Tracking Two-Way Human-Mink Transmission During an Outbreak of Severe Acute Respiratory Syndrome Coronavirus 2 on a Farm in Bulgaria

Introduction: During the COVID-19 pandemic, it was observed that SARS-CoV-2 (Severe Acute Respiratory Syndrome Coronavirus 2) can be transmitted from humans to various animals. Our study aims to examine the genetic evidence for transmission of SARS-CoV-2 from humans to mink and potentially back to humans in the first reported outbreak on a mink farm in Bulgaria. Materials and methods: Between October 2 and 29, 2023, 420 oropharyngeal samples from minks on a farm in Central Bulgaria were examined for SARS-CoV-2. Positive samples with Ct <32 were sequenced using Oxford Nanopore technologies. Results: On 18 October 2023, 98 of 118 mink samples tested positive for SARS-CoV-2. In addition, on 25 October, a mink caretaker from the same farm was confirmed to be infected with SARS-CoV-2. Phylogenetic analysis of the isolated SARS-CoV-2 revealed that the mink caretaker and two minks had a variant similar to FY.1.2 (37.5%), while five other minks were infected with a different variant similar to XBB.1.22 (62.5%). This suggests the presence of another source of infection on the farm with the XBB.1.22 variant. Furthermore, we identified a substitution at position I478K in the receptor-binding motif (RBM) of the receptor-binding domain (RBD) of S-protein in 2 mink samples. Conclusion: Based on the epidemiological and genetic analysis, our findings suggest a potential for human-to-mink and mink-to-human transmission of SARS-CoV-2. Tracing the route of transmission from an animal host to humans will help elucidate the route of origin and causality for the accumulation of mutations leading to the emergence of new human coronaviruses (HCoVs) and their variants with stronger or weaker pandemic potential.