SARS-CoV-2 in Danish Mink Farms: Course of the Epidemic and a Descriptive Analysis of the Outbreaks in 2020

Abstract

SARS-CoV-2 infection is the cause of COVID-19 in humans. In April 2020, SARS-CoV-2 infection in farmed mink (Neovision vision) occurred in the Netherlands. The first outbreaks in Denmark were detected in June 2020 in three farms. A steep increase in the number of infected farms occurred from September and onwards. Here, we describe prevalence data collected from 215 infected mink farms to characterize spread and impact of disease in infected farms. In one third of the farms, no clinical signs were observed. In farms with clinical signs, decreased feed intake, increased mortality and respiratory symptoms were most frequently observed, during a limited time period (median of 11 days). In 65% and 69% of farms, virus and sero-conversion, respectively, were detected in 100% of sampled animals at the first sampling. SARS-CoV-2 was detected, at low levels, in air samples collected close to the mink, on mink fur, on flies, on the foot of a seagull, and in gutter water, but not in feed. Some dogs and cats from infected farms tested positive for the virus. Chickens, rabbits, and horses sampled on a few farms, and wildlife sampled in the vicinity of the infected farms did not test positive for SARS-CoV-2. Thus, mink are highly susceptible to infection by SARS-CoV-2, but routes of transmission between farms, other than by direct human contact, are unclear.

Keywords: COVID-19, increased mortality; Neovision vision; SARS-CoV-2 prevalence; clinical signs; environment; seroprevalence.

Figure 1

Number of Danish mink farms tested positive for SARS-CoV-2 per day in the period from 15 June to 12 November 2020.

Figure 2

Days from when first clinical signs of SARS-CoV-2 in mink were observed to the observed peak of infection (left) or to when the last clinical signs (right) were observed by the farmer. The thick line illustrates the median, 50% of the observations are within the box, whiskers illustrates the largest value no further than 1.5 inter-quartile range from the box, and dots beyond the whiskers illustrate outliers. The small dots (jitters) within the box and whiskers indicate the observations (one for each farm).

Figure 3

Anti-SARS-CoV-2 seroprevalence (B,E) and virus prevalence (PCR, A, C, D) in Danish mink farms in the period from 15 June to 12 November 2020, at first and second sampling dates (A, B, D, E) and virus prevalence in dead mink (C). All dead mink were from the first sampling date. Farms are divided in farms where clinical signs were observed (ClinSigns), and farms without observations of clinical signs (noClinSigns). The number of farms included in each type of sampling is indicated as n. Most often, samples from 10 live mink were taken from each farm, but with some variation (1–30 samples), as well as samples from 5 to 10 dead mink. The thick line illustrates the median, 50% of the observations are within the box, whiskers show the largest values no further than 1.5 inter-quartile range from the box, and dots beyond the whiskers indicate outliers. The small dots (jitters) within the box and whiskers illustrate the observations (one for each farm).