Study population and setting
The study focused on three mink farms in the Northern Jutland region of Denmark where animals (Neovison vison) were infected with SARS-CoV-2-. The farms were chosen because there were PCR-confirmed human COVID-19 cases linked to them. Farms were visited at least twice, and mink were tested for evidence of SARS-CoV-2 infection either through an enzyme-linked immunosorbent assay for antibodies in serum samples from live animals or through PCR on swab samples from live or dead animals. Additionally, samples of exhaled air from mink or from within 1 m of mink enclosures were collected and tested for SARS-CoV-2 RNA. All sampling occurred between June 14 and July 2, 2020. Full SARS-CoV-2 genomes were sequenced from PCR-positive samples.
Summary of Main Findings
The pattern of SARS-CoV-2 infection differed across the three farms. Live animals from farms 1 and 3 had high seroprevalence (>66%) for SARS-Cov-2 antibodies, suggesting that the virus had been actively spreading in the mink population for some time before sampling. Seroprevalence at farm 2 was low at first sampling (3%) but increased to 97% more than a week later. SARS-CoV-2 RNA was detected in indoor air samples from farms 2 and 3 but not from farm 1; samples of outside air and mink feed tested negative for all farms. SARS-CoV-2 genome sequences from farmed mink and associated human cases were very similar and all clustered within the European clade 20B of the global SARS-CoV-2 phylogeny. Sequencing information suggests that a human index case from mid-May introduced SARS-CoV-2 to mink at farm 1 and to other individuals linked to farms 2 and 3 that spread the virus to the other farms.
Repeated visits to farms and testing for both SARS-CoV-2 RNA and antibodies provided information about the timing of virus introduction and transmission.
The study only investigated farms with human COVID-19 cases linked to them, so we do not know how many mink farms in Denmark have been infected. The virus from the human index case linked to mink farm 1 was not genotyped at one key nucleotide position, so its relationship relative to the viruses detected on mink farms is likely close but uncertain. The authors also note that some of the virus mutants detected on mink farms were absent or rare in human cases in Jutland and globally prior to 10 June 2020, but were seen in viruses from mink on a farm in the Netherlands. Whether this mutant is an adaptation to mink (e.g., with higher transmission efficiency) that emerged simultaneously in multiple locations is not clear from this work. Additionally, the increased frequency of the mutant in human cases after June 2020 cannot be definitively linked with the mink farm outbreaks and may have other plausible explanations. Finally, the paper described the human cases as being linked to mink farms but did not provide details on the occupation of these individuals and their direct exposure to infected mink, or whether these individuals were family members of mink farm workers.
This is the first report of SARS-CoV-2 infection on mink farms in Denmark and the first outside of the Netherlands. Since mink farms are present in other countries in Europe and North America, these results provide valuable information on the two-way transmission of SARS-CoV-2 between humans and mink on these farms and the connections with ongoing community transmission of virus variants.
This review was posted on: 19 December 2020