LELYSTAD, THE NETHERLANDS—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.)
The first two mink outbreaks were reported on 23 and 25 April at farms holding 12,000 and 7500 animals, respectively. More mink were dying than usual, and some had nasal discharge or difficulty breathing. In both cases, the virus was introduced by a farm worker who had COVID-19. Today, it has struck 12 of about 130 Dutch mink farms. Once COVID-19 reaches a farm, the virus appears to spread like wildfire, even though the animals are housed in separate cages. Scientists suspect it moves via infectious droplets, on feed or bedding, or in dust containing fecal matter.
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. Mortality was negligible at one farm and almost 10% at another. “That’s strange—we don’t really understand it,” says virologist Marion Koopmans of Erasmus Medical Center in Rotterdam. Feral cats roaming the farms—and stealing the mink’s food—were found to be infected as well. The researchers published a preprint about their work on 18 May; a paper in Eurosurveillance may come out soon.
The Netherlands is the only country so far to have reported SARS-CoV-2 in mink. In Denmark, the world’s largest mink producer, “We have not recorded any similar disease or outbreaks,” says Anne Sofie Hammer, a veterinary scientist at the University of Copenhagen. Neither has China, the second largest producer, says virologist Chen Hualan of the Chinese Academy of Agricultural Sciences. (Hubei, the province hardest hit by COVID-19, does not have mink farms, she notes.)
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. (Avian influenza, for instance, usually spreads as a mild disease in wild birds but can become highly pathogenic when it lands in a poultry barn.) Although SARS-CoV-2 is undergoing plenty of mutations as it spreads through mink, its virulence shows no signs of increasing.
Even so, the Dutch outbreaks have alarmed people in North Brabant province, where mink farms are concentrated. The region’s burgeoning goat industry caused the world’s largest human epidemic of Q fever between 2007 and 2009. Anxious citizens feared a repeat with SARS-CoV-2 and mink. But Coxiella burnetii, the bacterium that causes Q fever, forms hardy spores that wafted out of barns and blew off fields fertilized with goat manure. SARS-CoV-2 is far more fragile; environmental sampling has not turned up any virus outside mink sheds, says veterinary epidemiologist Arjan Stegeman of Utrecht University, who leads the research on mink outbreaks. Whereas farm workers should wear protective equipment, the population at large is at very low risk, Stegeman says.
Eventually, the virus seems to burn itself out at every farm, once more than 90% of the animals have contracted it and developed antibodies. Combined with the low mortality rate, that means the outbreaks are far less devastating for farmers than, for instance, bird flu in poultry or foot-and-mouth disease in cattle.
Even though just two of the Netherlands’s nearly 50,000 confirmed human COVID-19 cases have been linked to the farms, the government decided to cull the animals because the problem could become bigger in the months ahead. Female mink give birth in April and May, leading to a sixfold increase in populations. Antibodies in their mother’s milk probably protect pups for a while, but they might become vulnerable later to any virus lingering at the farm. “That could mean there’s a second wave in minks in the fall,” Van der Poel says—raising the risk of more human cases. The mink are culled by gassing them with carbon monoxide; the Dutch government will compensate farmers.
In the long run, their businesses were doomed anyway: A law approved by the Dutch parliament in 2012 bans mink farming as of 2024 for ethical reasons. The affected farmers may be allowed to reopen their farms for another 3 years if tests conclusively show the virus is gone—or they can decide to throw in the towel now.
sciencemag.org, 9 June 2020