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Ongoing outbreaks of avian influenza have decimated poultry flocks and wild birds across the United States and worldwide. The virus, known as H5N1, is also increasingly adapting to mammals and has been found in cats, goats, and raccoons. In the US, it has spread to at least 170 dairy herds across 13 states. And in April, health officials confirmed that a dairy worker had caught the virus from an infected cow. This was the first time the virus made the jump from a mammal to a human.
Now, the number of people becoming infected with bird flu is ticking upward. On July 25, the US Centers for Disease Control and Prevention confirmed an additional three human cases, bringing the total number of US cases to 13 since April. The infections occurred in people who were working directly with infected poultry at an egg farm in Colorado that had reported an outbreak of H5N1 among its birds. All three people have mild symptoms and have been offered Tamiflu, an antiviral drug. The CDC says the risk of H5N1 infection in the general public remains low.
“These cases are not entirely surprising given that these people were working with infected poultry,” says Stephen Morse, an epidemiologist at Columbia University in New York. “The good news is that so far, there’s no evidence that this has spread from person to person. At that point, we’d really have to ramp up the concern to the level of red alert.”
The CDC is looking into whether the workers in Colorado were wearing personal protective equipment, or PPE, such as gloves, coveralls, footwear, masks, and goggles. Historically, most human cases of bird flu infection have happened in people who were not wearing recommended PPE, according to the agency.
The new cases come shortly after another cluster of human infections was identified this month. On July 19, the CDC confirmed six human cases of bird flu among poultry workers at a different facility in Colorado. Those cases were in workers who were involved in the culling of birds infected with H5N1. Once the virus is found on a farm, poultry producers must cull entire flocks. With the latest three infections, Colorado now has nine confirmed cases of bird flu.
The other four cases—one in Texas, two in Michigan, and one in Colorado—have been linked to exposure to infected dairy cows. The virus likely spread to the workers through raw milk. A study published in May found that the virus can remain stable on milking equipment for at least an hour, increasing its potential to infect people and other animals. Pasteurizing milk, however, kills the H5N1 virus.
So far, all the US cases this year have resulted in mild symptoms, but in the past, H5N1 has had a lethality rate of around 50 percent. From 2003 to 2023, a total of 878 people tested positive for the virus, and 458 deaths were reported.
The last time H5N1 caused a major outbreak among US poultry flocks was in 2015, when it wiped out 50.5 million birds. It wasn’t until April 2022 that the US saw its first reported human case of bird flu, in a poultry worker in Colorado. No further cases were reported until this year. “Something has changed,” says Anice Lowen, a flu researcher at Emory University in Atlanta, Georgia. “Whether it’s due to changes in the virus or changes in the circumstances of exposure is hard to know without more information.”
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The CDC’s sequencing of the virus from the sixth human case reported from Colorado shows that it is related to the viruses detected in recent poultry outbreaks and infected dairy cattle herds. It contains a marker of mammalian adaptation that has been identified in more than 99 percent of dairy cow sequences, as well as in the first Michigan human case.
It’s possible there are more human infections that are going undetected. The US has tested only around 200 individuals for the virus, while more than 4,100 people are being monitored. Lowen says ideally everyone working with infected animals should be tested. “It’s really important that we know the extent of spillover,” she says. “I think this country certainly has the resources to be doing more testing.”
Matthew Miller, codirector of the Canadian Pandemic Preparedness Hub at McMaster University in Ontario, says proper PPE is critically important for groups at high risk of exposure. “Giving the virus an opportunity to infect humans is giving it an opportunity to learn how to transmit between humans,” he says. Flu viruses undergo constant changes in a host as they replicate, and it’s possible that H5N1 could eventually develop mutations that would make human transmission more likely.
In the case of the first Colorado facility, US health officials acknowledged that high summer temperatures may have played a role in those six infections. “At the time transmission is thought to have occurred, Colorado was experiencing 104-plus-degree heat,” said Nirav Shah, principal deputy director of the CDC, in a press conference on July 16. “The barns in which culling operations occur were no doubt even hotter. All of this makes PPE use a challenge.” There were also large industrial fans in the barns, which could have blown virus particles through the air, increasing the risk of infection.
Alongside rising cases in people, the virus has also spread to dozens of mammal species. A study published in Nature on July 25 found that the spillover of avian flu from birds to dairy cattle led to mammal-to-mammal transmission—between cows, and from cows to cats and a raccoon—raising concerns that H5N1 could eventually adapt to spread among people.
As H5N1 spreads to more species, scientists are worried about its potential to spark a pandemic. Some countries have tried vaccinating poultry against avian flu, but Morse says the problem is making enough doses at scale. Commercially raised chickens have short lives, and in the US, facilities may raise thousands of birds. “A lot of animals would need to be vaccinated, and in practical terms, it’s very hard to do,” Morse says.
Finland has started vaccinating high-risk workers at fur and poultry farms, and Morse thinks the US should follow suit. Several H5N1 vaccines have been licensed and the US has stockpiled a supply of these, but because the virus has a high mutation rate, it’s not known whether those vaccines would be effective against the currently circulating virus. The US recently awarded Moderna $176 million to develop an mRNA vaccine against H5N1. The company, which made one of the Covid-19 mRNA vaccines, is currently testing its bird flu vaccine in an early-stage trial.
