The bird flu epidemic puts mink farms back in the spotlight

At the beginning of October last, the mink on a fur farm in Spain suddenly started to get sick. They stopped eating and started salivating excessively. They became clumsy, began to feel tremors, and developed bloody muzzles.

At first, experts suspected the coronavirus might be to blame. It was a reasonable assumption; Since the start of the Covid-19 pandemic, the virus has repeatedly spread through mink farms, causing large animal outbreaks, triggering mass mink culls and prompting temporary moratoriums on farming of minks.

But it wasn’t the coronavirus that had infiltrated Spanish mink farming, scientists quickly discovered. It was H5N1, a highly pathogenic strain of avian flu.

In recent years, a new variant of H5N1 has spread widely among wild and domestic bird populations around the world. It has taken an unusually heavy toll on wild birds and has repeatedly spread to mammals, such as foxes, raccoons and bears, which may feed on infected birds.

But the mink farming epidemic was a new and troubling development, the scientists said. In Spain, the virus appears to be spreading from mink to mink. It also contained an unusual mutation that may be a sign of mammalian adaptation, scientists reported in a recent paper in the journal Eurosurveillance.

The outbreak “confirmed a fear I had” that the virus could spread effectively among mammals, said Dr Thijs Kuiken, a veterinary pathologist at Erasmus University Medical Center in the Netherlands.

There is no evidence the mink, which were all culled, transmitted the virus to humans, and experts have stressed the outbreak is not a cause for panic. But it serves as 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 with the outside world – and underscores the need for more proactive disease surveillance and other precautions, experts said. .

“Should we panic about this? No,” said Dr. Chrissy Eckstrand, a veterinary pathologist at Washington State University College of Veterinary Medicine. “But do we need to stay alert and prepared? I think we absolutely should.

Mink Mortality

In Spain, the first signs of trouble appeared in the first week of October when the death rate soared at a mink farm in Carral. At first, deaths were limited to a subset of the farm’s barns, which collectively housed more than 50,000 mink. But in the weeks that followed, the epidemic spread throughout the farm.

“The mechanism of transmission inside the farm is still unknown, but it is clear that the virus was able to move,” said Dr Isabella Monne, veterinarian at the European Union Reference Laboratory for avian influenza and Newcastle disease, and author of the Eurosurveillance article.

Laboratory tests revealed that the minks were infected with H5N1, and all the animals were subsequently slaughtered.

The precise way the virus entered the mink remains unknown. Farmed mink, including those from the Spanish farm, are often fed raw poultry, which poses a potential risk.

“If given poultry and poultry by-products infected with a strain of bird flu, these mink could potentially catch bird flu,” said Dr Casey Barton Behravesh, who heads 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 provided feed to the mink farms had experienced outbreaks of bird flu, and scientists said the most likely source of the virus was a wild bird. .

In the weeks leading up to the mink farming outbreak, the virus was detected in wild birds in the area. And the minks on the Spanish farm were housed in barns that weren’t completely closed on the sides. It’s a common feature of mink barns, which are usually left partially open to improve air circulation, said Dr Kuiken, who has studied the possibility of coronavirus transmission between wild animals and mink. breeding on Dutch mink farms.

“It was really very disturbing for us to see how open they were to the environment,” said Dr Kuiken, “and how easy it was for mammals and wild birds to enter these farms. of minks and to have contact with minks.”

Wild birds and other animals may be particularly attracted to mink food, a meaty porridge or paste that is typically smeared on top of the animals’ wire cages, experts said.

“It’s like a free buffet for these animals that come in to eat,” Dr Barton Behravesh said.

(Dr Monne pointed out that wild birds were also “victims” of the virus, however, and should not be blamed or targeted.)

Creature containment

Mink are usually housed in high densities, with their cages close together. This housing arrangement, combined with a lack of genetic diversity among farmed mink, could facilitate the rapid spread of a virus that enters a mink on a farm, scientists have said.

And once a virus begins to spread, it begins to pick up new mutations and adapt to its new hosts. Indeed, the researchers found that the influenza virus they isolated from mink in Spain had multiple mutations that distinguished it from sequences isolated from birds. One of these mutations, in particular, has already been 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 ended up on the farm, the scientists warned.

Globally, the H5N1 variant that spread among birds has led to fewer than 10 known cases in humans since December 2021, and there have been no documented cases of human-to-human transmission, according to the CDC. .

“The H5 virus is not well adapted to humans,” said Dr. Jim Lowe, a veterinarian at the University of Illinois College of Veterinary Medicine at Urbana-Champaign.

The fact that the virus emerged in a mink farm is not particularly surprising, he said, and is not necessarily alarming. “This is 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 pose a greater potential risk to humans. “It is more likely that such a virus spreads more easily and efficiently among humans,” Dr Kuiken said.

Eleven farm workers had contact with the mink; all tested negative for the virus, Dr. Monne and colleagues reported. This fact is “reassuring”, said Dr Monne. “But clearly what is worrying is that this virus is spreading everywhere.” This means 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 to spread among mink could exit 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, transmitting a mink-mutated version of the virus to humans or wild animals. In a recent study, Dr. Barton Behravesh and colleagues used GPS collars to track the movements of free-ranging cats living in or around several Utah mink farms that had experienced coronavirus outbreaks. The cats roamed widely, the researchers found.

“They made frequent visits to mink sheds, moved freely around affected farms, visited residential properties and surrounding neighborhoods on several 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, spokesperson for the Animal and Plant Health Inspection Service at the United States Department of Agriculture.

But with the virus so widespread, more proactive flu monitoring — including regular sampling of animals for asymptomatic infections — is needed on mink farms, the scientists said.

The mink is “certainly an animal that deserves increased attention”, said Dr Barton Behravesh.

Ensuring mink have clean food and water sources and that farm workers follow basic hygiene and sanitation practices can also help reduce risk on mink farms, said experts.

But Dr Kuiken said more drastic changes may be needed. “You also have to think first if you want to have mink farms,” he said. “We need to think a lot more about our human activities in ways that try to prevent the problems we see, for example, with the emergence of infectious diseases, rather than trying to mitigate them or fix them after they’ appeared.