Editor’s note: This story was published on April 30, 2024. For further updates on this rapidly changing situation, we recommend following STAT News and CIDRAP at the University of Minnesota.
This past March, dairy cows in Texas stopped eating well and their milk became thick and discolored; some of the cows spiked fevers. They weren’t extremely sick, but, in the parlance of the dairy industry, they “ain’t doing right,” said Meghan Davis, an associate professor of environmental health and engineering at the Johns Hopkins Bloomberg School of Public Health and a former dairy veterinarian.
The reason soon made headlines: for the first time, H5N1, a highly pathogenic avian influenza, was identified in cows, less than a week after the first case of bird flu was discovered in livestock among baby goats at a Minnesota farm.
The news made infectious disease doctors and researchers take notice, since one warning sign that bird flu could be evolving in a dangerous way is the increased ability to infect new species. But what made them really pause was the update that a dairy worker had also contracted the virus, raising fears that the virus had mutated to become more transmissible to people—an even more worrisome step.
So, what are the risks to humans now, and what are the warning signs that risks could be shifting?
Key points you should know:
- While U.S. officials caution that the risk to people is low, scientists are sounding the alarm that continued transmission in animals greatly increases the risks of spillover to people.
- H5N1 usually doesn’t infect people, but when it does, mortality is high—greater than 50 percent. However, it’s not clear how much less-severe or asymptomatic disease flies under the radar. The only case recorded in a person during this outbreak, at the Texas dairy farm, is an example of illness that may only have been recognized due to more monitoring in recent years.
- Low rates of testing among herds, and recommendations against testing people who have come into contact with the sick animals but haven’t developed symptoms yet, could be obscuring the true rate of transmission.
- In this outbreak, there have been no cases observed yet in pigs, which can help bird flu transform into a virus better adapted to people. However, regulatory agencies seem to be relying on typical surveillance testing, without looking more closely at pigs—even though there’s evidence that a dairy farm spread the virus to a separate chicken farm.
- People with Long Covid should protect themselves against all infections, since they can cause relapses or worsening of symptoms. Many of the same precautions that work against Covid work against bird flu as well.
The current risks
H5N1 was first identified in 1959, but it didn’t make waves until an outbreak in China in 1996 spread to people with an eventual mortality rate of more than 50 percent. Since 2003, a total of 889 people have been sickened by H5N1, with 463 of those people dying.
Bird flu is particularly contagious and virulent among birds, but it has also been detected in scores of other animals, especially since the outbreak reignited in 2020.
Yet over the past few decades, compared to how much avian flu has spread among birds and other animals, “the number of cases that get into humans is quite small, so that’s a very rare event,” Davis said. The U.S. also randomly samples a handful of flu tests every week in order to track which flu variants are in circulation.
It’s important to note, however, that flu infections are typically examined for H5N1 in fairly limited circumstances: when people become very sick and have had close contact with animals, especially birds, or when they develop symptoms after close contact with animals known to have the virus. So it’s possible some infections are not being detected.
It’s still not clear how the virus was introduced to dairy cows and how it’s spreading—whether between cows or whether birds or other animals shared the virus with cows, which could indicate the virus mutated to become better at infecting cows. Recent genetic analysis indicates the virus may have begun spreading in cows in late 2023.
Officials in the U.S. have said that the risk to most people at this point is low, but scientists continue to sound the alarm about the risks of a continually spreading and evolving virus. “This remains, I think, an enormous concern,” Jeremy Farrar, chief scientist at the World Health Organization (WHO), told reporters recently. Every time the virus adapts to a new host, it could pick up mutations that make it more transmissible to humans—and the more sick animals there are, the more likely people are to come into contact with them.
A lack of transparency—and testing
U.S. officials have been slammed for not releasing information in a timely manner, which makes it difficult to judge the risks.
There are clear echoes of Covid-19 failures in this outbreak response, experts say: Data need to be released as quickly and comprehensively as possible, officials should make statements based on currently available evidence, and testing needs to be frequent and widespread, even among animals and people with no symptoms.
Cows in a herd in North Carolina recently tested positive despite not showing any symptoms, raising concerns that H5N1 may be spreading more among cows—and potentially in other animals—than previously believed. It’s not clear how many cows are sick, because until last week the U.S. Department of Agriculture (USDA) was only reimbursing the costs of tests for 20 cows per farm, and only for cows with symptoms.
The USDA posted an update indicating cows may be spreading the virus to chickens on other farms, yet the agency is not currently testing pigs—which are well-known vectors for influenza variants among people—more than usual.
H5N1 viral fragments were found last week in milk being sold at grocery stores, but so far, scientists have not been able to culture the virus—indicating these fragments are likely not contagious, but rather show how widespread infections are. Officials say there are currently no risks from consuming pasteurized milk and other dairy products.
On Friday, testing labs received an “urgent” notice from the Centers for Disease Control and Prevention (CDC) advising that a batch of H5 tests had a faulty component, but the agency said it did not affect the accuracy of previous tests.
The CDC does not recommend testing everyone who has been exposed to infected animals. The agency only recommends monitoring for symptoms and testing if symptoms develop. Even close contacts of an infected person may not be tested. This lack of testing reminds some experts of the early Covid-19 response, when SARS-CoV-2 spread widely before people without symptoms were widely tested; although there may be differences in how many people are asymptomatic with Covid-19 versus the flu, we could still be missing cases now.
There have only been two known cases of H5N1 among people in the United States. In 2022, an inmate tested positive after killing a flock of infected chickens in Colorado. The person’s reported symptom was fatigue and they recovered, according to David Daigle at the CDC. The dairy worker this year reported conjunctivitis (or pinkeye), and that person was also recovering as of April 16, Daigle said.
Monitoring and detection of bird flu has improved in recent years, said William Schaffner, a fellow with the Infectious Diseases Society of America and professor of infectious diseases at the Vanderbilt University Medical Center. “We’re looking harder, and we’re finding more.”
We’re looking harder, and we’re finding more.
william schaffner
What happens next?
The more H5N1 spreads in birds, the more opportunities it has to infect new species, and the more chances it has to evolve and potentially pick up new mutations that could make it more transmissible and virulent among people.
Yet testing animals for potentially dangerous illnesses like avian influenza usually only happens for very sick or dead animals. Animals with more mild illnesses—like the cows in North Carolina—might never be tested, which means the virus could be spreading, and mutating, at a greater rate than we realize.
Wastewater monitoring is an economical and unobtrusive way to track infections of people and animals. It can help detect outbreaks before symptoms are even reported, and it can also serve as a measure of how much virus is circulating at a given time. The U.S. is not yet testing wastewater for H5N1, according to the CDC, but some scientists are exploring the possibility globally.
The concern level would ratchet up once more if the virus were detected in pigs, making it all the more puzzling that pigs near dairy farms are reportedly not being monitored more than usual. Pigs can be infected with both avian and human influenza strains at the same time; through a process called “reassortment,” they can swap out pieces of both strains and potentially create a new variant that is better at infecting humans.
The biggest red flag would be evidence that the virus has gotten into people and begun spreading from person to person. The virus could spread from farmers to their friends and family, then into their broader communities, Davis said. “If we start to see that, then that raises a lot more concern because now you have multiple sustained transmission events.”
At the moment, that doesn’t seem to be happening. Across the country, doctors’ visits and hospitalizations for influenza-like illness continue to decline as flu season ends. And there have been no reports of additional sick workers at cow, chicken, or pig farms, which would be “visible” even if officials are slow to release information, said Ziyad Al-Aly, chief of the Research and Education Service at the Veterans Affairs St. Louis Health Care System.
But the current lack of human-to-human spread makes it a good time to sound the alarm before a new pandemic takes root.
The world is changing: climate change and humans are reshaping the planet, causing more animals to come into contact with previously distant species, including people. “We’re invading animal habitats” where animals lived, isolated from humans, for thousands of years, Al-Aly said. Monitoring at all points in the transmission chain “is really not happening as comprehensively as it should be happening.”
That’s important, he said, because “the virus that’s going to start the next pandemic already exists in nature.”
Melody Schreiber is a freelance health and science journalist, and the editor of What We Didn’t Expect: Personal Stories About Premature Birth.
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