SARS-CoV-2 Transmission between Mink (Neovison vison) and Humans, Denmark

Severe acute respiratory syndrome coronavirus 2 has caused a pandemic in humans. Farmed mink (Neovison vison) are also susceptible. In Denmark, this virus has spread rapidly among farmed mink, resulting in some respiratory disease. Full-length virus genome sequencing revealed novel virus variants in mink. These variants subsequently appeared within the local human community.

reads were trimmed versus the ARCTIC protocol primers and end-trimmed (30 nt). Consensus sequences were generated with a minimum depth of 10 and use of quality-based vote consensus building. All positions with less than 90% site coverage were excluded from the analysis.
A subset of 567 reference sequences used in Nextstrain on June 26, 2020 were downloaded from GISAID; these were selected based on quality and matching the proportionate distribution of the major clades to the set of sequences used in Nextstrain.
The mink-derived sequences and linked human cases belonged to European Clade 20B. This is specifically defined by the mutations G28881A, G28882A, and G28883C and supported by the presence of the mutations C3037T and C14408T ( Table 2 in the main article).

Farm Sampling
Sampling and Testing of Mink in Farm 1 Farm 1 was in northern Jutland. It had ≈1,800 adult females and >9,000 kits, born around the beginning of May 2020. No excess deaths had occurred but some mild respiratory distress had been observed in a few adult mink. Initially, fecal and nasal swab samples as well as blood samples were collected ( Table 1 in the main article). Five of 30 live mink and all 4 deceased mink scored positive by RT-qPCR (Table 1). Furthermore, 29 of 30 animals scored positive by ELISA for anti-SARS-CoV-2 antibodies (Table 1). In contrast, 11 stored mink serum samples, collected in 2016 before the emergence of SARS-CoV-2 in 2019, scored negative in this assay.
In follow-up sampling, 4 of 30 mink kits tested scored SARS-CoV-2 RNA positive (Table 1). Serum samples from all 30 kits and 30 adults scored positive by ELISA. In retesting of 4 adult females that tested positive by RT-qPCR in the initial testing, 3 again tested positive and all 4 were again scored as seropositive ( Table 1). Samples of mink feed and of air samples collected from the farm all proved negative in RT-qPCR assays.
Sampling and Analysis of Mink at Farm 2 Farm 2, located ≈12 km from farm 1, had ≈700 adult female mink and 3,500 kits. One person on this farm, who had contact with the mink, tested positive for SARS-CoV-2 and was linked to infected persons connected to farm 1. Initial sampling from the mink occurred on June 18, 2020, when no clinical signs or excess deaths had been observed. Serum samples from 30 adult mink were assayed by ELISA and one scored positive (Table 1). In addition, samples from 8 live and 8 dead mink were tested by RT-qPCR; 1 throat swab tested strongly positive (Ct = 20.4) but none of the rectal swabs did (Table 1). During a follow-up visit on June 22, 40 of the 50 swab samples from mink kits were scored positive by RT-qPCR (Table 1). However, only 1 of the serum samples from these 50 kits tested positive by ELISA. In the adult animals, 46 of 50 throat swab samples were RT-qPCR positive but only 3 serum samples from these animals scored positive by ELISA (Table 1). One of 3 sick mink, with respiratory symptoms, scored strongly positive from testing the throat swab sample (Ct = 17.6) and was also seropositive. In addition, an air sample, collected directly from the exhaled air of this mink, scored positive (Ct = 31.4).
In the following days, several mink displayed respiratory disease symptoms. On June 30, adult females sampled previously were resampled and some had clear respiratory symptoms and distress. In a period of 8 days, the seroprevalence among the adult mink increased from 6% to >95% and 35 of 37 throat swab samples were again positive by RT-qPCR (Table 1). During June 22-30, 4 of the 50 animals had died. Three of these had high levels of SARS-CoV-2 RNA (Ct <25) in throat swabs; the fourth was unavailable. Three separate mink feed samples were scored negative.
Sampling and Analysis of Mink at Farm 3 Farm 3, with ≈1,000 adult females and almost 4,500 kits is located <1 km from farm 1, and 1 person in contact with the mink had tested positive for SARS-CoV-2. Some clinical signs of respiratory disease had been apparent and 20-30 mink had died in the 2 weeks before initial sampling on June 29, 2020. By ELISA, 20 of the 30 serum samples tested positive (Table 1), suggesting that the infection had been introduced >1-2 weeks before this sampling. Throat swabs from 30 live mink were tested in pools; all 6 pools were positive by RT-PCR, as were throat swabs from 5 dead mink. On July 2, during follow-up, 23 of the 30 adult mink were seropositive, as were 24 of the 30 mink kits. All the throat swabs and most of the nasal swabs scored positive by RT-qPCR (Table 1). Air samples collected from near the mink (<1 m from cages) were also positive, but air samples from outside the open buildings (2-3 m from cages) were negative. The mink on farms 1, 2, and 3 have all been culled.