In September of 2020, a captive mink fell ill. According to a report from the World Organization for Animal Health, they stopped eating, struggled to breathe, and bled from the nose. There were two thousand animals that died.
The coronaviruses were found in the mink.
The Centers for Disease Control and Prevention dispatched a team of outbreak investigators, who collaborated with other agencies, to sample farm workers and other animals to determine how the virus had spread.
The director of the C.D.C.'s One Health Office said they tried to leave no stone unturned.
The C.D.C. confirmed last month that four Michigan residents, including two farm employees, had been exposed to the same coronaviruses variant that was found in the mink. It was the first instance of possible animal-to-human transmission in the United States.
When did the variant first emerge? How did a taxidermist have no connection to the farm contract? Could there be a link between the white-tailed deer variant discovered in Ontario and the Michigan mink outbreak?
It feels like a puzzle, according to Dr. Samira Mubareka, a researcher at the University of Toronto.
When the coronaviruses tore through fur farms, scientists were worried that it could become a long-term source of new strains.
coronaviruses have been found on 18 American farms, the most recent in Wisconsin in February. Even though Congress is considering a ban on mink farming, there is still no national system to keep an eye on the farms. Some of the details of the outbreak investigations are reported for the first time.
Scientists say it is difficult to determine how much of a risk a farm poses. It threatens to leave experts blind to the emergence of worrisome new variant that could spill back into humans.
A veterinary microbiologist at Penn State University said that we could really benefit from increased transparency regarding spillover and spillback risk.
In the spring and summer of 2020, the Netherlands andDenmark reported farm outbreaks of mink. It appeared that humans had transmitted the virus to mink, that the virus had changed as it moved among the animals, and that the animals then spread the altered virus back to humans.
The doctor at the hospital said that all of that jumping back and forth over the fence was what they saw.
The Dutch and Danes took swift and decisive action, according to a PhD student at the University of London. Dutch authorities required farmers to submit carcasses for examination and to report respiratory symptoms in animals. Both nations shuttered their mink farms after the virus proved difficult to control.
The United States came up with a set of voluntary guidelines to help farmers keep the virus at bay, including asking farm workers to wear masks and notifying authorities of suspected cases.
There was no national screening program and federal officials relied on farm owners to self-report outbreaks.
When invited, the C.D.C. investigated the outbreak. Dr. Barton Behravesh said that some owners of affected farms declined to participate and that field teams only performed on-site investigations on eight farms.
C.D.C. investigators worked with the U.S.D.A. and state agencies to test for the disease on the Michigan farm. All but two of the 159 animals on the farm were actively infectious.
There were no other animals tested around the farm, but one dog tested positive for the disease.
Two of the farm's employees were affected by the same version of the virus that was spreading among the animals. The variant had two different variations that had been found in Europe and people connected to mink farms.
A fourth person was connected to a Michigan resident who had a sample collected nearly two months after the outbreak. The third case was a local taxidermist, according to internal health department emails obtained by the Documenting Covid-19 Project and the Detroit Free Press. The Department of Health and Human Services declined to confirm these details for privacy reasons. Neither had any connection to the farm.
The experts concluded that there was likely a person spreading the virus to other people.
The data doesn't exist so we have to collect more samples from farm workers, local residents and animals.
It was difficult for Americans to access testing in 2020. Until this spring, federal officials recommended against routinely testing animals for the virus.
There was a shortage of certain supplies after widespread testing wasn't available.
Without more samples, it's impossible to rule out the possibility that the variant emerged in humans, who then spread it to mink.
The taxidermist and his wife have a bigger puzzle. The most likely scenario is that the variant was spread more widely in the human population than was known, and that a couple caught it from another person.
It is possible that they picked up the variant from another animal.
Lynn Su said that testing any animals they may have been in contact with after the two fell ill was either not feasible or not indicated.
According to the health department emails obtained by the Documenting Covid-19 Project and the Free Press, the pair had close contact with deer while hunting.
Studies suggest that humans have introduced the virus to white-tailed deer, which then transmit it to themselves. The taxidermist and his wife could have been exposed to the mink variant from the deer.
The deer might have picked up the virus from the mink that had escaped from the farms. Animals on farms that have tested positive for the virus may act as a breeding ground for wildlife.
Deer might come into contact with farm waste. Dutch researchers found that on farms with an outbreak, airborne dust, as well as the straw and hay, can be highly contaminated with virus.
Scientists said a deer link is intriguing. Canadian researchers have found a unique coronaviruses variant in deer. The deer variant was different from other known ones, but the closest matches were collected from people in Michigan.
There is a chance that the new variant of the virus was spread among humans and mammals and then evolved into deer.
The taxidermist and his wife's case was not believed to have been caused by deer. He asked if there were other animals we were missing.
The data is not always clear-cut. In the fall of 2020, the U.S.D.A. tested a dozen cattle on a Wisconsin farm. Three of the cattle had low levels of immunity to the virus, according to a U.S.D.A. spokesman.
The findings did not meet the criteria for a positive result, according to Mr. Weger. Experiments show that cattle are not susceptible to the disease.
It is difficult to draw conclusions without more analysis, and the findings suggest a need to monitor livestock, according to outside experts.
Some are concerned that officials have not disclosed the findings from the investigations.
More details of the investigations will be published in scientific journals, according to Dr. Barton Behravesh, of the C.D.C.
Dr. Dutcher said that the U.S.D.A. would like to do more active monitoring on mink farms.
The U.S.D.A does not have any reports of active infections after 2020. There were two farms in Wisconsin and one in another unnamed state where researchers found anti-mink antibodies. Mr. Weger said in an email that there was no evidence of an outbreak on either farm.
The presence of antibodies suggests that the virus was undetected on the farms.
The director of veterinary sciences at the Center for a Humane Economy said that without surveilling, how would you know?
The vaccine may help slow the transmission on farms. It is possible that vaccinations could make infections more likely to be present.
She said that the United States should be testing both farm and mink employees regularly, as well as communicating the results in a timely way.
The web of transmission that may be going on with wildlife, farmed animals and humans needs to be understood.