SARS-CoV-2 infections on French mink farms after one year of COVID-19

By Dr. Liji Thomas, MDFeb 3 2023Reviewed by Aimee Molineux

The coronavirus disease 2019 (COVID-19) pandemic was declared by the World Health Organization (WHO). As it spread over the whole world, reports of infection in farmed mink began to appear, raising fears that they could spread it to humans – reverse zoonosis. A recent preprint describes the situation in France, which has only four mink farms.

*Important notice: bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Introduction

Farmed American mink (Neovison vison) were first found to be harboring the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in April 2020, followed by Denmark and then several other European countries. The Danish outbreak spread rapidly both within and among farms, with a variant called cluster S being identified as having emerged from mink infections in November 2020.

The mink were considered to have acquired the infection from farm employees, but reverse transmission was also identified in both the Netherlands and in Poland.

In France, no clinical symptoms of mink SARS-CoV-2 infection have been reported. Mink are typically slaughtered when they are almost a year old, that is, when they become young adults. All animals of the same batch are included in the slaughter at the end of each year.

The current paper, which appears on the bioRxiv* preprint server, describes the investigation of all French mink farms towards the end of 2020. This was to maximize the chances of identifying SARS-CoV-2 infection in these animals at any time over the past year.

The researchers used serology for anti-nucleocapsid (anti-N) antibodies and nucleic acid-based methods.

What does the study show?

The study included almost 2000 mink born in 2020. About 1600 swabs were collected from the throat and trachea, with 60 or more animals per building being sampled.

The results showed that one farm currently harbored infected mink, with SARS-CoV-2 ribonucleic acid (RNA) being detected in the swabs. Approximately 87% of samples from the infected farm were seropositive or doubtful, vs. ~1% in the non-infected farms. The positive samples showed neutralizing titers from ~80-2200, indicative of ongoing virus circulation.

Of the 162 samples from the infected farm, just less than 55 were positive for SARS-CoV-2 RNA, and 33 had low levels. Random positive samples were confirmed by seroneutralization assay, while negative samples from the other three farms were also verified by this method.

None of the animals were symptomatic and none died, suggesting a mild illness that was missed by the farm.

Genomic sequencing was carried out on samples with a low cycle threshold (Ct). However, the variant was different from that detected on any other farm in the Netherlands or Denmark by 13 SNPs (Single Nucleotide Polymorphisms). Four of these led to changes in the amino acid sequence in the spike protein (silent mutation in S (S477N) or ORF1b.

Yet, several different variants were found to be circulating within this clade during the sampling period. All these variants belonged to the 20A clade, and all were found in human samples collected in France during the same period.

The other farms showed no sign of SARS-CoV-2, but a few samples were positive for mink Alphacoronavirus, mink-coronavirus sequence, and Caliciviridae were identified. These can cause asymptomatic infection, but the former is also known to cause epizootic gastroenteritis (ECG). The coexistence of these viruses could cause infected mink to show atypical symptoms of SARS-CoV-2 infection.

What are the implications?

The results of the study indicate that reverse zoonosis happened on this mink farm, with the same 20A clade being identified from all mink samples and human samples collected from the wave of human cases at this time. This clade has not been found in any other mink sequences worldwide.

The period of circulation of the virus on the farm appears to have been limited to less than three months, since significant divergence was not observed among the genomic sequences from the farm mink samples. Human-to-mink transmission is thus likely to have occurred shortly before the outbreak. This is supported by the relatively low Ct values in some mink, especially since throat samples are positive for viral RNA from the second to 17^th day after infection.

Mink-to-mink transmission has been shown to be rapid by earlier researchers, who reported that the prevalence of the infection among mink went up from a mere 4% to 97% on one Danish farm.

Finally, the existence of the alphacoronavirus infection on two farms, could trigger increased viral recombination with the betacoronavirus, SARS-CoV-2, circulating at the same time. This has been reported to occur in animals on farms and in the wild, and in humans. This could cause novel coronaviruses to emerge due to the combination of unpredictable phenotypic features and viral fitness.

The presence of other silent potential pathogens on mink farms might alter the clinical presentation of SARS-CoV-2 in mink, accounting for the variability in clinical features of mink infection in various countries.

*Important notice: bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.