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Mink farmer Henrik Nordgaard Hansen killed the entire herd at his farm near Næstved, Denmark, last week.

MADS CLAUS RASMUSSEN/Ritzau Scanpix/AFP via Getty Images

Mutant coronaviruses found in mink spark massive culls and doom a Danish group’s research

Sciences COVID-19 reporting is supported by the Pulitzer Center and the Heising-Simons Foundation.

Jens Malmkvist’s life’s work came to a dramatic end this week. An ethologist at Aarhus University, Malmkvist studies the behavior and welfare of farmed mink, with the aim of giving them a better life as they are raised for fur. But on Monday and Tuesday, all 6350 minks at the Aarhus facility were gassed as part of a nationwide cull ordered on 4 November by the Danish government.

Denmark is seeking to stop the spread of what it deems a dangerous strain of SARS-CoV-2, the virus that causes COVID-19, that’s circulating in mink and infecting humans as well. Scientists say mutations in the virus, described this week in a short “working paper,” might reduce the efficacy of COVID-19 vaccines.

Enforcement of the countrywide cull was temporarily halted yesterday after the Danish minister for food, agriculture and fisheries, Mogens Jensen, acknowledged the government lacked the legal authority to implement it. In a statement, Jensen said the process had a “regrettably messy start,” but noted the government has submitted a bill to the Danish Parliament that would give it the power it needs.

But Malmkvist’s research herd is dead already; he says it may be the right decision but mourns the impact on research and the personal toll on his team. “It’s incredibly sad and shocking, just happening overnight like that.”

Peter Ben Embarek, an expert in food safety and zoonoses at the World Health Organization, calls the government’s decision to cull a “brave” and “dramatic” step.

The move reflects a growing concern about the spread of SARS-CoV-2 in mink, reported in six countries. Four—Spain, Sweden, Italy, and the United States—have responded by culling populations at affected farms. The Netherlands went further by expediting a complete ban on mink farming, originally planned for 2024, and now effective by March 2021. The explosive spread of the virus in mink and several documented cases of mink-to-human transmission made the Dutch government fear farms might become a permanent virus reservoir.

The Danish government has an even more troubling concern. SARS-CoV-2 has remained “quite stable” since its emergence in the human population, Ben Embarek says, but spillovers into animals can trigger mutations as the virus adapts to a new host. That has happened in mink. The working paper, posted online yesterday by the Statens Serum Institute, a government lab, reported clusters of genetic mutations in the virus infecting the animals. One strain, now called the ΔFVI-spike mutant, has four mutations in the gene coding for the spike protein, which helps the virus enter host cells.

Experiments with the ΔFVI strain showed plasma from recovered COVID-19 patients or rabbits immunized with the virus did not neutralize it as efficiently as the unmutated virus. Because many COVID-19 vaccines aim to elicit an immune response to the spike protein, the worry is that those vaccines might not work well against the ΔFVI mutant, which so far has been found in 12 people in Denmark as well.

The findings are preliminary, but the government considered them serious enough to order the culling of all 12 million remaining minks in Denmark, the world’s largest mink producer. About 80% of the animals were scheduled to meet their end this month anyway—and become fur—but the decision means breeding and laboratory stocks will be destroyed as well. Mink farming will be banned at least until the end of 2021.

Biologist David Kennedy of Pennsylvania State University, University Park, agrees mutations in the spike protein “could completely undermine” vaccine efficacy, especially if they occur in the part of the protein that binds to the human receptor, as one of the four mutations in ΔFVI does. He says more data are needed to document the mutations’ impact, but in the meantime, it’s important to keep the strain from spreading. “If the human-to-human transmission of this variant isn’t stopped, stopping further introductions of this variant from the mink won’t matter.”

Researchers in the Netherlands have also studied SARS-CoV-2 evolution in mink. In a paper published online by Science yesterday, they reported finding the mutation affecting the receptor binding site—but not the other three present in ΔFVI—at four farms. “It did not continue spreading, and we haven’t seen it since,” says veterinary epidemiologist Arjan Stegeman of Utrecht University. (The team did find that 68% of farmworkers and their contacts had antibodies to the virus, underscoring the risk of mink-to-human transmission.)

At least nine countries that have mink farms have not reported SARS-CoV-2 infections in mink. But they may well have undetected outbreaks, says Ilaria Capua, a veterinary virologist at the University of Florida’s One Health Center of Excellence. The virus may have also spilled over into other farmed or wild mustelids, a family that includes weasels, otters, badgers, martens, and ferrets. “We should look at this as the first of what would be many spillovers in animal populations,” Capua says. “I would strongly invite [scientists] to go look at animals.”

Although mink farming is controversial in Europe, it is likely to survive the pandemic, Malmkvist says, so research into mink welfare remains necessary. His own studies have shown, for instance, that young animals are happiest living in male-female pairs and that they don’t particularly care for big cages but love having a two-story cage so they can climb up and down. Other papers have shed light on the animals’ preferred diet and bedding materials.

Work by his team also influenced Danish legislation that requires breeders to select for less fearful animals, and farms within the European Union can no longer trade internationally without certification based in part on the group’s research. “I felt part of a team, making a difference,” he says.

The fact that his research herd had not seen outbreaks makes the mass slaughter all the more painful. “But we cannot take any risk regarding human health,” Malmkvist says. “I must trust that the Danish leadership has made the right decision.”