Now that cases of coronavirus have been identified on mink farms, questions have arisen about the role of this animal in causing the current Covid-19 pandemic. Is it the intermediate host scientists have been looking for?
How SARS-Cov-2 infected humans still unclear
How did SARS-Cov-2 get into humans? That is a question for which there is not yet a clear answer. It seems more likely, based on current knowledge, that the ancestor of SARS-CoV-2 responsible for Covid-19 evolved in bats, particularly those of the genus Rhinolophus. From there, the coronavirus infected another species, in which it replicated and mutated to better adapt. Over time and through human-animal contact, the coronavirus crossed the species barrier and caused the current pandemic.
Research is now focused on the identity of this intermediate host. Since the beginning of the pandemic, several hypotheses have been proposed, the most serious being the Pangolin hypothesis. Coronavirus strains isolated from this endangered mammal are close to SARS-Cov-2 but too distant to be directly related. If not the pangolin, then what animal acted as a mediator between bats and humans? Many infection-prone animals in close contact with humans could have played this role. The virus has already been isolated from dogs, cats, ferrets, lions, and tigers living in zoos, as well as from mink.
Recently, an outbreak of coronavirus was detected at a mink farm in Denmark, resulting in the slaughter of millions of animals. Scientists from the Dutch Public Health Service have investigated similar cases in the Netherlands. There, the first cases of SARS-Cov-2 infections associated with mink farms were reported as early as spring 2020. Using a genome sequencing technique called whole genome sequencing (WGS), Dutch scientists sought to trace the evolutionary history of strains isolated from these farms.
Mink, a possible intermediate host
Their study, published in Science, focused on the first 16 farms that experienced an outbreak in the Netherlands. The first occurred on April 23, 2020, when four of the five farmworkers developed respiratory symptoms. Coronavirus was subsequently isolated from the minks. On the second farm investigated, the outbreak was confirmed on April 25, 2020. The eight employees were tested five days later, on April 30. They all had antibodies to SARS-CoV-2 in their serum.
In total, the scientists obtained 18 usable genomes, isolated from the minks and employees, from seven different farms of the 16 visited. The genomes were divided into 5 groups, graded from A to D according to their sequence. For example, group 1 coronaviruses share up to 9 nucleotide polymorphisms with the human coronavirus, which circulates mainly in the Netherlands (within a 19 km radius of farms). Some mink-derived strains carry the D614G mutation, the dominant variant in Europe, while others show the same profile as the historical strain that infected the first inhabitants of Wuhan.
The scientists also point out another fact: 68% of employees on mink farms caught covid-19. Antibodies were identified in 51% of cases and PCR tests were positive in the remaining 49%.
The WHO investigation
Why do mink farms seem to be so favorable for coronavirus occurrence? Animal density is very high in these mink farms. In addition, mink can sneeze, which facilitates the spread of a respiratory disease like Covid-19 between animals and humans.
But are they the cause of the global pandemic? Although it is impossible to draw a formal conclusion at this point, the question deserves to be asked. China breeds these animals en masse for their fur, but also for their meat, products that could be sold in the exotic market of Wuhan, the assumed source of the pandemic.
China has just authorized scientists from the World Health Organization (WHO) to investigate the origin of the coronavirus in its territory. They are expected to begin their work Thursday in hopes of gathering enough clues to complete the year-long investigation.