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Thousands of minks are dying of Covid-19 in the United States. Scientists say the mass mink casualties can be attributed to the animals’ natural susceptibility to the virus and the large number of minks kept in close proximity on farms.
J. Scott Weese, director of the Centre for Public Health and Zoonoses at Ontario Veterinary College, said viruses typically affect certain species more than others. Some animals, like cows and pigs, seem resistant to Covid-19. Others — including humans, cats and minks — seem to be more vulnerable.
These differences can be explained in part by how the virus attacks cells, said Weese. SARS-CoV-2, the virus that causes Covid-19, attaches to ACE2 receptors on cells. These receptors are found in all animals, but their form varies somewhat between species. Some animals seem to have receptors that are easier for the virus to attach to, he said.
Weese said animals have more ACE2 receptors in their lungs than in other parts of their body. This is why Covid-19 is primarily a respiratory disease.
The mink farms provide a “perfect storm” for the virus because minks are vulnerable to Covid-19, and thousands are kept in close proximity, said Weese. Even though only a small percentage of minks might die from Covid-19, the sheer number of cases on farms leads to a lot of deaths.
If 10,000 cats were raised in a colony, there would likely be a lot of sick cats and a reasonable number of dead ones, he said.
“It comes down to the susceptibility of the species and the chance of the virus to spread around,” he said.
That interpretation was shared by Wim van der Poel, the lead researcher on emerging and zoonotic viruses at Wageningen Bioveterinary Research in the Netherlands. His team published a paper in June about Covid-19 infections in minks.
“Our impression is that farms are highly at risk due to the large numbers of susceptible animals in the farms,” he said.
He said his research suggests minks do not always develop clinical symptoms, and their fatality rate is less than 5%. But in some cases, the animals can develop severe interstitial pneumonia.
In some farms in the Netherlands, the virus seems to have circulated in the farm for quite some time before being detected, van der Poel said. Farm workers on all mink farms are now advised to use personal protection gear.
It is possible that the specific virus killing the minks would be less suited to attacking humans.
Kaitlin Sawatzki, a researcher studying Covid-19 in animals at the Cummings School of Veterinary Medicine, said research she is working on suggests there are two viral mutations of Covid-19 that are advantageous to infection in minks and ferrets but not humans. Her paper is currently a pre-print, meaning it has not been peer reviewed.
“Viruses in these animals are pushed to acquire specific mutations, or they're outcompeted and essentially die off,” she said. “This is not surprising and in line with what we expect for zoonotic viruses.”
The ACE2 receptors could be one of the differences that the virus is adapting to, she said.
“One small silver lining is that most animals seem to have little to no disease themselves and so far, mink are the exception and not the rule,” said Sawatzki.
Although there is evidence that different ACE2 receptors influence vulnerability to Covid-19, scientists aren’t able to predict with certainty how different species will respond.
Angela Bosco-Lauth, a biomedical researcher at Colorado State University who has studied Covid-19 in cats and dogs, said she finds the mink situation confusing since ferrets, which are close relatives to minks, seem to be less susceptible. She said scientists don’t yet have all the answers for why some species get sicker than others.
Pigs have ACE2 receptors that seem structurally good for Covid-19, but pigs don’t seem to be vulnerable in the real world, said Weese. He said scientists were worried about outbreaks in pigs but it doesn’t seem like that will be a problem.
