In a sad sideshow to the COVID-19 pandemic, authorities in the Netherlands began to gas tens of thousands of mink on 6 June, most of them pups born only weeks ago. SARS-CoV-2 has attacked farms that raise the animals for fur, and the Dutch government worries infected mink could become a viral reservoir that could cause new outbreaks in humans.

The mink outbreaks are “spillover” from the human pandemic—a zoonosis in reverse that has offered scientists in the Netherlands a unique chance to study how the virus jumps between species and burns through large animal populations.

But they're also a public health problem. Genetic and epidemiological sleuthing has shown that at least two farm workers have caught the virus from mink—the only patients anywhere known to have become infected by animals. SARS-CoV-2 can infect other animals, including cats, dogs, tigers, hamsters, ferrets, and macaques, but there are no known cases of transmission from these species back into the human population. (The virus originally spread to humans from an as-yet-unidentified animal species.)...

...That mink are susceptible wasn't a surprise, because they are closely related to ferrets, says Wim van der Poel of Wageningen University & Research, which has an animal health laboratory here. (Both mink and ferrets can also contract human influenza viruses.) Like humans, infected mink can show no symptoms, or develop severe problems, including pneumonia...

...The Dutch outbreaks are giving scientists a chance to study how the virus adapts as it spreads through a large, dense population. In some other animal viruses, such conditions trigger an evolution toward a more virulent form, because the virus isn't penalized if it kills a host animal quickly as long as it can easily jump to the next one....