As a deadly strain of avian influenza continues to decimate bird populations around the world and spread among other animals, some scientists are warning that mammal-to-mammal transmission has emerged as a real possibility with potentially catastrophic consequences for humans.
Over the past year, officials in the United States, the United Kingdom, and Canada have detected cases of the highly pathogenic H5N1 bird flu in a variety of species, including bears, foxes, otters, raccoons, and skunks. Last month, a cat suffered serious neurological symptoms from a late 2022 infection, according to French officials who said that the virus showed genetic characteristics consistent with adaptation to mammals.
Most of these infections are likely the result of mammals eating infected birds, according to Jürgen Richt, director of the Center on Emerging and Zoonotic Infectious Diseases at Kansas State University.
More alarming, multiple researchers argue, was the large outbreak of H5N1 on a Spanish mink farm last October, which could mark the first known instance of mammal-to-mammal transmission.
"Farmworkers began noticing a spike in deaths among the animals, with sick minks experiencing an array of dire symptoms like loss of appetite, excessive saliva, bloody snouts, tremors, and a lack of muscle control," CBC Newsreported Thursday. "Eventually, the entire population of minks was either killed or culled—more than 50,000 animals in total."
"A virus which has evolved on a mink farm and subsequently infects farmworkers exposed to infected animals is a highly plausible route for the emergence of a virus capable of human-to-human transmission to emerge."
A study published two weeks ago in Eurosurveillance, a peer-reviewed journal of epidemiological research, described the outbreak and its public health implications. Notably, the authors wrote that their findings "indicate that an onward transmission of the virus to other minks may have taken place in the affected farm."
As CBC News noted, "That's a major shift, after only sporadic cases among humans and other mammals over the last decade."
Michelle Wille, a University of Sydney researcher who focuses on the dynamics of wild bird viruses, told the Canadian outlet that "this outbreak signals the very real potential for the emergence of mammal-to-mammal transmission."
It's just one farm and none of the workers—all of whom wore personal protective equipment—were infected. However, Dr. Isaac Bogoch, a Toronto-based infectious disease specialist, warned Thursday that if the virus mutates in a way that enables it to become increasingly transmissible between mammals, including humans, "it could have deadly consequences."
"This is an infection that has epidemic and pandemic potential," Bogoch told CBC News. "I don't know if people recognize how big a deal this is."
A "mass mortality event" involving roughly 2,500 endangered seals found off the coast of Russia's Caspian Sea last month has also raised alarm.
A researcher at Russia's Dagestan State University, Alimurad Gadzhiyev, said last week that early samples from the seals "tested positive for bird flu," adding that they were still studying whether the virus caused the die-off.
Peacock warned there have been mixed reports from Russia about the seals, which could have contracted the virus by eating infected seabirds.
But if the seals did give bird flu to each other it "would be yet another very concerning development," he added.
"The mink outbreaks, the increased number of infections of scavenger mammals, and the potential seal outbreak would all point to this virus having the potential to cause a pandemic" in humans, he said.
Among birds, the mortality rate of H5N1 can approach 100%, ravaging wild bird populations and poultry farms alike. The World Organization for Animal Health told BBC News on Thursday that it has recorded almost 42 million cases of H5N1 in wild and domestic birds since the current outbreak started in October 2021. Another 193 million domestic birds have been culled in an attempt to curb transmission.
The highly pathogenic strain of avian flu also frequently causes death in other mammals, including humans. According to the World Health Organization (WHO), nearly 870 cases of H5N1 were reported in humans from 2003 to 2022 and they resulted in at least 457 deaths—a fatality rate that exceeds 50%.
The virus has "not acquired the ability for sustained transmission among humans," the WHO stated last month. "Thus the likelihood of human-to-human spread is low."
However, a December report from the U.K. Health Security Agency warned that the "rapid and consistent acquisition of the mutation in mammals may imply this virus has a propensity to cause zoonotic infections," meaning that it could jump to humans.
Dr. Wenqing Zhang, head of the WHO's global influenza program, told BBC News on Thursday that the threat posed by the virus spilling over "is very concerning and the risk has been increasing over the years as reflected in the number of outbreaks in animals as well as a number of infections in humans."
"We're closely related to minks and ferrets, in terms of influenza risks... If it's propagating to minks, and killing minks, it's worrisome to us."
As CBC News reported this week: "Most human infections also appeared to involve people having direct contact with infected birds. Real-world mink-to-mink transmission now firmly suggests H5N1 is now 'poised to emerge in mammals,' Wille said—and while the outbreak in Spain may be the first reported instance of mammalian spread, it may not be the last."
Wille warned that "a virus which has evolved on a mink farm and subsequently infects farmworkers exposed to infected animals is a highly plausible route for the emergence of a virus capable of human-to-human transmission to emerge."
Louise Moncla, an assistant professor of pathobiology at the University of Pennsylvania, told the outlet that viruses often adapt to new host species through an "intermediary host."
"And so what's concerning about this is that this is exactly the kind of scenario you would expect to see that could lead to this type of adaptation, that could allow these viruses to replicate better in other mammals—like us," Moncla explained.
The alarm bells sounded this week echo long-standing warnings about the growing prospects of a devastating bird flu pandemic.
In his 2005 book, The Monster at Our Door, the late historian Mike Davis wrote that "the essence of the avian flu threat... is that a mutant influenza of nightmarish virulence—evolved and now entrenched in ecological niches recently created by global agro-capitalism—is searching for the new gene or two that will enable it to travel at pandemic velocity through a densely urbanized and mostly poor humanity."
Alluding to the "constantly evolving nature of influenza viruses," the WHO recently stressed "the importance of global surveillance to detect and monitor virological, epidemiological, and clinical changes associated with emerging or circulating influenza viruses that may affect human (or animal) health, and timely virus-sharing for risk assessment."
To avert a cataclysmic bird flu pandemic, scientists have also emphasized the need to ramp up H5N1 vaccine production, with Wille pointing out that "a very aggressive and successful poultry vaccination campaign ultimately stopped all human cases" of the H7N9 strain of the virus in the early 2010s.
Others have also criticized the global fur farming industry, citing the spread of bird flu as well the coronavirus among cruelly confined minks.
"We're closely related to minks and ferrets, in terms of influenza risks," Dr. Jan Hajek, an infectious diseases physician at Vancouver General Hospital, told CBC News. "If it's propagating to minks, and killing minks, it's worrisome to us."