Study population and setting
The study reports on two investigations of SARS-CoV-2 infection outbreaks on mink farms in the Western Macedonia region of Greece. Farm A, located in the regional unit of Kozani, had approximately 6,500 minks on the property at the beginning of the outbreak in November 2020 when the animals were in the fattening period of their growth prior to pelting. Animals were housed in seven open-sided sheds and animal care was solely managed by two farmers. Farm B was in the regional unit of Kastoria and had 738 animals going through a slimming down phase prior to the outbreak on the property in early 2021.
Summary of Main Findings
The outbreak of SARS-CoV-2 infection on farm A started following a procedure where both farmers took blood samples from all the animals on the farm over the course of three days to test for Aleutian mink virus disease, a common infection in farmed mustelids. Neither farmer wore a mask during blood sampling and one farmer began experiencing COVID-19 symptoms two days after the start of blood sampling; both farmers were positive for SARS-CoV-2 by rapid antigen test and PCR when sampled 12 days later. Minks on farm A began exhibiting signs of illness (reduced food intake) one day after the completion of blood sampling, followed by additional severe signs. Between day 5 and day 26 after observation of reduced food intake, 548 total deaths (8.4% of the population of 6,500) were recorded. A similar introduction event could not be identified for farm B: farmers did not have prolonged contact with animals during the time prior to the outbreak and none of the farmers or farm workers were positive for SARS-CoV-2 RNA or antibodies when tested over the course of the outbreak (days 1–22). Deaths on farm B were first recorded in a single shed one day after the observation of clinical signs and then spread to neighboring sheds. Unlike the more unimodally distributed pattern of deaths on farm A, deaths on farm B were more variable over time. At day 28 when the last death was recorded, 74 minks had died (10% of the population of 738). Ten visibly ill animals were tested from each farm and tested for SARS-CoV-2 via PCR and all oropharyngeal swabs were positive; four samples from each farm were subjected to full-genome sequencing, revealing amino acid substitutions in the spike protein commonly observed in other mink outbreaks (e.g., Y453F) and that the outbreaks resulted from separate introductions of genetically distinct viruses. Serological testing after the end of the outbreaks of 172 animals on farm A revealed 160 (93%) seropositive and 84 of 90 (93.3%) were seropositive on farm B. Taking into account the number of animals that were infected and died, the authors estimated that 93.6% of animals on farm A were infected, 9.01% of infected animals died, and the reproductive number (R0) at the beginning of the outbreak was 2.9; similar numbers were observed for farm B: epidemic size of 94% and infection fatality rate of 10.7%. Additionally, SARS-CoV-2 RNA was detected on surfaces above mink enclosures and from the air on both farms when sampled in February 2021.
Because the investigations on the farms were initiated rapidly after the first onset of clinical signs in animals, and the outbreak on farm A was started when hundreds of animals were exposed to an infected farmer, the researchers were able to estimate more epidemiological parameters than other studies of mink outbreaks in Europe. This included the number of infections and deaths in mink attributed to the introduction event, the final outbreak size, and the infection fatality rate. In contrast to other studies in Denmark and the Netherlands, the outbreaks on the two farms in Greece were not ended via culling of animals, allowing the researchers to test whether the outbreaks would resolve after mink populations developed herd immunity.
Without an identifiable introduction event for farm B, it was not possible to estimate some epidemiological parameters, including the number of infections and deaths due to the introduction event, R0, the epidemic growth rate, the doubling time, or the generation time. It was also unclear why the distribution of deaths over time was so different between farms A and B. While this could be attributed to less intensive exposure of animals on farm B to infected human index cases as compared to farm A or limitation of exposure to a smaller number of animal sheds, greater scrutiny of the locations of infections in sheds over time might help to explain the stochasticity in daily deaths. While the two outbreaks were very similar in the number of mink deaths and the proportion of the population infected despite the different growth stages of the animals on each farm (fattening vs. slimming down), potential differences in other epidemiological parameters due to the different growth stages and their potential effects on mink health and immunity were not investigated.
This article adds to the existing evidence that SARS-CoV-2 is readily transmissible between humans and minks on farms where animals are raised intensively for their fur. The observation that infections and deaths in minks eventually ended on both farms without culling indicates that a herd immunity threshold may exist in mink populations, but at the cost of high mortality (~10%).
This review was posted on: 18 October 2021