Wisconsin’s No. 1 mink farming industry now seen as a COVID-19 risk
he 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 Taylor County, Wisconsin, mink farms 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 in, 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.”
It wasn’t hard to guess the cause. Wisconsin was a coronavirus hotspot from late summer on, and workers at mink ranches had already tested positive. 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 outbreaks shone light on an industry that has for years operated so discreetly in Wisconsin — the nation’s top pelt producer — that even the officials in charge of animal health didn’t know where all of the state’s 19 mink farms were. Those farms are neither regulated nor licensed by the state.
Officials have caught up fast amid concerns that a mutation of the species-hopping virus could pose danger to humans. In fact, the state just added mink farmers to the category of residents next in line for vaccination along with teachers, child care workers and grocery store employees.
Wisconsin is not, however, contemplating a Denmark-style moratorium on mink farming or industry-wide cull, according to state veterinarian Darlene Konkle. Wisconsin has yet to find a mink-to-human leap of the virus. And since mink aren’t officially considered livestock, Wisconsin doesn’t compensate farmers whose mink are killed to prevent disease spread, she said.
“It would be no small thing to take the Denmark approach and say we’re going to depopulate all the mink farms,” Konkle said. “Because that’s a situation where you’re taking away somebody’s livelihood.”
The mink detectives
The first infections of U.S. farmed mink, in Utah in August, 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, marking the “first free-ranging, native wild animal confirmed with SARS-CoV-2,” said Thomas DeLiberto and Susan Shriner of the USDA’s Animal and Plant Inspection Service. Soon after that, two more mink — both Oregon farm escapees — also tested positive.
Of all the animals that have been infected with the coronavirus, including dogs, cats and a few other mammals, the only species to have suffered large-scale casualties so far is the American mink living on mink ranches around the world.
Since the first mink got sick on a Dutch ranch in April, millions of the animals have died or been culled on nearly 400 ranches across Denmark, the Netherlands, Sweden, Lithuania, Greece, Italy, France, Spain and Canada. The United States has seen 16 ranch outbreaks since August: 12 in Utah, two in Wisconsin, one each in Oregon and Michigan as of late January.
“It’s a top priority in human and veterinary diagnostic labs,” said Keith Poulsen, director of the Wisconsin veterinary diagnostic lab, which has been running COVID tests on farmed mink and people, operating from 5 a.m. to 2 a.m. to keep up.
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 the fight against the pandemic — particularly if it mutated along the way into something deadlier, more transmissible or harder for modern medicine to attack.
No disease is an island
Such zoonotic outbreaks happen constantly, and disease reservoirs are everywhere. Raccoons harbor rabies; rodents across the American West harbor the bacterium that causes plague. People are a reservoir of tuberculosis for cattle.
Viruses adapt to new hosts by mutating. Some 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: an apparently super-contagious variant first discovered in Britain is expected to become the dominant U.S. strain by March.
No evidence has emerged yet that farmed mink have infected people in the United States, though the investigation is ongoing. In Denmark, which was the world’s biggest mink producer with 17 million animals, hundreds of farms were affected.
There, researchers found the virus passed from people to mink and back again, mutating as it went. Some people got a genetic variant dubbed Cluster-5 that looked extra nasty, because the virus’s spikes had changed in ways that made it harder for monoclonal antibody treatments to recognize the virus, at least in the lab.
Danish and Dutch governments ordered culls of their mink for fear of creating a new reservoir for the disease. By mid-November, Cluster-5 cases had stopped turning up and had, scientists guessed, dead-ended.
The mink variant turned out not as bad as feared — but the next one could be worse.
That is why a state advisory committee of health care experts in January recommended that an estimated 300 mink farm workers be included in the next phase of the vaccine rollout, citing the potential for dangerous mutation.
The mysteries of mutation
When 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 in Madison, a lab that has been testing wild animals trapped near mink farms.
Ip and other scientists knew early on that mink might be susceptible to COVID-19, because they share some immune response similarities and a key lung receptor with people.
Said Poulsen: “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 it produced a million pelts, one-third of the U.S. total.
CDC teams began collecting samples from the people and the mink on affected farms, while U.S. Wildlife Services live-trapped wild animals nearby. The two farms that were quarantined would not be released until all their tests were negative.
Tracking who gave what to whom will take some time. “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 culling controversy
Animal health authorities in the United States could order mink culls, like in Europe. Wisconsin officials have done so for other outbreaks, like on deer farms stricken with chronic wasting disease, but never for a whole industry. That would require a complex legal process, and a mass quarantine of all farms.
The European situation differs in important ways, the industry says. Denmark had way more mink, and more farms — north of a thousand — in an area “a third the size of Wisconsin” Hildebrandt said, with a lot more people around.
By contrast, Wisconsin’s few farms are mostly in sparsely populated rural areas.
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. Meanwhile, China, also the biggest market for mink fur, has seen the European culls as a market opportunity and stepped up breeding.
Officials also fear mink ranch escapees could infect the wild population. Hildebrandt said the harsh economics of fur farming mostly solved the escape issue. 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. Most ranches are now owned by large foreign companies that erect very secure fences.   
But research by wildlife disease biologist Jeff Bowman and his Ontario Ministry of Natural Resources colleagues has found that 64% of the mink they trapped near ranches were either captive or wild-captive hybrids.
“Our studies showed that there are potential pathways for (disease) spread from farms to other wildlife,” Bowman said.
Germ concerns heightened
Because mink live in such tight quarters and are so vulnerable to disease, ranchers were careful about germs long before COVID-19 struck, Hildebrandt said. Some already required workers to shower before and after shifts.
Farms are now limiting, as much as possible, how much they move or handle the animals. At some ranches everyone arriving is photographed, to assist with contact tracing. Workers and visitors are distanced and masked up; some wear the paper suits that health care workers don.
The CDC sent a team to Wisconsin to run through safety procedures for workers and held a national webinar for mink ranchers. Government guidelines are all voluntary, except at the two quarantined farms.
Scientists are still assessing how much risk mink carcasses pose, Taylor said at the CDC’s December webinar on zoonotic diseases. He said Utah requires the bodies be buried immediately in landfills, predators and scavengers kept away, and lined trucks disinfected.
The animals most at risk right now are captive mink. For them, people are the disease reservoir. Three companies are working on mink vaccines that may be ready by the spring, Hildebrandt said.
Some mink, he said, will be vaccinated before many of us are.
A version of this story first appeared in Sierra magazine. Wisconsin Watch investigations editor Jim Malewitz contributed to this report. Wisconsin Watch (wisconsinwatch.org) collaborates with WPR, PBS Wisconsin, other news media and the University of Wisconsin-Madison School of Journalism and Mass Communication. All works created, published, posted or disseminated by Wisconsin Watch do not necessarily reflect the views or opinions of UW-Madison or any of its affiliates.