Amid a global bird die-off from avian flu, officials have also noticed the deadly virus strain, called H5N1, infecting a growing number of mammals. This week, the World Health Organization (WHO) urged authorities to remain vigilant—but not panic—about the virus’s potential risk to humans.
“The recent spillover to mammals needs to be monitored closely,” Tedros Adhanom Ghebreyesus, the WHO’s director-general, told reporters on Wednesday, according to the Agence France-Presse (AFP). But “for the moment, WHO assesses the risk to humans as low.”
Avian flu is not adapted to infect people, making human cases rare and person-to-person transmission even more difficult. But experts say that the more H5N1 spreads among animals, the more likely it is to evolve into a variant that can jump to humans, per the AFP.
Despite the current low risk to public health, officials must prepare “to face outbreaks in humans, and be ready also to control them as soon as possible,” Sylvie Briand, director of Global Infectious Hazard Preparedness and Emergency Preparedness at the WHO, tells Fortune’s Erin Prater.
H5N1 was first detected in domestic waterfowl in 1996 and spread to migratory birds around 2005. Then, these long-distance fliers carried the virus across the world, writes Science’s Kai Kupferschmidt. Over that time, the virus has infected relatively few humans—but those cases have proven deadly. According to the WHO, there were 868 global cases of H5N1 in humans between January 2003 and November 2022, 457 of which were fatal.
Currently, a massive outbreak of bird flu is taking place around the world. The United States is experiencing the worst avian flu outbreak in its history, with the virus directly or indirectly leading to 58 million bird deaths in the past year, per Fortune. Europe is also experiencing its most severe outbreak, according to the AFP.
“With high levels of transmission we are seeing unprecedented numbers of dead birds and outbreaks,” Michelle Wille, a bird flu researcher at the University of Sydney in Australia, tells the Sydney Morning Herald’s Liam Mannix.
H5N1 doesn’t tend to infect mammals, because they have fewer of the receptors in their upper airways that the virus binds to.
But during this year’s outbreak, foxes, raccoons, bears and other mammals have caught the virus. In the U.S., mammalian infections have been detected in nine different states, according to USA Today’s Adrianna Rodriguez. In Peru, at least 585 sea lions have been found dead, likely due to bird flu. Other infected animals include dolphins and opossums, per Fortune.
Most of these cases are probably caused by a mammal eating an infected bird, Jürgen Richt, who studies avian flu at Kansas State University, tells USA Today. But in a paper published in January in the journal Eurosurveillance, researchers document evidence that the virus might have spread between minks on a farm in Spain last October. Genetic sequencing revealed a genetic change known to make some influenza viruses more capable of reproducing in mammals, writes Nature News’ Saima May Sidik.
The mink outbreak “confirmed a fear that I had” that bird flu could spread efficiently in mammals, Thijs Kuiken, a veterinary pathologist at Erasmus University Medical Center in the Netherlands, tells the Times.
“We’ve never seen mammal-to-mammal transmission, ever. It has never happened,” Wille says to the Sydney Morning Herald. “Now it’s no longer just a hypothetical. Now we’ve actually seen it happen.”
Experts say this development is not a cause for alarm. “It’s not, in my mind, a particularly worrisome situation for human health,” Jim Lowe, a veterinarian at the University of Illinois at Urbana-Champaign, tells the Times. “Obviously it’s not very good for the mink.”
The tightly packed, caged minks could have transmitted H5N1 due to their conditions rather than a fundamental change in the virus, Frank Wong, a bird flu expert at Australia’s Commonwealth Scientific and Industrial Research Organization, tells the Sydney Morning Herald. “It’s still a bird-adapted virus.”
But evidence of spread between mammals is also a warning sign, others say. “This outbreak signals the very real potential for the emergence of mammal-to-mammal transmission,” Wille told the CBC News’ Lauren Pelley in an email.
“We need to be vigilant to make sure that spread in animals is contained,” Briand tells the AFP. “The more the virus circulates in animals, the higher is the risk for humans as well.”