Bird flu, at this point, is somewhat of a misnomer. The virus, which primarily infects birds, is circulating uncontrolled around much of the world, devastating not just birds but wide swaths of the animal kingdom. Foxes, bobcats, and pigs have fallen ill. Grizzly bears have gone blind. Sea creatures, including seals and sea lions, have died in great numbers.
But none of the sickened animals has raised as much concern as mink. In October, a bird-flu outbreak erupted at a Spanish mink farm, killing thousands of the animals before the rest were culled. It later became clear that the virus had spread between the animals, picking up a mutation that helped it thrive in mammals. It was likely the first time that mammal-to-mammal spread drove a huge outbreak of bird flu. Because mink are known to spread certain viruses to humans, the fear was that the disease could jump from mink to people. No humans got sick from the outbreak in Spain, but other infections have spread from mink to humans before: In 2020, COVID outbreaks on Danish mink farms led to new mink-related variants that spread to a small number of humans.
As mammals ourselves, we have good reason to be concerned. Outbreaks on crowded mink farms are an ideal scenario for bird flu to mutate. If, in doing so, it picks up the ability to spread between humans, it could potentially start another global pandemic. “There are many reasons to be concerned about mink,” Tom Peacock, a flu researcher at Imperial College London, told me. Right now, mink are a problem we can’t afford to ignore.
For two animals with very different body types, mink and humans have some unusual similarities. Research suggests that we share similar receptors for COVID, bird flu, and human flu, through which these viruses can gain entry into our bodies. The numerous COVID outbreaks on mink farms during the early pandemic, and the bird-flu outbreak in Spain, gravely illustrate this point. It’s “not surprising” that mink can get these respiratory diseases, James Lowe, a veterinary-medicine professor at the University of Illinois at Urbana-Champaign, told me. Mink are closely related to ferrets, which are so well known for their susceptibility to human flu that they’re the go-to model for flu research.
Mink wouldn’t get sick as often, and wouldn’t be as big an issue for humans, if we didn’t keep farming them for fur in the perfect conditions for outbreaks. Many barns used to raise mink are partially open-air, allowing infected wild birds to come in contact with the animals, sharing not only air but potentially food. Mink farms are also notoriously cramped: The Spanish farm, for example, kept tens of thousands of mink in about 30 barns. Viral transmission would be all but guaranteed in those conditions, but the animals are especially vulnerable. Because mink are normally solitary creatures, they face significant stress in packed barns, which may further predispose them to disease, Angela Bosco-Lauth, a biomedical-sciences professor at Colorado State University, told me. And because they’re often inbred so their coats look alike, an entire population may share a similar genetic susceptibility to disease. The frequency of outbreaks among mink, Bosco-Lauth said, “may actually have less to do with the animals and more to do with the fact that we raise them in the same way … we would an intensive cattle farm or chickens.”
So far, there’s no evidence that mink from the Spanish farm spread bird flu to humans: None of the workers tested positive for the virus, and since then, no other mink farms have reported outbreaks. “We’re just not very susceptible” to bird flu, Lowe said. Our bird-flu receptors are tucked deep in our lungs, but when we’re exposed, most of the virus gets caught in the nose, throat, and other parts of the upper respiratory tract. This is why bird-flu infection is less common in people but is often pneumonia-level severe when it does happen. Indeed, a few humans have gotten sick and died from bird flu in the 27 years that the current strain of bird flu, known as H5N1, has circulated. This month, a girl in Cambodia died from the virus after potentially encountering a sick bird. The more virus circulating in an environment, the higher the chances a person will get infected. “It’s a dose thing,” Lowe said.
But our susceptibility to bird flu could change. Another mink outbreak would give the virus more opportunities to keep mutating. The worry is that this could create a new variant that’s better at binding to the human flu receptors in our upper respiratory tract, Stephanie Seifert, a professor at Washington State University who studies zoonotic pathogens, told me. If the virus gains the ability to infect the nose and throat, Peacock, at Imperial College London, said, it would be better at spreading. Those mutations “would worry us the most.” Fortunately, the mutations that arose on the Spanish mink farm “were not as bad as many of us worried about,” he added, “but that doesn’t mean that the next time this happens, this will also be the case.”
Because mink carry the receptors for both bird flu and human flu, they could serve as “mixing vessels” for the viruses to combine, researchers wrote in 2021. (Ferrets, pigs, and humans share this quality too.) Through a process called reassortment, flu viruses can swap segments of their genome, resulting in a kind of Frankenstein pathogen. Although viruses remixed in this way aren’t necessarily more dangerous, they could be, and that’s not a risk worth taking. “The previous three influenza pandemics all arose due to mixing between avian and human influenza viruses,” Peacock said.
While there are good reasons to be concerned about mink, it is hard to gauge just how concerned we should be—especially given what we still don’t know about this changing virus. After the death of the young girl in Cambodia, the World Health Organization called the global bird flu situation “worrying,” while the CDC maintains that the risk to the public is low. Lowe said “it’s certainly not very risky” that bird flu will spill over into humans, but is worth keeping an eye on. H5N1 bird flu is not new, he added, and it hasn’t affected people en masse yet. But the virus has already changed in ways that make it better at infecting wild birds, and as it spreads in the wild, it may continue to change to better infect mammals, including humans. “We don’t understand enough to make strong predictions of public-health risk,” Jonathan Runstadler, an infectious-diseases professor at Tufts University, told me.
As bird flu continues to spread among birds and in domestic and wild animal populations, it will only become harder to control. The virus, formally seasonal, is already present year-round in parts of Europe and Asia, and it is poised to do the same in the Americas. Breaking the chain of transmission is vital to preventing another pandemic. An important step is to avoid situations where humans, mink, or any other animal could be infected with both human and bird flu at the same time.
Since the COVID outbreaks, mink farms have generally beefed up their biosecurity: Farm workers are often required to wear masks and protective gear, such as disposable overalls. To limit the risk to mink—and other susceptible hosts—farms need to reduce their size and density, reduce contact between mink and wild birds, and monitor the virus, Runstadler said. Some nations, including Mexico, Ecuador, have recently embraced bird-flu vaccines for poultry in light of the outbreaks. H5N1 vaccines are also available for humans, though they aren’t readily available. Still, one of the most obvious options is to shut mink farms down. “We probably should have done that after SARS-CoV-2,” Bosco-Lauth, at Colorado State, said. Doing so is controversial, however, because the global mink industry is valuable, with a huge market in China. Denmark, which produces up to 40 percent of the world’s mink pelts, temporarily banned mink breeding in 2020 after a spate of COVID outbreaks, but the ban expired last month, and farms are returning, albeit in a limited capacity.
Mink are far from the only animal that poses a bird-flu risk to humans. “Frankly, with what we’re seeing with other wildlife species, there really aren’t any mammals that I would discount at this point in time,” Bosco-Lauth said. Any mammal species repeatedly infected by the virus is a potential risk, including marine mammals, such as seals. But we should be most concerned about the ones humans frequently come into close contact with, especially animals that are raised in high density, such as pigs, Runstadler said. This doesn’t pose just a human public-health concern, he said, but the potential for “ecological disruption.” Bird flu can be a devastating disease for wildlife, killing animals swiftly and without mercy.
Whether or not bird flu makes the jump into humans, it isn’t the last virus that will threaten us—or mink. The era we live in has become known as the “Pandemicene,” as my colleague Ed Yong has called it, one defined by the regular spillover of viruses into humans, caused by our disruption of the normal trajectories of viral movement in nature. Mink may never pass bird flu to us. But that doesn’t mean they won’t be a risk the next time a novel influenza or coronavirus comes around. Doing nothing about mink essentially means choosing luck as a public-health strategy. Sooner or later, it will run out.