Epidemiologic and Genomic Evidence for Zoonotic Transmission of SARS-CoV-2 among People and Animals on a Michigan Mink Farm, United States, 2020

Abstract

Farmed mink are one of few animals in which infection with SARS-CoV-2 has resulted in sustained transmission among a population and spillback from mink to people. In September 2020, mink on a Michigan farm exhibited increased morbidity and mortality rates due to confirmed SARS-CoV-2 infection. We conducted an epidemiologic investigation to identify the source of initial mink exposure, assess the degree of spread within the facility's overall mink population, and evaluate the risk of further viral spread on the farm and in surrounding wildlife habitats. Three farm employees reported symptoms consistent with COVID-19 the same day that increased mortality rates were observed among the mink herd. One of these individuals, and another asymptomatic employee, tested positive for SARS-CoV-2 by real-time reverse transcription PCR (RT-qPCR) 9 days later. All but one mink sampled on the farm were positive for SARS-CoV-2 based on nucleic acid detection from at least one oral, nasal, or rectal swab tested by RT-qPCR (99%). Sequence analysis showed high degrees of similarity between sequences from mink and the two positive farm employees. Epidemiologic and genomic data, including the presence of F486L and N501T mutations believed to arise through mink adaptation, support the hypothesis that the two employees with SARS-CoV-2 nucleic acid detection contracted COVID-19 from mink. However, the specific source of virus introduction onto the farm was not identified. Three companion animals living with mink farm employees and 31 wild animals of six species sampled in the surrounding area were negative for SARS-CoV-2 by RT-qPCR. Results from this investigation support the necessity of a One Health approach to manage the zoonotic spread of SARS-CoV-2 and underscores the critical need for multifaceted public health approaches to prevent the introduction and spread of respiratory viruses on mink farms.

Keywords: COVID-19; Neogale vison; One Health; SARS-CoV-2; animal; coronavirus; mink; zoonotic disease.

Figure 1

Timeline of events. Epidemic curve of mink mortalities * in grey, overlaid by onset of illness (crosses), sample collection, and diagnostic results (slashes and open circles) of the mink herd and seven staff present on a Michigan mink farm confirmed positive for SARS-CoV-2, 23 September–19 October 2020. Red bracket indicates dates of One Health field investigation, 13–16 October. * Mink mortality data were only collected until 17 October 2020.

Figure 2

Nextstrain divergence tree where blue tree tips indicate background Michigan human community sequences selected March 2020 to January 2021, red, orange, and yellow are mink sequences from September, October, and November, respectively, and teal are outbreak-associated human sequences.

Figure 3

Closeup view of mink clade and associated sequences blue tree tips indicate Michigan human community samples taken March 2020 to January 2021, red, orange, and yellow are mink sequences from September, October, and November, respectively, and teal are outbreak-associated human sequences.

Figure 4

Nextstrain molecular clock tree where blue tree tips indicate Michigan human community samples taken March 2020 to January 2021, red, orange, and yellow are mink sequences from September, October, and November, respectively, and teal are outbreak-associated human sequences.