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A majority of genome sequences from human cases had clear phylogenetic relationships to sequences recovered from mink samples. Sequences from humans and mink from the same mink farms clustered closely together, suggesting within-farm human-to-mink and/or mink-to-human transmission. In sequences from two human samples from one of the negative mink farms the mutation Y453F, considered as an adaptation to mink, was observed.

“Between mid-October and mid-November, the National Veterinary Institute received 74 submissions of between 3–5 dead mink, representing between 1 and 4 submissions per farm. Thirteen farms gave positive results for SARS-CoV-2 nucleic acids using qRT-PCR. All positive farms were located in Sölvesborg, the County of Blekinge, in the south-eastern part of the country. None of the positive farms had reported increased morbidity or mortality before testing positive but, retrospectively, a slight increase in daily mortalities could be observed in the records from several of the farms.

All sequences from mink belonged to sub-lineage B1.1.39, a sub-lineage only seen once in Sweden before the outbreak. From the serological screening, 24 samples per farm were received from 26 out of the 28 mink farms that remained after the pelting. Specific SARS-CoV-2 antibodies were detected in the vast majority of samples from 23 farms, including in all farms that previously had been tested positive for SARS-CoV-2 nucleic acids. In the remaining three farms, all samples tested negative.

A total of 100 persons have been registered in the program, but due to the seasonal mode of work, and changes in the workforce, the number of participants has varied. The 317 samples that have been taken and analysed for ongoing viral infection within the surveillance program have resulted in 8 positive persons. In addition to samples from these persons another 14 samples from mink farmworkers that was tested positive before the surveillance was launched were collected. All 22 samples were whole-genome sequenced (WGS). In the serological survey, 78 persons participated, among whom 27 tested positive.

The resulting sequences from WGS were analysed using pangolin. Generally, two main groups were seen, one representing sequences with a pangolin classification similar to that of sequences recovered from WGS of samples from minks (B.1.1.39) and the second group representing sequences with a pangolin classification identical to those circulating in Sweden at the time. The sequences were further analysed by aligning them towards the reference sequence NC_045512. A phylogenetic tree was calculated, and the subtree representing sequences with the pangolin classification B.1.1.39 was studied separately as new sequences were added.

A majority of sequences from human cases had clear phylogenetic relationships to sequences recovered from mink samples. Sequences from humans and mink from the same mink farms clustered closely together, suggesting within-farm human-to-mink and/or mink-to-human transmission. In sequences from two human samples from one of the negative mink farms the mutation Y453F, considered as an adaptation to mink, was observed.”

From: Mink-associated infections with SARS-CoV-2 –

Surveillance of infectious diseases in animals and humans in Sweden 2020 (PDF file)

 

** Update ** Mink farming was suspended for 2021 in Sweden, but on the basis of this report, mink farming should be banned entirely, not just in Sweden, but internationally. Mink farms risk becoming major reservoirs of Sars-CoV-2 capable of spilling back into the human population. The advantages of mink farming do not remotely outweigh the risks.

 

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