The Wild World of Mink and Coronavirus

Mink on the lam and corona’s reverse spillover

By Kate Golden

January 7, 2021

Dark brown furry wild mink staring at the camera against a snowy white background.

Wild American mink in the snow. | Photo by Carol Hamilton/iStock

The first sign of trouble was that the mink stopped eating, said Hugh Hildebrandt, one of two main mink vets in Wisconsin. Next came coughing and sneezing, lethargy and labored breathing. Hildebrandt had worked with mink for 30 years. He wrote the Merck Veterinary Manual section on mink. But he had never seen anything like this. 

Captive mink have a flu season in the fall, just like people—they get it from us, in fact. But what appeared in the two mink farms in Taylor, Wisconsin, that saw outbreaks in October was not flu, which tends to sicken the weakest animals. This took out the strongest mink, the mature adult females. Over a few days, it killed hundreds per day and about 5,500 total on the two ranches. It whipped through by coat color, light to dark: The lighter-coat mink, ranch-bred to bring out recessive genes, have long been more delicate. 

Five to seven days into the outbreak, the ranchers thought that most of the mink were going to die, said Hildebrandt. “And they wake up the next morning, and it's just stopped. They all start eating. They eat more than they ever did before.”

The mink detectives

It wasn’t hard to guess the cause. Wisconsin was a hotspot of the pandemic from late summer on, and workers at mink ranches had already tested positive for SARS-CoV-2. But the Wisconsin Veterinary Diagnostic Laboratory in Madison confirmed the suspicion within days. The mink almost certainly got it from farmworkers, a jump called “reverse zoonosis.” 

The first infections of US farmed mink,  in Utah in August, had triggered a national investigation involving wildlife and human health experts across local, state, and federal agencies. Their questions: How did the virus get there, where would it go next, and what could it do? In Europe, the virus had spread from farm to farm, and also jumped back to humans. 

Then, in mid-December, a wild mink trapped near a Utah mink farm was confirmed to have the virus. 

“To our knowledge, this is the first free-ranging, native wild animal confirmed with SARS-CoV-2,” Thomas DeLiberto and Susan Shriner, of the USDA’s Animal and Plant Inspection Service, said. Soon after that, a second mink—an Oregon farm escapee—tested positive too.

Global industry meets global pandemic

Of all the animals that have been infected with SARS-CoV-2 (mostly dogs and cats, plus a few other mammals), the only species to have suffered large-scale casualties so far is the American mink, Neovison vison, living on mink ranches around the world. Since the first mink got sick on a Dutch mink ranch in April, millions of the animals have died or been preemptively culled on nearly 400 ranches across Denmark, the Netherlands, Sweden, Lithuania, Greece, Italy, France, Spain, and Canada. The US has seen 16 ranch outbreaks since August: 12 in Utah, two in Wisconsin, one each in Oregon and Michigan.

“It’s a top priority in human and veterinary diagnostic labs,” Keith Poulsen, director of the Wisconsin veterinary lab, said. One of about a dozen such labs nationwide, the WVDL has been running COVID tests on farmed mink and people, in addition to its usual tests on cows, chickens, and other animals, and is now operating from 5 A.M. to 2 A.M. Epidemiologists use the term “One Health” to describe why investigating zoonotic disease outbreaks is critical to the health of people, livestock, wildlife, and the environment. Outbreaks on European mink ranches demonstrated the risks. If the virus managed to establish a reservoir of disease in farmed mink or their wild cousins, it could hamper our fight against the pandemic, harm wildlife, or even threaten ecosystems. Particularly if it mutated along the way into something deadlier, more transmissible, or harder for modern medicine to attack.

No man is an island, and neither are diseases

Zoonotic outbreaks happen constantly, and disease reservoirs are everywhere. In one recent week, ProMed, the bulletin of the Massachusetts-based International Society for Infectious Diseases, emailed reports of avian influenza in Chinese people and Polish poultry, influenza in an Oregon horse, anthrax in Croatian cattle, and Australian bat lyssavirus in Queensland. Raccoons harbor rabies; rodents across the American West harbor the bacterium that causes plague. Diseases can jump the other way too. People are a reservoir of tuberculosis for cattle.

Whenever a virus jumps to a new host species, it adapts by mutating. Some of these mutations can help the virus spread faster, worsen the severity of disease, make it harder for the body to fight, or make therapeutics or vaccines less effective. That has come to pass recently: a new, apparently super-contagious variant first discovered in Britain has since been found in dozens of countries.  

No evidence has emerged yet that farmed mink have infected people in the US, though that investigation is still ongoing. But Denmark’s outbreaks showed the potential. Hundreds of farms were affected—Denmark, the world’s biggest mink producer, had 17 million animals.  

Researchers found the virus passed from people to mink and back again, mutating as it went. A genetic variant dubbed Cluster-5 looked extra nasty, because the virus’s spike proteins had changed in a way that made it more difficult for the monoclonal antibodies used to treat some COVID patients (President Donald Trump was given monoclonal antibodies when he got sick) to recognize the virus, at least in the lab. At least 367 people got infected with mink variants, and 12 of those people had Cluster-5. 

But it wasn’t those few human cases that prompted the Danish and Dutch governments to order culls of their mink. Instead, as Danish virologist Marion Koopmans wrote in a November letter to The Lancet, it was the risk of establishing a new reservoir for the disease, she wrote, and the unknown consequences. 

By mid-November, a lucky break emerged from the Danes’ surveillance: Cluster-5 sequences had stopped turning up in human cases and had, they guessed, dead-ended. It was noteworthy enough that Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID), weighed in. It didn’t look like Cluster-5 had spread far enough to reduce vaccine efficacy, he said, though it was important to keep an eye on it.

Cluster-5 turned out not as bad a variant as feared, but the next one could be worse.

The mysteries of mutation

When it comes to deciding how much to freak out about a zoonotic virus, the details—like what hosts they prefer, how long they stick around, or how fast they can mutate—make all the difference. 

“Some viruses hardly ever change, but a host can remain infected for decades. Others mutate at a furious rate and change to outpace our immune response,” wrote Hon Ip, a virologist at the National Wildlife Health Center, a lab that has been testing wild animals trapped near mink farms. 

Speed is essential for any zoonotic disease response. In the spring of 2020, scientists at both the National Wildlife Health Center and the veterinary lab hurried to create validated genetic tests for SARS-CoV-2. They knew mink might be susceptible, because they share some immune response similarities and a key lung receptor with people. Ferrets, which are closely related to mink, are used as animal models in human respiratory health studies.

“We have a pretty extensive infrastructure to be able to do this very, very quickly,”  said Keith Poulsen, director of the Wisconsin veterinary lab. “We’d been at this super-high alert level. And they (mink farmers) were seeing thousands die per day.”

They knew, too, that Wisconsin had a heck of a lot of mink. Last year its farmers produced a million pelts, one-third of the US total and the most of any state. In the past few years, Taylor County, Wisconsin, produced the most mink pelts in North America, according to Hildebrandt.

CDC teams began collecting samples from the people and the mink on affected farms, while US Wildlife Services live-trapped wild animals nearby. Farms that went under quarantine would not be released until all their tests were negative, “and we’re sure we’re not making a wildlife reservoir,” Poulsen said. 

Tracking who gave what to whom will take some time. (That effort is being led by the National Veterinary Services Laboratory in Ames, Iowa.) Comparing genomic sequences is a tricky chicken-and-egg problem, but some initial results have emerged.

“Some of them are very, very clear,” Poulsen said. “People became sick from community-acquired infection, and they gave it to the mink. But that’s not all the cases.”

The Mink Princesses are no more, but the farms remain

A billboard once stood in the middle of town that proclaimed Medford, Wisconsin, as the Mink Capital of the World. That crossroads is now the home of the Taylor County Museum, where by coincidence, Mary Schultz of the Taylor County Historical Society, a longtime area resident, just finished curating an exhibit about the annual Medford Mink Festival once held there.

Schultz described the exhibit: a small space, full of mink-covered things. “It was a big thing back in the day,” she said. The weeklong festival, first held in 1964, included a pageant to choose a Miss Medford Mink Princess from among the local high-schoolers. (The exhibit includes scrapbooks of all the princesses.) The pageant included a swimsuit competition, though, thankfully, no mink bikinis. Many items that would not spring to mind as wanting a bit of fur were made of or topped with a bit of fur, including toothpicks, a toothbrush, a men’s tie and a bowtie, cufflink mice, and a wallet. “I don’t know who in the world would wear that tie,” Schultz said, amused.

The mink billboard came down, Schultz said, around when the mink festivals stopped—the last Mink Princess picture is from 1979. Today, even in Medford, mink ranches operate discreetly, as they have done ever since the anti-fur campaigns of the 1980s.

“There’s a great big mink ranch up on a highway northeast of us,” Schultz said. “There’s no sign out there that says what this is, and I don’t even know who owns it.”

An invisible industry

In her Lancet letter, Danish virologist Koopmans wrote that the outbreaks in her country should be a lesson for the fur sector worldwide. “(T)here is currently no global overview of the location of such farms, and no mandatory surveillance programme. In view of our observations, that is urgently needed.” 

That will be no easy task. In Wisconsin, the mink industry was so under the radar that the Wisconsin state veterinarian, Darlene Konkle, had to ask the Fur Commission USA how to reach the farmers and how many there were.

“Ordinarily, with mink farms, we don’t have a lot of contact with them,” said Konkle, who works at the Wisconsin Department of Agriculture, Trade, and Consumer Protection. Just 19 of the state’s nearly 65,000 farms are mink farms.

Mink farmers today typically have thousands of animals—hundreds of thousands, even. But mink are not considered livestock by Wisconsin law. Mink farms do not need a license to operate. DATCP does not inspect mink farms, register them, or survey them. 

A similar situation pertains in Utah, where Utah state veterinarian David Taylor said in a December One Health conference call that investigators weren’t welcome at the start.

“This industry is one that has innately a little bit of mistrust, because of the fact that they have been targets, oftentimes,” Taylor said. “So they are very, very closed to having any outside people come onto their farms and see their operations.” 

Nor were they keen on sacrificing more animals to assist the scientists with their mortality studies of the virus, just after they had lost so much, said Taylor. “So we had to find some compensation for mortality studies before we could even begin. And then they were also holding out for compensation for their losses.” With some “creative financing” by the Utah state legislature of CARES Act money, and diplomacy on the part of the government officials, Taylor said, the ranch gates began to open.

“When they’re losing that volume of their business in just literally one week’s time, and they’ve lost 50 percent of their breeders and several thousand mink, having someone come in and just understand that that’s life-shattering for them—that was what opened up the door for communication.”

The culling controversy

Animal health authorities in the US could order mink culls, like those in Europe. Wisconsin authorities have ordered culls in the past for other outbreaks, like on individual deer farms stricken with chronic wasting disease, but never for a whole industry. But ordering a cull would require a complex legal process, and a mass quarantine of all farms.

Suzanne Gibbons-Burgener, a public health veterinarian and infectious diseases epidemiologist at the Wisconsin Department of Health Services, wrote, “The decision to depopulate a group of animals is rarely the first response in a disease event and would typically only be necessary if the risk to human or animal health is significant, and other mitigation steps are insufficient to substantially reduce the risk.”

Animal rights groups see the mink outbreaks as another reason to ban the industry. The Humane Society Veterinary Medical Association, in a statement, called farmed mink a “significant public health threat.”

Those in the mink industry called the European cull and calls for bans an overreaction. But they also said the European situation differed in important ways. Denmark had way more mink, more farms—north of a thousand—in an area “a third the size of Wisconsin,” Hildebrandt said, with a lot more people around. Wisconsin’s few farms are mostly in sparsely populated rural areas. European farms are also managed more cooperatively, which may have offered more opportunities for the farm-to-farm spread that was documented there. Such spread has not been documented in America.

The Danish cull faced immediate backlash, prompted the resignation of the minister who ordered it, and was halted partway through. But on December 21, Denmark’s parliament banned mink farming until 2022 and retroactively provided a legal basis for the cull, underscoring the seriousness of the risks. Across Europe, millions of mink have died or been killed, the Dutch hastened a previously planned ban on fur farming, and France announced it would close its last four farms. In contrast, China, also the biggest market for mink fur, has seen the European culls as a market opportunity and stepped up breeding.

The ballad of the wandering mink

How fluid are the boundaries between mink farms and the wild? Pre-pandemic, American mink that escaped or were intentionally released from European farms were considered some of the worst invaders on the continent. They established themselves quite handily, out-competed the native European mink, and caused all sorts of ecological damage. But in North America, there’s not much accounting of how often farmed mink escape or what they have done with themselves. This is partly because American mink already live in the wild, partly because mink farms are very discreet and not closely regulated, and most of all because very few people have looked.

The few include wildlife disease biologist Jeff Bowman and his colleagues at the Ontario Ministry of Natural Resources, who have long been researching the spread and impact of ranch escapees in their province.

It is usually obvious, Bowman said, which mink are wild and which are ranch-born, even in the trap. Your wild mink is small, chocolate-brown, and bitey. It does not come in sapphire, blue iris, white, palomino, or any other color that brings to mind a celebrity baby name, as your farmed mink does (these colors, incidentally, span white, gray, brown, and black). A farmed mink has been raised on a twice-daily splat of high-quality agricultural leftovers, such as cheese, eggs, and sausage trimmings, is at least twice the size of a wild mink, and is more docile. (In one YouTube video at a minkery, a white mink the size of a large housecat, in the arms of its handler, gently waves its paws and squints at the light.)

Brown wild American mink poking its head out of a rock crevice.Wild American mink wearing a radio collar. | Photo courtesy of Larissa Nituch

Black domestic American mink wearing a radio collar against a green, lush landscape.

Domestic mink found living in the wild during a study conducted by Jeff Bowman and Larissa Nituch. | Photo courtesy of Larissa Nituch

Near Ontario mink ranches, 64 percent of the mink trapped were either escapees or captive-wild hybrids. When it comes to the coronavirus, Bowman said, these studies may be instructive. “Our studies showed that there are potential pathways for spread from farms to other wildlife,” he said. Some of the wild mink that Bowman tested were positive for Aleutian mink disease, which is a known problem on local mink farms, where it can cause reproductive failures and death. The wild mink had their own strains of the Aleutian virus too—it could potentially go back and forth, Bowman said.

Bowman found that mink clearly escaped from certain problem farms much more than others. When the researchers trapped farther afield, they found far fewer ranch-bred and hybrid mink, just 18 percent of the total. It seemed the Ontario mink farm escapees hadn’t managed the sort of widespread invasion that was most worrisome. 

Wisconsin Department of Natural Resources officials haven’t studied the issue of mink farm escapees like Canadian researchers have (though they are now considering more surveillance as part of the One Health investigation). But Arnold Groehler, president of the Wisconsin Trappers Association, said he has been catching the odd mink-ranch escapee for many years. In the 1970s, he said, “it was not uncommon to catch mink that had every phase of color there was. Jet black, pure white, some that looked like dairy cows.” 

Hugh Hildebrandt, the Wisconsin mink vet, said the harsh economics of fur farming mostly solved the escape issues of yore. The price of a pelt, which always fluctuated with high fashion’s whims, has for the past few years not even covered the roughly $35 cost of raising the animal. The industry in Wisconsin began with small family farms over a century ago, and those remaining are still all family farms, even if some are now owned by large foreign companies. But they are mostly large, with very secure fences. The fences exist to keep wildlife like raccoons, skunks, and wild mink from helping themselves to the food supply or passing diseases to the captive mink. Whenever Hildebrandt makes his rounds to a ranch, he checks the guard fence.

Because mink live in such tight quarters and are so vulnerable to disease, ranchers were careful about germs long before COVID struck, Hildebrandt said. Some already required workers to shower before and after shifts. It is also common to leave ranch clothes on the ranch, because mink are smelly.

Hildebrandt said mink operations are now limiting, as much as possible, how much they move or handle the animals. Pelting, or mink harvesting, could not be put off, because it must be done as soon as the mink fur out each fall. But most of that is done now. At some ranches everyone arriving is photographed, to assist with contact tracing. Workers and visitors are distanced and masked up, if not wearing the paper suits that health care workers don. “They wouldn’t last long with mink,” Hildebrandt said. 

The CDC sent a field team to Wisconsin that ran through safety procedures for workers and ran a national webinar for mink ranchers. Government guidelines are all voluntary, except at the affected farms, which were quarantined.

At quarantined farms, anything coming in or out is controlled, including dead mink. Scientists are still assessing how much risk the bodies pose, said Utah state veterinarian David Taylor, at the CDC’s December webinar on zoonotic disease updates. Hot composting can kill pathogens, but it has to be done right.

“After we went onto these farms and saw what they considered to be composting, which really were just piled-up mink, we made the decision here in Utah to just have these buried at landfills,” he said. To limit risk of the virus spreading further, the bodies must be buried immediately, predators and scavengers kept away, the lined trucks disinfected. “We just felt like right now that was the quickest, easiest way to dispose of these animals and the most safe.”

Currently, mink farmers are hanging onto this year’s pelts while scientists work out whether fur can spread the virus and if so how to decontaminate it. (Pelts currently on the market are all pre-pandemic, from last year’s mink.) So far the swabs haven’t turned up any virus, said Caitlin Cossaboom, a CDC veterinarian, in a December web presentation.

The question remains of how many farmed mink are out there, on the lam. Even in recent years, trapper Groehler has found a few mink with obvious “heavy ranch genetics,” which he suspects came from a few old-school ranches left in the area—the sort that might be hand-feeding mink in small sheds. Some escapees, he said, had become the bane of a trout hatchery near a small-scale mink ranch. 

“A lot of people have a mindset that if the ranch mink would escape, it wouldn’t survive in the wild—they’re used to getting their food twice a day; they wouldn’t know how to hunt,” he said. “But it’s interesting how things survive. They adapt. If you are hungry, you eat anything smaller than you. And when they find a wonderful food source like a fish hatchery, that’s like heaven on earth for them, and they’re staying put.”

Maybe mink are better at pandemics than we are?

Groehler worries about what SARS-CoV-2 might do among thousands of captive mink. “It can mutate from the mink, and what will it turn into next? What other animal species will it affect next? I don’t think anybody knows yet.”

The animals most at risk right now are the mink on farms—and perhaps their fellow captive mustelids, the endangered black-footed ferret. 

These ferrets, once spread across much of the American West, have been reintroduced from near-extinction over the past 40 years through a painstaking captive breeding program, which includes artificial insemination and even training the kits how to hunt.

In northern Colorado, the National Black-Footed Ferret Conservation Center, which houses about 170 adult ferrets, or two-thirds of the captive breeding population, has locked down like a mink farm—no nonessential visitors allowed; handwashing, PPE, temperature checks, disinfection of cages. The population of ferrets has been split into pods, much like American schoolchildren. 

Before SARS-CoV-2, the ferrets’ biggest threat was another zoonotic disease: sylvatic plague, from the same bacterium that causes bubonic plague. It was also introduced by people, via ships. 

For mink or ferrets, people are the disease reservoir. Though perhaps there is relief in sight. Three companies are presently working on mink vaccines that may be ready by the spring, Hildebrandt said. Some mink will be vaccinated before many of us are. About 120 black-footed ferrets at the Colorado captive center have already been inoculated with an experimental vaccine created at the National Wildlife Health Center. 

There are also some reasons for hope in the nature of the virus and the mink. As infectious as it is, SARS-CoV-2 doesn’t seem to stick around long outside its hosts—unlike Aleutian mink disease virus, which lasts for months, or the chronic wasting disease-causing prions that can persist in soil for years.

The wild American mink, too, naturally follows CDC guidelines better than many of us have, preferring solitude to the company of its conspecifics.

As Groehler, the trapper, put it: “Wild mink socially distance very well.”