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Genomic evidence suggests animal-to-human transmission of SARS-CoV-2 within mink farms, triggering investigations into animals’ possible involvement in the COVID-19 pandemic
In December 2019, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was first identified as the cause of a pneumonia outbreak and named COVID-19. Now, as of February 2021, SARS-CoV-2 has spread across the globe to infect 110 million and take the lives of over 2 million people (Johns Hopkins Coronavirus Resource Center 2021).
When analyzing the new virus in comparison to the rest of the coronavirus family, a zoonotic origin of the outbreak was linked to a Wuhan live animal market where various animals – including seafood, poultry, and exotic animals – were sold. It is well known that many infectious agents, especially those that cause evolving diseases like COVID-19, infect a host species. These host populations that are epidemiologically connected to the target population of the pathogen are called natural reservoirs (Haydon et al. 2002). Managing reservoirs of multihost pathogens is key to effective disease control.
Currently, humans act as a reservoir for SARS-CoV-2, but they are potentially not the only reservoir for this virus. On top of discovering the origin of the original outbreak in a live animal market, many different species have been used to model SARS-CoV-2 susceptibility and transmission, such as cats, tree shrews, hamsters, and ferrets. Recently, SARS-CoV-2 was detected in farmed mink (Neovison vison) in the Netherlands, with animals displaying significant increases in mortality. However, it was unknown if SARS-CoV-2 first infected humans then minks, or if the virus jumped from minks to humans. Additionally, it was unclear to what extent animals were involved in the COVID-19 pandemic. Therefore, Oude Munnink et al. (2021) investigated the origins of the first 16 SARS-CoV-2-affected mink farms in the Netherlands by combining data from SARS-CoV-2 diagnostics, genomic sequences, and in-depth interviews.
To reconstruct the sequence of transmission of SARS-CoV-2, the authors tested and interviewed 97 employees and contacts at 16 mink farms. They found that 68% of mink farm employees and contacts had evidence of SARS-CoV-2 infection. To determine if this high percentage was caused by human-to-mink or mink-to-human transmission, they also used whole-genome sequencing to map out the genomes of SARS-CoV-2 found in humans and minks. Whole-genome sequencing is the process of determining the complete DNA sequence of an organism’s (like the pathogen SARS-CoV-2) genome at a single time (National Cancer Institute 2012). Oude Munnink et al. found that the genomes of SARS-CoV-2 in minks carry a mutation that originated from the SARS-CoV-2 virus in humans, which suggests human-to-mink transmission on mink farms. They even found some employees with SARS-CoV-2 genome sequences that carried a small amount of animal SARS-CoV-2 sequence, which is evident of an additional but reverse event: mink-to-human transmission.
The authors next established the potential risk of SARS-CoV-2 infection for people who live near mink farms by comparing the genetic sequences of the minks infected, the employees of the farms infected, and comparing those sequences with whole-genome sequences in a national database. To discriminate between community-acquired and mink farm-related infections, whole-genome sequencing was also performed on SARS-CoV-2-positive samples from people who live in the same geographic area as the farms. Oude Munnink et al. found that the samples taken from the mink farms reflected the wide diversity of SARS-CoV-2 genomes seen in the Netherlands national data, suggesting no spillover events of the virus to the people living near the mink farms occurred.
Lastly, by comparing individual mink farms, the authors hoped to identify common factors that might explain farm-to-farm spread and why minks were a host population for SARS-CoV-2. Considering variables such as farm owner, animal feed supplier, mink population size, and shared employees, they found no common factor for most farms. Moreover, clustering of farms by similar genome sequences could not be explained by geographic distance. In most cases, employee sequences were nearly identical to the mink sequences from the same farm.
Consequently, Oude Munnink et al. speculates that transmission of SARS-CoV-2 on mink farms occurred between humans and minks and back again to humans. Their research also found evidence of ongoing SARS-CoV-2 transmission in mink farms and that contact with SARS-CoV-2-infected mink is a risk factor for COVID-19. More research will need to be conducted to further investigate if mink and other similar species are at risk of becoming reservoirs for SARS-CoV-2 and what commonalities predict transmission.
Why was such a high level of diversity observed in the whole-genome sequences from some mink farms? The authors hypothesize that multiple generations of viral infections in minks happened before the increase in respiratory symptoms and mortality was noticed. This could be indicative of a faster evolutionary rate of the virus in the mink population. However, one limitation to the data is the moment of SARS-CoV-2 introduction to the farms is not known, so no conclusions can be drawn. Regardless, since publication of this study, SARS-CoV-2 infections have been uncovered in mink farms in Spain, Denmark, and the United States, suggesting this animal-to-human transmission was not an isolated event.
Even though the authors did not identify any common factors that could explain farm-to-farm spread, their research is indicative of possible animal-to-human transmission of SARS-CoV-2 which has critical importance to the future of disease control. We cannot guarantee that COVID-19 will be the last epidemic caused by SARS-CoV-2. But, we can better monitor animals’ health and collaborate between public health agencies and the fur production industry to prevent future spillover of SARS-CoV-2 to humans by mink and other natural reservoirs.
Rachel Hendricks is a senior at Davidson College expected to graduate with a Bachelor’s degree in Biology in May 2021. Contact her at rahendricks@davidson.edu
RESOURCES
Haydon D. T., S. Cleaveland, L. H. Taylor, and M. K. Laurenson, 2002 Identifying Reservoirs of Infection: A Conceptual and Practical Challenge. Emerging Infectious Diseases 8: 1468–1473. https://doi.org/10.3201/eid0812.010317
Johns Hopkins University of Medicine, 2021 COVID-19 Data in Motion. Johns Hopkins Coronavirus Resource Center.
National Cancer Institute, 2012 Definition of whole-genome sequencing. NCI Dictionary of Genetics Terms.
Oude Munnink B. B., R. S. Sikkema, D. F. Nieuwenhuijse, R. J. Molenaar, E. Munger, et al., 2021 Transmission of SARS-CoV-2 on mink farms between humans and mink and back to humans. Science 371: 172–177. https://doi.org/10.1126/science.abe5901
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