The first identified cases of Covid-19 (coronavirus disease 2019) in mink were detected in April 2020 in the Netherlands and later in other European countries including Spain, Denmark, Italy, Sweden, Greece, France and Lithuania (15, 19). In addition, infected mink have been detected in several states in the USA (Utah, Michigan, Oregon, and Wisconsin) and in Canada (19). In most of these cases, animals had an asymptomatic disease course, although there were sometimes observed mild respiratory symptoms and diarrhoea as well as pneumonia and increased mortality (3, 14, 16, 17, 19). The source of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on infected mink farms has not been identified, but in a few cases in the Netherlands and Denmark it has been clearly demonstrated that the virus was transmitted from infected humans and then spread among mink (9, 15, 16). However, the biggest threat was that SARS-CoV-2 would spill back from mink into the human population and that during the passage through mink the virus could generate mutations in its genome. Mutations in the gene coding the spike protein are particularly threatening because they are considered to jeopardize the potential efficacy of vaccination and antibody therapy (7). Preliminary studies by the Danish Statens Serum Institut indicated that SARS-CoV-2 Cluster 5 variant was less effectively neutralised by antibodies from humans infected with unmutated virus (10). The implemented biosecurity procedures on mink farms in most affected countries (especially in the Netherlands and Denmark) turned out to be insufficient, infections in mink have spread on a large scale, and the occurrence of SARS-CoV-2 in people connected to mink farms has also increased (3). Therefore, the European Centre for Disease Prevention and Control, World Health Organization and World Organisation for Animal Health (OIE) recommended close surveillance of mink farms (5, 18). In response to these recommendations, in the autumn of 2020 the Polish General Veterinary Inspectorate introduced official monitoring of mink farms. Additionally in December 2020, the Minister of Agriculture and Rural Development changed the law to recognise SARS-CoV-2 infection in mink as a notifiable disease (6). After the preventive elimination of mink farms in Denmark, Poland became the largest producer of mink pelts in Europe, although the number of active farms had decreased from 354 in 2020 to 266 at the beginning of 2021 (some farms have suspended or definitively terminated their activities).
The swabs from 27 farms tested negative. However, in the examination of 20 swabs from one farm, 4 showed the presence of SARS-CoV-2–specific amplicons of all gene fragments (4) (Table 1).
Results of SARS-CoV-2 testing for E, N and RdRp genes of individual throat swabs collected from infected mink
Positive throat swab | 3 | 8 | 11 | 12 |
---|---|---|---|---|
E gene | 19.4 | 23.9 | 22.3 | 23.9 |
N gene | 24.9 | 29.6 | 27.9 | 28.9 |
RdRp1 | 29.9 | 37.8 | 34.3 | 36.2 |
RdRp2 | 28.8 | 36.9 | 33.6 | 35.5 |
The numbers indicate Ct values (defined as the number of cycles where the fluorescence generated by the presence of target sequence exceeds the background)
The infected farm was located in the Pomorskie voivodeship (in the north of Poland) and kept about 5,850 breeder minks which were healthy. After detection of SARS-CoV-2 and prior to culling and proper carcass disposal 120 blood samples were taken. Together with 20 throat swab samples from mink carcasses from this farm, they comprised sample set 1. Throat swabs were also collected from 30 additional animals and blood samples were simultaneously taken from the same 30 animals, these being sample set 2. In total 50 throat swabs and 150 serum samples were tested. The results are presented in Table 2.
Summary of further sample testing from the infected mink
throat swabs | serum samples | |||
---|---|---|---|---|
sample set | ||||
studied | positive | studied | positive | |
1 | 20 | 20 | 120 | 30 |
2 | 30 | 15 | 30 | 15 |
total | 50 | 35 | 150 | 45 |
(70%) | (30%) |
Viable numbers of mapped reads were obtained from two samples, 210209_3596 and 210209_3597, and were 10,103 and 12,848, respectively. The respective average coverage extents of the SARS-CoV-2 genome were 9.1 (210209_3596) and 13.8 (210209_3597). Variant calling was carried out and consensus sequences were found using the BCTtools package (13). Two full-length SARS-CoV-2 genomes were obtained from throat swabs nos 11 and 12 (Table 1) and consisted of 29,891 nucleotides each. They were submitted to the GISAID EpiCoV database with the strain IDs EPI_ISL_984305 and EPI_ISL_984307. Phylogenetic analysis indicated that these viruses belong to the 20B Nextstrain clade (Fig. 1).
Fig. 1
Phylogenetic tree of SARS-CoV-2 sequenced in the present study. The sequences are available in the EpiCoV GISAID database. The SARS-CoV-2 isolates detected in mink in January 2021 are marked with the red box

According to GISAID classification, both strains were of B.1.1.279 lineage, although of two different clades, O and GR. This discrepancy probably results from relatively low sequence coverage in one of the regions used by the GISAID tool for virus classification. In addition, many nucleotide mutations were found compared to the genome sequence of the reference strain and some of them were nonsynonymous (Table 3).
Summary of amino acid substitutions identified in two SARS-CoV-2 genomes detected in infected mink
SARS-CoV-2 gene | EPI_ISL_984307 (210209_3597/2021) | EPI_ISL_984305 (210209_3596/2021) |
---|---|---|
C1247F | C1247F | |
D614G | D614G | |
S | G75V | G75V |
Y453F | Y453F | |
M177T | ||
D343Y | D343Y | |
N | G204L | G204L |
R203K | R203K | |
NS3 | R126M | |
NS7b | L22stop | L22stop |
Q275H | Q275H | |
NSP2 | S138L | S138L |
V541I | V541I | |
D218E | D218E | |
NSP3 | ||
E869D | E869D | |
G37E | G37E | |
NSP9 | P57S | P57S |
NSP12 | P323L | P323L |
NSP13 | K94I | K94I |
A274S | A274S | |
NSP14 | M49I | M49I |
NSP15 | M218T | M218T |
Strain classification | ||
GISAID clade | O | GR |
PANGO lineage | B.1.1.279 | B.1.1.279 |
Nextstrain clade | 20B | 20B |
SARS-CoV-2 was identified as an emerging coronavirus in humans in December 2019. It is now well documented that the SARS-CoV-2 virus can be transmitted between humans and animals and subsequently circulate in infected animals and be transmitted back to humans, posing a risk to public health (9, 15, 16). Like the ferret, the mink, belongs to the
In the spike protein of SARS-CoV-2 detected on a Polish mink farm, four or five amino acid substitutions were identified, depending on the isolate. Among the identified changes, only one substitution of tyrosine to phenylalanine (Y453F) is located in the receptor-binding domain (8). This location is not regarded as critical for binding to ACE2 receptors but it is described as responsible for increased binding affinity to cellular receptors in mink (1, 11). This substitution was detected in people who had no contact with mink and even in countries not connected with the mink industry, such as South Africa, the Faroe Islands, Norway and Switzerland. It also appears that this change was present in SARS-CoV-2–infected Danes several months earlier than it was detected in minks. So far it has not been identified in people in Poland (GISAID database:
In summary, in this report we confirmed the infection of mink with SARS-CoV-2 on a farm in Poland and presented the subsequent analysis of virus sequences from two isolates. These data can be useful for future assessments of the epidemiological situation of SARS-CoV-2 in Poland to maintain vigilance for public health.
Fig. 1

Summary of further sample testing from the infected mink
throat swabs | serum samples | |||
---|---|---|---|---|
sample set | ||||
studied | positive | studied | positive | |
1 | 20 | 20 | 120 | 30 |
2 | 30 | 15 | 30 | 15 |
total | 50 | 35 | 150 | 45 |
(70%) | (30%) |
Summary of amino acid substitutions identified in two SARS-CoV-2 genomes detected in infected mink
SARS-CoV-2 gene | EPI_ISL_984307 (210209_3597/2021) | EPI_ISL_984305 (210209_3596/2021) |
---|---|---|
C1247F | C1247F | |
D614G | D614G | |
S | G75V | G75V |
Y453F | Y453F | |
M177T | ||
D343Y | D343Y | |
N | G204L | G204L |
R203K | R203K | |
NS3 | R126M | |
NS7b | L22stop | L22stop |
Q275H | Q275H | |
NSP2 | S138L | S138L |
V541I | V541I | |
D218E | D218E | |
NSP3 | ||
E869D | E869D | |
G37E | G37E | |
NSP9 | P57S | P57S |
NSP12 | P323L | P323L |
NSP13 | K94I | K94I |
A274S | A274S | |
NSP14 | M49I | M49I |
NSP15 | M218T | M218T |
Strain classification | ||
GISAID clade | O | GR |
PANGO lineage | B.1.1.279 | B.1.1.279 |
Nextstrain clade | 20B | 20B |
Results of SARS-CoV-2 testing for E, N and RdRp genes of individual throat swabs collected from infected mink
Positive throat swab | 3 | 8 | 11 | 12 |
---|---|---|---|---|
E gene | 19.4 | 23.9 | 22.3 | 23.9 |
N gene | 24.9 | 29.6 | 27.9 | 28.9 |
RdRp1 | 29.9 | 37.8 | 34.3 | 36.2 |
RdRp2 | 28.8 | 36.9 | 33.6 | 35.5 |