A fermenting, bubbling culture vessel, in which the virus acquires an average of two new mutations per month, and constantly jumps back and forth between minks, employees and the cats that roam around. That is the image that scientists affiliated with ten Dutch research institutes are sketching out the corona outbreak on the mink farms.
The team analyzed the genetic family tree of the virus on the first 16 Dutch fur farms where the virus was diagnosed – that was until the beginning of June, but that number has now more than doubled. On the 16 farms, 66 people and 11 cats were found who had contracted a ‘mink variant’ of the virus.
At least five jumps
‘You usually don’t know whether they all got the virus directly from the mink, or whether they were infected with each other,’ says professor of virology Marion Koopmans (Erasmus MC), leader of the study. But given that the scientists found five ‘clusters’ of mink viruses, it is likely that the virus made the leap from mink to humans at least five times by June.
Epidemiologist Lidwien Smit (Utrecht University) also points out that no fewer than two thirds of all employees tested had traces of contamination. ‘That is even a higher percentage than you see in family situations. That alone makes it plausible that many more people were directly infected by minks than those few we already knew about. ‘
After the first finding of infected mink in two stables in April, Minister of Agriculture Carola Schouten called the chance of transmission to humans ‘negligible’. A month later, the first case came to light of an employee who had most likely become ill from a mink. Another month later, another case was added. That was only the tip of the iceberg, it now appears.
No danger to local residents
Incidentally, there does not appear to have been any direct danger to local residents. The researchers found no case in which a mink virus ‘leaked’ into the neighborhood.
That makes it immediately puzzling how the virus ended up on the farms, and how it went from stable to stable at least eleven times. In one case, it is clear that the virus was introduced by an infected worker. In other cases this is a mystery: perhaps a commuting employee or supplier has moved the virus. In any case, the virus has not spread through the air, says Smit. ‘We see companies that are right next to each other, yet have a different cluster of viruses.’
The virus is so eager to spread among minks that Koopmans does not even rule out that the fur industry has played a role in the transmission of the virus to humans in China. “This could be a plausible intermediate step in the path the virus has traveled from bats to humans,” she says. “Maybe this is one of the links we still miss.”
Although a striking number of infected cats were found on the mink farms, cat owners need not worry, says Smit. After all, the farms were feral cats who lived among the minks. The domestic cats of the farmers, the cats of local residents and eleven dogs from the neighborhood all tested negative.
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