SARS-CoV-2 monitoring in mink farms in Poland

Introduction. Many countries have reported SARS-CoV-2 infections in mink and the transmission back to humans has raised the concern of mink being reservoirs for the emergence of novel variants. The implemented monitoring system in mink farms in Poland enabled the first detection of SARS-CoV-2 infection in these animals in January 2021. Ongoing monitoring of mink farms was carried out consequently. Material and methods. Samples collected from 11853 mink from a total of 594 farms located in different regions of Poland were tested in the frame of the SARS-CoV-2 molecular monitoring system. Selected samples with the highest loads of viral genetic material from positive farms were subjected to sequencing and phylogenetic analysis. Additionally, serological studies were carried out for one positive farm in order to follow the antibody response after infection. Results. SARS-CoV-2 RNA was detected in minks on 11 farms located in 8 out of 16 voivodeship administrative regions of Poland. Whole genome sequences were obtained for 19 SARS-CoV-2 strains from 10 out of 11 positive farms. These genomes belonged to 4 different variants: VOC-Gamma (20B), VOC-Delta (21J), VOC-Alpha (20I) and VOC-Omicron (21L) and seven different Pango lineages: B.1.1.464, B.1.1.7, AY.43, AY.122, AY.126, B.1.617.2 and BA.2. These viruses reflected the diversity of SARS-CoV-2 variants circulating in humans at the time of virus detection in the mink with the exception of one farm where a virus was identified that had been prevalent among humans more than a year earlier, suggesting its persistence on the farm over a long period of time. Several nucleotide and amino acids mutations specific for persistent strains were found in the analyzed samples including the Y453F mutation responsible for potential adaptation towards the mink host. Serological testing of blood samples revealed a high rate of seroprevalence on the single studied mink farm. Conclusion. Farmed mink are highly susceptible to infection with SARS-CoV-2 belonging to different lineages including the Omicron BA.2 VOC. As SARS-CoV-2 infections identified on farms in Poland were asymptomatic, mink may become an unnoticeable virus reservoir generating new, potentially dangerous variants threating of human health. Thus, real-time monitoring of mink is extremely important in the frame of "one health" approach.