Early last October, the mink on a fur farm in Spain suddenly began to fall ill. They stopped eating and began salivating excessively. They became clumsy, started to experience tremors and developed bloody snouts.
At first, experts suspected that the coronavirus might be to blame. It was a reasonable assumption; since the beginning of the Covid-19 pandemic, the virus has repeatedly found its way onto mink farms, sparking large animal outbreaks, triggering mass mink culls and prompting temporary moratoriums on mink farming.
But it was not the coronavirus that had infiltrated the Spanish mink farm, scientists soon discovered. It was H5N1, a highly pathogenic strain of avian influenza.
Over the last few years, a new variant of H5N1 has spread widely through wild and domestic bird populations around the world. It has taken an unusually heavy toll on wild birds and repeatedly spilled over into mammals, such as foxes, raccoons and bears, that might feed on infected birds.
But the mink farm outbreak was a new and troubling development, scientists said. In Spain, the virus appeared to spread from mink to mink. It also contained an unusual mutation that might be a sign of adaptation to mammals, scientists reported in a recent paper in the journal Eurosurveillance.
The outbreak “confirmed a fear that I had” that the virus could spread efficiently among mammals, said Dr. Thijs Kuiken, a veterinary pathologist at Erasmus University Medical Center in the Netherlands.
There is no evidence that the mink, which were all culled, transmitted the virus to humans, and experts stressed that the outbreak was not a cause for panic. But it is a reminder of some of the risks posed by mink farms — places in which large numbers of susceptible animals are housed in facilities with porous borders to the outside world — and highlights the need for more proactive disease surveillance and other precautions, experts said.
“Should we freak out about this? No,” said Dr. Chrissy Eckstrand, a veterinary pathologist at the College of Veterinary Medicine at Washington State University. “But should we stay vigilant and prepared? I think absolutely we should.”
In Spain, the first signs of trouble came during the first week of October, when the mortality rate spiked on a mink farm in Carral. At first, the deaths were confined to a subset of the farm’s barns, which collectively housed more than 50,000 mink. But in the weeks that followed, the outbreak spread throughout the entire farm.
“The mechanism of transmission inside the farm is still unknown, but it’s clear that the virus was able to move,” said Dr. Isabella Monne,a veterinarian at the European Union Reference Laboratory for Avian Influenza and Newcastle Disease, and an author of the Eurosurveillance paper.
Laboratory testing revealed that the mink were infected with H5N1, and all the animals were subsequently culled.
Precisely how the virus got into the mink remains unknown. Farmed mink, including those on the Spanish farm, are often fed raw poultry, which presents a potential risk.
“If they were to be given infected poultry and poultry byproducts with an avian influenza strain, those mink could potentially get avian influenza,” said Dr. Casey Barton Behravesh, who directs the One Health Office at the Centers for Disease Control and Prevention.
But in this case, there was no evidence that the poultry farms that supplied feed to the mink farms had experienced avian influenza outbreaks, and scientists said that the most likely source of the virus was a wild bird.
In the weeks before the mink farm outbreak, the virus was detected in wild birds in the region. And the mink on the Spanish farm were housed in barns that were not completely enclosed on the sides. That is a common feature of mink barns, which are generally left partially open to improve airflow, said Dr. Kuiken, who has studied the possibility of coronavirus transmission between wild animals and farmed mink on Dutch mink farms.
“It was really quite disturbing to us to see how open they were to the environment,” Dr. Kuiken said, “and how easy it was for both mammals and wild birds to get into these mink farms and have contact with mink.”
Wild birds and other animals may be especially attracted by the minks’ food, a meaty mush or paste that is typically smeared across the top of the animals’ wire cages, experts said.
“It’s like a free buffet for these animals to come and eat,” Dr. Barton Behravesh said.
(Dr. Monne stressed that wild birds were also “victims” of the virus, however, and should not be blamed or targeted.)
Mink are typically housed in high densities, with their cages close together. This housing arrangement, combined with a lack of genetic diversity among farmed mink, could make it easier for a virus that finds its way into a mink to spread quickly through a farm, scientists said.
And once a virus starts to spread, it begins picking up new mutations and adapting to its new hosts. Indeed, researchers found that the flu virus they isolated from the mink in Spain had multiple mutations that set it apart from sequences isolated from birds. One of these mutations, in particular, has been previously shown to help influenza replicate better in mammalian cells.
Still, the significance of some of the mutations remains unknown, and researchers cannot rule out the possibility that they were present in the virus before it found its way onto the farm, scientists cautioned.
Globally, the H5N1 variant that has been spreading in birds has led to fewer than 10 known cases in people since December 2021, and there have been no documented instances of human-to-human transmission, according to the C.D.C.
“The H5 virus is not well adapted to humans,” said Dr. Jim Lowe, a veterinarian at the College of Veterinary Medicine at the University of Illinois at Urbana-Champaign.
The fact that the virus showed up on a mink farm is not particularly surprising, he said, and not necessarily cause for alarm. “It’s not, in my mind, a particularly worrisome situation for human health,” Dr. Lowe said. “Obviously it’s not very good for the mink.”
But a mink-adapted version of the virus could present a greater potential risk to people. “It’s more likely that such a virus will be more easily efficiently spread among humans,” Dr. Kuiken said.
Eleven farm workers had contact with the mink; all tested negative for the virus, Dr. Monne and her colleagues reported. That fact is “reassuring,” Dr. Monne said. “But clearly, what is worrisome is that this virus is spreading everywhere.” That means that there will be more opportunities for the virus to infect, and potentially spread, in mink and other mammals.
The permeability of mink farms also means that a virus that begins spreading in mink could make its way off the farm. Mink sometimes escape from farms, and dogs and cats on mink farms with coronavirus outbreaks have also been infected with the virus, scientists have found.
These animals could potentially act as intermediate hosts, passing a mutated mink version of the virus on to humans or wild animals. In one recent study, Dr. Barton Behravesh and her colleagues used GPS collars to track the movements of free-roaming cats living on or around several Utah mink farms that had experienced coronavirus outbreaks. The cats roamed widely, the researchers found.
“They made frequent visits to the mink sheds, moved freely around affected farms, visited surrounding residential properties and neighborhoods on multiple occasions,” Dr. Barton Behravesh said.
Highly pathogenic avian influenza has not been detected on any mink farms in the United States to date, said Lyndsay Cole, a spokeswoman for the for the Animal and Plant Health Inspection Service at theU.S. Department of Agriculture.
But with the virus so widespread, more proactive influenza surveillance — including regularly sampling animals for asymptomatic infections — is needed on mink farms, scientists said.
Mink are “definitely an animal that warrants heightened attention,” Dr. Barton Behravesh said.
Ensuring that mink have clean food and water sources and that farm workers adhere to basic hygiene and sanitation practices can also help reduce the risks on mink farms, experts said.
But Dr. Kuiken said that more sweeping changes might be needed. “You have to also think in the first place whether you want to have mink farms,” he said. “We need to be thinking much more about our human activities in a way that we try to prevent the problems that we’re seeing, for example, with the emergence of infectious diseases, rather than trying to mitigate them or solve them after they’ve appeared.”