has continued to grow, even after the CDC’s recommendations. “Everybody in California loves farm-to-table and natural foods,” Chin-Hong said, but he adds that if folks are thinking of consuming raw milk, they should also assess the real risks that come with doing so — especially now, as the U.S. watches for any spread of H5N1.

“Is the juice worth the squeeze?” he asked, “If you are very old or very young, particularly babies under six months or those who are older, that risk might be too high at this moment.”


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