A new study by the Vilnius University Life Sciences Center (VU LSC) and an international team of researchers, published in the peer-reviewed journal Virus Evolution, has revealed that the coronavirus responsible for the COVID-19 pandemic spread undetected for an extended period in Lithuania’s mink farms. The findings show that the virus was transmitted at least four times from mink to humans and 14 times from humans to mink in Lithuania. The results highlight the importance of monitoring viruses circulating in animal farms for public health and expose significant gaps in this area in Lithuania.
The study was led by Dr. Gytis Dudas, a senior researcher at the Vilnius University Life Sciences Center, and conducted by Martynas Smičius, a doctoral student at the same center. In Lithuania, the research also involved scientists and representatives from the Vilnius University Life Sciences Center, Vilnius University Hospital Santaros Klinikos, the Lithuanian University of Health Sciences, Hospital of the Lithuanian University of Health Sciences Kauno Klinikos, the State Food and Veterinary Service, the State Data Agency, and the Government. International partners included researchers from the Erasmus Medical Center in the Netherlands, Rigshospitalet in Denmark, and the University of Cambridge in the United Kingdom.
Phylogenetic analyses revealed that some SARS-CoV-2 lineages circulated within the mink population for nearly a year – even after they had disappeared among humans. This hidden transmission came to light only because of Lithuania’s well-developed human coronavirus surveillance programme, when the virus was detected in humans again in late 2021, most often among mink farm workers.
Researchers identified several viral lineages that entered mink populations at different stages of the pandemic, ranging from early variants (such as B.1.343 and B.1.177.60) to later Alpha (B.1.1.7) and Delta (B.1.617.2) variants.
The genetic evidence presented in the study suggests that some infections detected in Lithuania may have been of zoonotic origin. The variants responsible for these infections had often already disappeared from the human population. At the same time, their closest genetic relatives had previously been identified in both humans and mink in Lithuania during earlier outbreaks. In addition, these variants frequently carried mutations associated with more efficient transmission in mink populations. Importantly, the infected individuals worked in mink farms. Taken together, these findings allowed researchers to rule out alternative explanations for the origin of these infections, such as transmission linked to international travel or prolonged infections in immunocompromised individuals.
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| a) Human COVID-19 cases in Lithuania coloured by variant frequency; b) estimated dates of fourteen SARS-CoV-2 spillover events from humans to mink in Lithuania, coloured by variant; c) size of the farmed mink population in Lithuania. |
The authors of the study note that, in Lithuania, disease surveillance in mink farms has largely relied on passive monitoring, with farmers expected to report potential outbreaks themselves. However, the nearly year-long undetected spread of SARS-CoV-2 in Lithuanian mink farms suggests that such a system may be insufficient.
Most coronavirus genomes from mink were obtained only at the end of 2021, following the inspection of all mink farms in Lithuania and in response to suspicious human infection cases. At that time, SARS-CoV-2 infection was detected in 25 out of 57 farms.
According to Dr. Gytis Dudas, the coronavirus is a non-host-specific virus capable of infecting a wide range of animals, including mink, deer, cats, and dogs. Animal populations that can sustain ongoing transmission may become reservoirs from which the infection can spill back into humans.
He emphasises that, in the future, it is important to strengthen animal disease surveillance in Lithuania, ensure closer cooperation between veterinary and public health institutions, and enable earlier detection of potential zoonotic infections in humans.
He also points out that this will require training a new generation of specialists in the field. Contributing to this effort will be a new Master’s programme in bioinformatics at the Vilnius University Life Sciences Center, set to launch in the autumn.
The study once again demonstrates that the “One Health” principle – integrating human, animal, and environmental health – is essential for better preparedness for future outbreaks, epidemics, and pandemics of infectious diseases.