LONDON – The Danish government has ordered a mass cull of 15 million mink following evidence the animals are harboring a variant of SARS-CoV-2 with mutations in the spike protein through which the virus infects human cells.

The country’s infectious disease control agency, Statens Serum Institut (SSI), has shown that what it has named “SARS-CoV-2 mink variant” shows decreased susceptibility to neutralizing antibodies from people who have recovered from COVID-19, compared to the effect of the same antibodies on non-mink variants of the virus.

Since the beginning of June, 216 mink farms in Denmark have been infected with SARS-CoV-2. The mink variant first came to light in June when it was found in an outbreak in a care home close to a farm. Between August and Nov. 2, the mink variant was detected in 168 farms.

That has led to a pronounced spread in humans, with 214 cases confirmed among 5,102 samples that have been completely sequenced between May 25 and Oct. 18, according to an SSI statement on Nov. 5.

Of the 214 cases of mink variant infection, 200 were found in people in the North Jutland region, in 535 samples to have been sequenced. “This means that mink variants have been detected in 40% of the samples,” SSI said.

The 14 cases of infection with mink variants outside North Jutland were found in 4,568 viral sequences, corresponding to 0.3% of the samples.

“There is a risk that the effect of spike-based anti-COVID-19 vaccines may be affected when changes occur in this part of the genetic material,” SSI said in a risk assessment prepared for the Danish government.

In a press conference on Nov. 3, Mette Frederiksen, prime minister of Denmark, said there is a serious public health risk. “The discovery of a mutated infection in mink, which weakens the ability to form antibodies [means] resolute action is needed. All mink must be culled.”

Without immediate action there is a risk SARS-CoV-2 mink variation “may pose a risk to the effectiveness of a future vaccine,” Frederiksen said.

There have been outbreaks of SARS-CoV-2 on mink farms elsewhere in Europe, but no previous reports of mutations that could undermine COVID-19 vaccines. The Netherlands was to have banned mink breeding by 2024 on animal welfare grounds, but in June its parliament voted to cease mink breeding immediately after SARS-CoV-2 broke out on 17 farms.

The first case of a mink transmitting the infection to humans in the Netherlands was reported in May, when the Dutch government also raised concerns that mink can be asymptomatic, leading to late detection. There also has been a cull in northeast Spain.

In August, the U.S. Department of Agriculture (USDA) announced that two mink farms in Utah had had mink and farm workers test positive for SARS-CoV-2, but said, “There is currently no evidence that animals, including mink, play a significant role in spreading the virus to humans.” However, more studies are needed, USDA said.

Denmark is the world’s biggest producer of mink, selling 17 million pelts per annum. The trade is worth $785 million and there are more than 1,000 mink farms in the country.

Reservoir of infection

Alongside concerns about mutations in the virus, SSI is concerned that mink farms represent a huge reservoir of infection. The animals are bred in confined spaces, meaning the virus transmits rapidly. The infection has spread from humans to mink, and its fast replication in the animal host will likely generate more and more mutations, that can be transmitted back to humans, according to SSI.

To date, SSI has recorded seven different mutations in the spike protein of SARS-CoV-2 in mink, and has found as many as four of them in a single SARS-CoV-2 mink variant. The four-mutation variant has been detected on five farms in North Jutland and in 12 patients, four of whom had a direct connection to three of the farms.

The mutations in the spike protein seen in SARS-CoV-2 mink variant are different from those generated by human-to-human transmission/replication, according to SSI.

After SSI raised the alarm about SARS-CoV-2 infections in mink in June, more than a million animals were culled in a bid to stop the spread. Despite that, “there are currently no signs that infection in mink farms is decreasing over time,” SSI epidemiologists said in the risk assessment, dated Nov. 2.

SSI has a high scientific reputation; however, the DNA sequences of SARS-CoV-2 mink variant have not yet been published, causing some frustration with groups tracking the evolution of the virus.

The U.K.’s COVID-19 Genomics UK consortium issued a statement saying, “As far as we are aware, the details about the specific mutations in this SARS-CoV-2 lineage have not been made publicly available and therefore at this point it is not possible for researchers to provide a comment.”

With the changes in the spike protein yet to be evaluated by the international scientific community, the implications are unclear, said James Wood, head of the Department of Veterinary Medicine at Cambridge University. “It is too early to say that the change[s] will cause either vaccines or immunity to fail,” he said.

A high level of human infection with the mink variant is more likely to be driven by person-to person-transmission rather than from direct mink-to-person transmission. “Given this, culling mink may well not in itself cause the strain to disappear, but may stop further mutant strains from developing in [mink],” according to Wood.

Editor’s note: This story has been corrected to restate the number of human infections with SARS-CoV-2 mink variant that have been confirmed by viral genome sequencing, according to figures released by SSI on Nov. 5

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