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Preliminary report of an outbreak of SARS-CoV-2 in mink and mink farmers associated with community spread, Denmark, June to November 2020

Our take —

Between June and November 2020, SARS-CoV-2 was detected on hundreds of mink farms and in workers on those farms in Denmark. A genetic variant associated with mink was recognized in June 2020 and eventually spread to thousands of people in the general population. This study highlights that the large population of mink on these farms is not only a hazard to farm workers but could serve as possible reservoirs that seed new infections or viral variants into the human population. Animal health agencies in countries with carnivore farms should consider enhanced surveillance and biosafety procedures to detect and limit outbreaks early so that massive culling of animals is not necessary.

Study design

Other

Study population and setting

Beginning in June 2020 with the first reported outbreaks of SARS-CoV-2 infection in farmed mink populations in Denmark, the country began monitoring for additional outbreaks. Infected farms were identified by tracing the address of infected human patients to a mink farm, via tracing of patient contacts to identify individuals involved in mink production, surveillance of dead mink from all mink farms country-wide, and reported clinical signs of infection in minks identified by veterinarians. Mink-associated SARS-CoV-2 genetic variants circulating in the human population in Denmark were identified via whole genome sequencing of infected mink and a convenience sample of human cases. These data spanning June to November 2020 were then used to estimate the regional incidence of mink-associated variants.

Summary of Main Findings

Between June and November 2020, 290/1,147 (25%) mink farms in Denmark had detectable SARS-CoV-2 infection, corresponding to approximately 3–4 million animals. A total of 3,319 people were identified with a connection to mink farms, 643 (19%) of which were infected. The highest proportion of infected farms and the highest attack rate for purported mink-human infections was in the North Denmark region with 48% of farms infected (190/394) and 324/1,092 (30%) of individuals with a connection to mink farms testing positive. A Y453F amino acid substitution in the spike protein was identified associated with mink in Denmark in June 2020. Stratified estimates based on the occurrence of this variant in a convenience sample of genomes from confirmed human cases (13,355/65,872, 20%) indicate that approximately 4,000 people were infected with a mink-associated SARS-CoV-2 variant. Attempts to control spread of SARS-CoV-2 among mink farms via enhanced surveillance of farms, enhanced biosecurity and use of protective equipment, and culling of infected farms were largely unsuccessful due to the delayed detection of infected farms. The Danish government ordered all mink (~17 million animals) in the country to be culled in November 2020 and instituted a ban on mink farming until the end of 2021.

Study Strengths

The authors assessed the representativeness of the convenience sampling of SARS-CoV-2 genome from human patients by confirming that the proportion of samples sequenced that were connected to mink farms was similar to the proportion of cases connected to mink farms in the general population in each region. The estimates of the number of human cases of mink-associated SARS-CoV-2 variants were also stratified by week and by region to account for regional variation in the incidence of the mink variant and increasing sequencing efforts over time.

Limitations

Since it was not possible to identify employees living outside of mink farms using with the current surveillance model, the occupational risk of mink-human infections on farms may been underestimated.

Value added

Outbreaks of SARS-CoV-2 infection on mink farms have been reported in Canada, Denmark, France, Greece, Italy, Lithuania, the Netherlands, Spain, Sweden, and the Unites States. Until June 2020, Denmark accounted for roughly 40% of the world production of mink for fur. This study provides evidence of the rapid spread of SARS-CoV-2 among mink farms in Denmark and the circulation of mink-associated viral variants in the general human population. The failure of control measures to prevent spread should be of concern to mink farmers in other countries and potentially other farms supporting the fur trade, especially those with susceptible carnivore species (e.g., raccoon dog).

This review was posted on: 12 March 2021