The minks on Dutch fur farms first got sick in mid-April, showing symptoms ranging from runny noses to severe respiratory distress. They had caught the novel coronavirus from human handlers, the government later said, and soon farmed minks appeared to have passed it back to two other people, in the world’s first reports of animal-to-human transmission since the pandemic began.
The Netherlands has since culled more than 500,000 minks from 13 infected fur companies. The goal of the grim task, set to continue until the farms are virus-free, is to snuff out the possibility of the animals becoming a reservoir for the virus that causes covid-19, which could stymie efforts to end a pandemic that has killed nearly half a million people worldwide.
Some researchers say that although the chances of that happening appear minimal, the implications are too grave to dismiss. In a commentary published Thursday in the Lancet Microbe, researchers at University College London called for widespread surveillance of pets, livestock and wildlife. Studies on animal susceptibility have been small, limited and, in the case of pigs, conflicting, they wrote.
“We need to develop surveillance strategies to ensure we don’t get taken by surprise by a large outbreak in animals, which could pose a threat not just to animal health but to human health as well,” co-author Joanne Santini, a professor of structural and molecular biology, said in a statement.
The steps being taken in the Netherlands, which also include surveillance of cats at the farms and wild mink relatives called martens, are among the broadest efforts to understand how a zoonotic virus that originated in animals before hopping to humans may now be spreading back to animals. In the six months since the outbreak began, cases have been reported of human transmission to dogs, cats, tigers and lions in addition to minks. Laboratory experiments have found that ferrets, hamsters, monkeys and other mammals are also susceptible to the virus.
Scientists and public health officials emphasize that humans are the overwhelming drivers of the coronavirus’s spread and that the chance of becoming sickened by infected animals appears to be minute. But researchers say the lab results and the small but growing number of cases in animals are reason for far more extensive study of how the virus can move between species.
“We know that these viruses are capable of mutating,” said Peter Rabinowitz, a physician who directs the University of Washington Center for One Health Research, which is studying the virus in household pets. “There could be changes in the virus, and these human-animal transmission events could play more of a role in the future, and we have to be more vigilant.”
Although the novel coronavirus is a zoonotic disease believed to have originated in horseshoe bats, the Centers for Disease Control and Prevention and the World Health Organization initially played down the idea of transmission to animals, saying there was no evidence they could be infected. After reports of infected dogs emerged from Hong Kong, they said there was no evidence animals could transmit the virus to humans. Now, the CDC says there is no evidence animals “play a significant role” in transmission but advises socially distancing pets from non-household members and isolating pets from people with covid-19, the disease caused by the coronavirus.
The shifting messaging was imprudent and confusing, said J. Scott Weese, a professor at Ontario Veterinary College who in January argued on his blog that we should assume the virus could infect other species “until proven otherwise.” From the start of the outbreak, he said, patients should have been asked about contact with animals and quarantine protocols should have taken exposed animals into account, as was done in Hong Kong.
“It’s better to prevent problems, and if you don’t look, you can’t act,” Weese said in an email. “Hopefully the animal aspect is a niche thing that won’t have much relevant impact on humans. However, the potential risk of this virus affecting livestock, pets and wildlife was ignored.”
Without large numbers of cases, there have been concerns over whether warnings about the potential of transmission to or from animals might prompt people to abandon pets or harm other animals, researchers say.
“If everybody drops their dog and cat off at the animal shelter because they’re concerned they’re going to get covid from their pet, that becomes a real serious animal welfare issue,” said Katie Kuehl, a clinical instructor at Washington State University’s veterinary school.
Kuehl and Rabinowitz are co-leading one of a handful of studies now aimed at better understanding human-to-pet transmission. They’re collecting swabs and blood samples from cats, dogs, ferrets and hamsters owned by coronavirus patients in Washington state and asking owners questions about how they interact with their pets — Do they sleep together? Share dishes? — to determine risk factors.
Weese is leading a similar study in Ontario. Researchers at Tufts’s Cummings School of Veterinary Medicine have sampled about 400 animals — mostly dogs and cats but also bats, pigs and horses — as part of a two-pronged study that is also following human-animal pairs over time. All tests have been negative, said virologist Jonathan Runstadler.
Given the coronavirus’s animal origin, “It’s not too far a leap for all of us to say, ‘Well, what can it do now? Is it liable to infect another animal host?’ ” said Runstadler, who added that one motivation for the study was to learn whether staff at the school’s veterinary clinics might be at risk from infected patients.
Other researchers are looking at wildlife. In a recent paper published by the U.S. Geological Survey, experts on bats, wildlife, epidemiology and viruses concluded that there is a “non-negligible risk” of transmission of the coronavirus from humans to bats on this continent and a 33 percent chance the virus could spread within a North American bat population. That risk could be dramatically reduced if people who handle bats use personal protective equipment, according to the paper, which was based on preexisting knowledge, not surveillance or lab experiments.
For the time being, researchers say cats should be a focus, because studies have found they are highly susceptible to the coronavirus and because they are common pets and roam freely in many places. In a study published last month in the New England Journal of Medicine, scientists infected cats with the virus and found that those cats could infect other cats. No felines showed symptoms, but the amount of virus they shed in nasal swabs was similar to that shed by some humans, said co-author Peter Halfmann, a University of Wisconsin virologist.
“If a human can transmit to a human with this amount of virus being shed, it’s definitely possible for a cat to transmit to a human,” Halfmann said.
This is unlikely to be significant for most pet owners, because cats that stay indoors would only be infected by humans in the household. But it could be important for animal shelters that house many cats, which Halfmann said his team is starting to study.
Cats are also on the radar in the Netherlands, where the mink farm outbreaks began in a coronavirus hot spot in the nation’s south. Several cats that live on the farms tested positive for the virus or antibodies, and it is unclear what role they played in transmission, said Arjan Stegeman, a veterinary epidemiologist at Utrecht University who chairs an expert group that advises the Dutch government on animal diseases.
But the timing of infections and genetic sequencing of virus samples from sickened minks and farmworkers show the initial cases were brought into the barns by humans and provide “a very convincing case” that two humans were later infected by minks, said Stegeman, who helped lead the investigations. That the virus caught fire among minks was not surprising, because the animals are related to ferrets, which are known to be susceptible, Stegeman said. Denmark on Wednesday said that a mink farm there had also been infected and would be culled.
Stegeman said there’s no cause for concern about a cat reservoir yet, but he and colleagues are now studying cats owned by coronavirus patients anyway.
“It makes sense to do research on it now” and to stop the spread among mink, Stegeman said. Otherwise, he added, “Once we have very limited human-to-human transmission … you could get again a jump from an animal reservoir to humans. Things would start all over again.”