At the end of September 2020, a mink in captivity on a farm in Michigan suddenly fell ill. They stopped eating, had difficulty breathing and were bleeding from their noses, according to a report by the World Animal Health Organization. Two thousand animals died.
Laboratory tests soon confirmed that the mink were infected with the coronavirus.
The Centers for Disease Control and Prevention sent a team of outbreak researchers who collaborated with other agencies to rub minks, farm workers and menagerie from other animals, from rats to raccoons, to find out how the virus had spread.
“We tried not to leave a stone unturned,” said Dr. Casey Barton Behravesh, who heads the CDC’s Unified Health Service.
Last month, the CDC confirmed that four Michigan residents, including two farm employees, had been infected with the same unique variant of the coronavirus that was found in the mink. This was the first and only known case of possible animal-to-human transmission in the United States.
But many questions remain: When and with whom did the option first appear? How did a taxidermist with no connection to the farm negotiate it? Could there be a link between the Michigan epidemic in Michigan and a variant of the white-tailed deer that scientists recently discovered in neighboring Ontario?
“It really looks like a puzzle,” said Dr. Samira Mubareka, a virologist at the Sunnybrook Research Institute and the University of Toronto. “It’s not just pieces that are missing – neighboring, interconnected parts are missing.”
Since the early days of the pandemic, when the coronavirus destroyed fur farms, scientists have worried that mink could become a long-term reservoir for the virus and a potential source of new options.
To date, coronavirus infections have been found in mink on 18 US farms, the most recent in Wisconsin in February. Even when Congress is considering banning mink farming, there is still no national system for proactive surveillance of mink farms that are not required to report cases to federal authorities. And officials have not released much information about the epidemic investigations they have conducted; some of these details are reported here for the first time.
Together, secrecy and spot monitoring make it difficult to determine the risk to mink farms, scientists say. And it threatens to leave experts blind to the emergence of alarming new options that could spill back onto people, extending the pandemic.
“Combined with the desperate need for better systematic monitoring in humans and animals, we could really benefit from increased transparency regarding the risk of spillage and spillage,” said Vivek Kapoor, a veterinary microbiologist at Penn State University.
The investigation
Member of the operation for cleaning and destruction of mink in Denmark in 2020. Credit … Morten Stricker / Ritsau Scanpix, through Agence France-Presse – Getty Images
The Netherlands and Denmark were the first countries to report outbreaks of mink farms in the spring and summer of 2020. Scientists have compiled a disturbing chain of events: Humans appear to have transmitted the virus to mink; that the virus mutated while moving among the minks, and that the animals then spread the altered virus back to humans.
“All this jumping back and forth over the fence is what we saw,” said Dr. Marion Kupmans, a virologist at Erasmus University Medical Center in Rotterdam. “And that’s something you really don’t like as a virologist.”
The Netherlands and Denmark have taken “swift and decisive” action, said Adriana Diaz, a PhD student at the University of London who is studying the answers. The Dutch authorities conducted antibody tests on all farms and asked farmers to report respiratory symptoms in mink and to hand over carcasses for testing on a regular basis. However, the virus proved difficult to control, and both nations eventually closed their mink farms.
The United States has taken a different approach, developing a set of voluntary guidelines to help farmers keep the virus at bay, including asking agricultural workers to wear masks and notify authorities of suspicious cases.
But there was no national screening program, and federal officials relied on farm owners to report outbreaks themselves. “All of our federal oversight efforts are voluntary,” said Dr. Tracy Ducher, coordinator of science and biosecurity at the United States Department of Agriculture’s Animal and Plant Health Inspection Service.
The CDC investigates outbreaks only when officially invited. Some owners of affected farms refused to participate and field teams conducted on-site surveys on only eight farms, said Dr Barton Behravesh.
At the farm in Michigan, CDC investigators worked with the USDA and government agencies to test people and animals for the virus. They collected tampons and samples from 159 minks on the farm; all but two were actively infected, said Dr. Barton Behravesh.
None of the other animals tested around the farm – two dogs, a cat, raccoons, possums, striped skunks, rats, marmots and rabbits – were infected, but one dog tested positive for antibodies, officials said.
Two of the farm’s employees were infected with the same version of the virus that was spreading among the minks. The variant had two mutations that were also found in mink farmers in Europe and in people associated with mink farms.
Officials found the same mutations in a sample collected by another Michigan resident nearly two months after the mink epidemic and then in a fourth person linked to that resident. The third case is a local taxidermist, according to emails from the Internal Health Department, received from the Documenting Covid-19 Project and the Detroit Free Press, and the fourth case is the man’s wife, the organizations said later. (Michigan’s Department of Health and Human Services declined to confirm these details for reasons of confidentiality.) None of them had any known connection to the mink farm.
Unanswered questions
Deer at a research facility at the University of Texas A&M. Studies show that deer have adopted the coronavirus from humans, who could theoretically pass it back to deer. Credit … Sergio Flores for The New York Times
These findings suggest a plausible scenario, experts said: Man transmitted the virus to minks, and mutations occur when the virus spreads to animals, which then pass it back to farm workers. “We concluded that there was probably a spread of mink to humans on this particular farm in Michigan,” said Dr. Barton Behravesh.
But determining when and to whom the mutations first appeared requires many more virus samples from farm workers, locals and minks collected before and after the outbreak. “These data do not exist,” said Arinjay Banerjee, a virologist at the University of Saskatchewan.
Throughout 2020, testing was difficult for Americans, and few patient samples were sequenced. Animal surveillance was even worse; until this spring, federal officials explicitly recommended that animals not be routinely tested for the virus.
“There was no widespread testing available, then there was a shortage of certain supplies,” Dr Behravesh said. “So we didn’t want to have, you know, a crazy rush to test animals.”
Without more samples, it is impossible to rule out the possibility that the variant appeared in people who then spread it to the mink, scientists said.
The bigger puzzle is how the taxidermist and his wife got it. The most likely possibility, several experts said, was the option of circulating more widely in the human population than is known, and the couple caught it from another infected person.
Another, more speculative possibility is that they took the variant from another animal species. “Taxidermists are dealing with other dead animals,” said Linda Safe, a virologist and immunologist at Ohio State University.
But because the cases were discovered “weeks to months” after the two became ill, testing all the animals they may have been in contact with “is either not feasible or has not been shown,” said Lynn Sutfin, a DHHS spokesman. in Michigan.
The couple also had close contact with deer while hunting “at or very close to the onset of their own disease”, according to emails from the health department received from the Covid-19 and Free Press documentation project.
Studies show that humans have repeatedly introduced the white-tailed deer virus, which is then easily transmitted to each other. People could pass on the mink variant to deer, who may have passed it on to the taxidermist and his wife. “Given the very high viral loads that have been observed in white-tailed deer, the overflow to them could certainly occur from deer,” said Dr. Kapoor.
Alternatively, deer may have caught the virus directly from an infected mink known to have escaped from farms. Wild cats on mink farms have also tested positive for the virus and can act as vectors between captive minks and wildlife.
Or deer can come in contact with mink farm waste, Dr. Kapoor said. In farms with fires, dust in the air, as well as straw and hay on which mink beds can be heavily contaminated with the virus, Dutch researchers have found.
Another discovery makes the potential connection with deer intriguing, scientists said. Canadian researchers recently discovered a unique variant of the coronavirus circulating in deer in southwestern Ontario. Although the deer variant was strikingly different from other known variants, the closest matches were virus samples collected from humans and mink in Michigan in late 2020.
One possibility, still theoretical, is that whatever version of the virus has circulated among the minks and humans has made its way into the deer, where it has evolved into a new variant. “There may be interactions and interspecies transmission that were mysterious and we didn’t really understand them,” said Dr. Mubareka, author of the Ontario study.
Dr. Banerjee was skeptical that …
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