In November, a teenager in Fraser Valley, British Columbia sought medical care for conjunctivitis and a cough. Six days later, the teen was put on ventilator at the B.C. Children’s Hospital in Vancouver and remained in critical care for weeks.
An illness like this wouldn’t normally make headlines, but this child tested positive for a strain of bird flu, called H5N1, which infectious disease experts worry could fuel the next human pandemic.
The virus first emerged on poultry farms in Hong Kong in 1997, where it killed nearly 100 percent of chickens, causing internal bleeding and destroying multiple organs in a manner chillingly reminiscent of Ebola in humans. Since then, successive waves of infection, spread by wild birds, have plagued poultry farms around the world.
Recently, however, H5N1 took an unsettling evolutionary step in the direction of humans. In 2022, it tore through a population of sea elephants in Argentina, killing thousands with a mortality rate of 97 percent. It was the first time H5N1 is known to have taken hold in a mammalian species. Until then, people and other mammals who’d gotten sick had caught the virus through contact with birds. The sea elephants were passing it to one another.
(Bird flu is spreading from pole to pole. Here’s why it matters.)
By the time scientists got around to publishing their seal findings in June, H5N1 had infected another mammalian species: dairy cows. Since March, the virus has spread to more than 800 dairy herds in 16 states, including more than 500 in California, where it remains uncontrolled. On December 18, California Governor Gavin Newsome declared a state of emergency to respond to the outbreaks.
In the U.S., at least 61 people have caught the virus, most through direct contact with birds or cows. In December, a child in Marin County who drank raw (i.e. unpasteurized) milk, spiked a fever and vomited, later tested positive for H5N1. This week, the Centers for Disease Control and Prevention (CDC) confirmed the first “severe” bird flu case in the US; the patient had been exposed to sick and dead birds in a backyard flock.
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Every time a human gets sick, the virus has another opportunity to acquire the ability to spread from person-to-person. Once it passes that milestone, it could start a pandemic.
There is no evidence that H5N1 has passed that grim turning point. It may never make this leap. But “knowing what we know about these viruses, the trend is not good,” says Matthew Binnicker, a microbiologist specializing in respiratory diseases at Mayo Clinic in Rochester, Minnesota, adding “serious action” is needed.
Experts are worried about two main ways the virus could start spreading more easily between people. And they stress: It’s not too early, or unreasonable, to prepare for the worst.
Scientists are worried that pigs can be a breeding ground for a version of bird flu that can more easily spread among human.
Photograph By Edwin Remsberg/VWPics/Redux
1. Pigs could be the key to unlocking a bird flu pandemic
The presence of H5N1 in hundreds of cow herds is not a good development, but it’s not the barnyard animal scientists are most concerned about.
Should H5N1 start circulating in pigs, the chances of a human version arising would increase dramatically. That’s because pigs can be infected by both bird viruses and human viruses at the same time. This sets up a literal virus breeding ground.
Influenza viruses are extremely changeable in part because they’re made of RNA, a genetic molecule similar to DNA but with a major difference: RNA viruses have no proofreading mechanism during replication. So when an influenza virus reproduces inside a host cell, it is prone to making copying errors, increasing the rate of mutations. That means that an RNA virus such as H5N1 is particularly good at evolving to infect new species.
But influenza viruses have another tool that makes them still more dangerous: an ability to swap genetic material with other viruses. This process, known as reassortment, is a bit like shuffling two different decks of cards together—you wind up with a bit of both. If a pig catches H5N1 from a bird and catches, say, whatever seasonal influenza virus happens to be circulating among people, the two viruses will come into contact and, by reassortment, randomly acquire one another’s traits.
What happens next is up to chance. Many of these recombined viruses will die off without anyone ever noticing them. But occasionally, reassortment creates a virus whose genetic code gives it advantages that allow it to thrive. If those advantages include the ability to reproduce and spread among humans, and it gets the opportunity to start spreading in a population, it could become yet another new human pathogen. The 2009 H1N1 pandemic virus is thought to have started in domestic pigs in central Mexico.
On October 30, the Animal and Plant Health Inspection Service (APHIS), part of the U.S. Department of Agriculture, announced that it had found H5N1 on a small farm in Crook County, Oregon. Two pigs tested positive for a strain of H5N1 that is running rampant through wild birds, poultry and cattle, though small genetic differences suggest that the pigs acquired the virus from wild birds.
Although there’s no evidence that H5N1 is currently spreading in commercial pig farms, the Oregon case suggests that birds, pigs, cattle and other mammals are passing the virus among themselves more often than experts know about. “We have to be very cautious about under-interpreting findings like this,” says Binnicker. “Where there’s smoke, there’s fire. It’s not a cause for alarm, it’s not a cause for panic, but we can’t ignore it.”
2. An uncontrolled outbreak in dairy cattle puts us all at risk
Even if we avoid H5N1 infections in pigs, a human pandemic virus could arise from the raging dairy-cow epidemic. Like pigs, cattle can also be infected by human and bird viruses at the same time. Scientists think that reassortment is a bit more unlikely in cattle due to certain aspects of its physiology. In the case of dairy cattle, experts are more worried about humans becoming the breeding ground.
The presence of the virus in dairy farms exposes many people—farm workers and their families, friends and members of their communities—to the virus. And a human version of bird flu is perfectly capable of emerging, through reassortment, from a person infected with both bird flu and a seasonal flu virus.
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The coming flu season increases this risk. “We’re going to likely be having broad transmission and spread of human influenza viruses in the population,” says Binnicker. “If we have a farm worker who is infected with a human strain of influenza and they’re also working with an infected dairy cow that has avian influenza, then the reassortment event could potentially happen in the human if they become infected with both viruses at the same time.”
Farms have struggled to contain outbreaks—but some progress is being made
Containing the outbreak among cattle is important for reducing the potential threat to public health. The fewer cows infected, the fewer opportunities the virus has to get into other farm animals, like pigs, or humans.
But the cattle industry and its regulators have struggled to do so. Unlike poultry farmers, who have decades of experience with H5N1, the dairy industry was caught flat-footed. “We haven’t had this kind of challenge from a virus for many generations,” says Jaime Jonker, chief science officer of the National Milk Producers Federation, an industry group. “We don’t have that well-oiled mechanism of jumping into action.”
The cattle industry has been playing catch up since the outbreak began, most likely in late 2023 on farms in the Texas panhandle, after a wild bird infected with H5N1 somehow transmitted the virus to the mammary gland of a cow. “Everybody was surprised, because it has never been seen in any species that I’m aware of in the milk,” says Jim Roth, director of the Center for Food Security and Public Health at Iowa State University. “It was a very unusual situation.”
The virus seems to spread among cows mainly through contact with milking equipment. It then collects in such high concentrations in the milk of infected cows that it’s extremely hard to keep it from spreading. Farmers have tried using disinfectants on milking equipment and even directly on the cows’ teats, to no avail. “So much virus is being produced in the milk that it’s hard to stop,” says Roth.
Large farms employ a small army of workers to milk thousands of cows two or three times a day, seven days a week. It’s hard to avoid getting some virus-laden milk on coveralls and boots. Proper use of goggles, face shields, masks, gloves, boot covers, coveralls and other personal protective equipment (PPE) can, in theory, provide excellent protection for workers, experts at the CDC told me, but only if worn consistently and strictly adhering to protocols. That can be difficult in the often humid and wet conditions of the typical dairy farm. Many infections among farm workers present as conjunctivitis (i.e. pink eye), perhaps from workers reaching under protective goggles to rub their eyes.
Despite the containment difficulties, some progress is being made.
In July, Colorado started requiring testing of milk held in “bulk tanks” prior to shipment from farms. It also issued a quarantine order for infected cattle and required tougher biosecurity measures, such as routine sterilizing of the tires of vehicles, restricting visitors and establishing strict biosecurity protocols for workers.
The latest measures seem to have been effective—Colorado has reported no new infections in more than a month. In October, the USDA, citing Colorado’s success, began a national program of bulk-milk testing. (On pig farms, surveillance is left up to farmers and their veterinarians.)
The stricter measures are coming late for California, where the outbreak has spread throughout farms in Central Valley. The vast size of its dairy industry—the state has 1.7 million dairy cattle, compared to Colorado’s 200,000—and the close proximity of its farms to one another pose a challenge to containment efforts, experts say. “We are at a stage where we are don’t have the virus under control in California,” says Jonker.
The USDA has approved seven field trials of H5N1 vaccine candidates for dairy cattle. However, the agency, in a written response to questions, wouldn’t guess as to when a vaccine will be available “or whether a successful vaccine will ever be developed.”
What would cause bird flu alarm bells to ring? And what should we do when we hear them?
There’s evidence H5N1 is quickly adapting to human physiology. A single genetic mutation to the dairy-cow strain is enough to give it the ability to attach easily to cells in human airways, according to a study published this month in the journal Science. That mutation was found in the virus sample taken from the teen in British Columbia, and may be what made him so ill. Still, scientists say there’s still no evidence of human-to-human transmission.
Overall, the risk to public health of H5N1 is currently “low,” according to the CDC. That could change in an instant with another single spillover event of a strain capable of spreading from person to person. It would probably first appear as a small cluster of illnesses and gradually spread, slowly at first, then quickly. It’s impossible to predict how severe it will be: it could cause mild illness, like the 2009 influenza pandemic, or severe illness, like the 1918 influenza, which killed more than 50 million people, or something in between.
Regardless of the severity, rapid detection and quick response are key to containing such an outbreak. The U.S. currently has two candidate vaccines for H5N1 and plans to manufacture 10 million doses by April, according to the CDC.
Should human-to-human transmission arise, those doses could vaccinate a ring of people around a cluster of cases. Such a strategy could contain an outbreak, if officials respond quickly before the virus infects too many people.
In the meantime, the best thing most people can do is get their seasonal flu shot, which would help reduce the level of seasonal virus in circulation, and the chance of spillover. Public health experts also advise against drinking raw milk. (Grocery store milk is safe to drink, as it goes through a pasteurization process.)
The CDC currently focuses its “active surveillance” on people most likely to be exposed, such as farm workers. For instance, in one survey of 115 farm workers, eight tested positive for antibodies to H5N1, meaning at some point they had caught the virus, and four had developed symptoms.
In the general population, by contrast, prevalence is “vanishingly small,” says Eduardo Azziz-Baumgartner, a medical epidemiologist at the CDC. For this reason, he says, wider testing would be inefficient, expensive and result in too many false positives. So far, the CDC has administered more than 60,000 tests for H5N1 and only 61 have tested positive. (All but two got it from animals. And while experts don’t know where the other two got it from, there’s no evidence of human-to-human transmission.)
Maggie Bartlett, program director of the Global Virus Network and a virology professor at the Johns Hopkins School of Public Health, believes that the consequences of a human H5N1 virus are potentially so grave that greater vigilance is called for. She advocates making rapid-tests for H5N1 widely available and a more systematic monitoring of the virus among animals and people. She worries that the true number of people who have gotten H5N1 are far higher than the 61 we know about. “We’re not doing sufficient surveillance in the human population to know the [total number] of human cases,” she says. “That’s something that scientists have been lamenting for months.”
There’s no shortage of things to worry about. When and where spillover will occur—or if it will ever happen at all—is hard to predict. What we do know is that the chance of a human H5N1 virus emerging is higher now than it has ever been.
