Could an intensive livestock operation cause the coronavirus to mutate and fuel another pandemic? That’s the fear and debate now raging in Denmark.
Already Danes have killed more than two million farmed minks infected with COVID-19 since an outbreak began in October.
Last week the Danish government ordered its military and police to destroy up to 17 million more minks on 1,000 farms in the northern peninsula of Jutland after finding animals at a quarter of the operations had been infected.
But over the weekend, Prime Minister Mette Frederiksen reversed that decision, halting passage of emergency legislation authorizing the cull after intense opposition emerged from scientists, farmers and lawyers.
Now, little Denmark, which exports 40 per cent of the world’s farmed mink fur but suddenly is considering banning the industry, is the focus of intense global attention.
“We have a great responsibility towards our own population, but with the mutation that has now been found, we have an even greater responsibility for the rest of the world as well,” Frederiksen told a news conference on Nov. 5. She described the situation as “very serious.”
England has banned travellers from Denmark as a precaution.
Danish health authorities called for the mass killing after discovering as many as five different clusters of viral mutations in 214 Danish citizens who work or live near mink farms.
But the Danish government has conceded that it has no authority to order mink breeders to kill their animals unless “either infection has been found among the animals or that the mink farm is located in an area with disease outbreaks.”
Fear of ‘spillback’
The scientific debate about the risks posed by infected mink for vaccine development is not settled, but the outbreaks in Denmark highlight how quickly the virus may change when it jumps species.
According to preliminary studies by Denmark’s State Serum Institute, the nation’s public health agency, the new mink variants of COVID-19 show reduced sensitivity to antibodies in Danes who have already been infected by the virus.
“This is serious as it may mean that a future COVID-19 vaccine will be less effective against infection with these variants,” said the institute. It concluded that a “continued mink breeding would entail a significant risk of recurrence of a large spread of infection among mink and humans… and this would pose a major risk to public health.”
But the agency has released no genomic data (DNA sequences that track the virus’s evolution), and many scientists have raised questions about its conclusions because viruses naturally mutate all the time.
“So far there have not been any data demonstrating that COVID-19 caused by mink-derived variants is substantially different from human-derived variants,” tweeted U.S. virologist Angela Rasmussen at Columbia University. “‘Weak’ antibody responses are seen in mild or asymptomatic COVID-19 cases too.”
She added that without knowing “what those mutations are, it’s unclear what the implications are for vaccines.”
“The thing I’m most concerned about was not really addressed by the Danish authorities: spillback into a new reservoir species,” said Rasmussen.
By that she means the mink farm outbreaks raise concerns that even if the coronavirus can be successfully battled in humans, that victory might be short-lived if the virus resides, and mutates, in reservoirs of animal populations and then re-emerges in a different form to reinfect humans and trigger another pandemic.
The current pandemic is assumed to have begun when the SARS-CoV-2 novel virus spilled over from wild bats into another as yet unidentified mammal species or directly into humans sometime in the fall of 2019 in Wuhan, China.
Since then the scientists have learned the virus can infect a variety of animals including ferrets, cats, rabbits, dogs, tree shrews, hamsters and non-human primates such as macaques. And that infected cats, tree shrews, hamsters and ferrets can spread the virus to people.
Tries at infecting pigs and several poultry species with SARS-CoV-2 have so far proven unsuccessful.
But the mustelids, an expansive animal family, seem particularly susceptible to the virus and vulnerable to viral spillover. These carnivores include ferrets, badgers, raccoons, raccoon dogs, otters, martens, wolverines, mink and nearly 50 other species.
Researchers have been inoculating ferrets with influenza virus for decades, because the animals get just as sick as people. A ferret with the flu will lose weight, avoid food, sneeze, have a runny nose and even experience neurological complications. Recent studies on ferrets have also shown how COVID-19 spreads through droplets and aerosols in the air.
Outbreaks of COVID-19 have already occurred on mink farms in the Netherlands, Spain, Sweden and Italy; and in Utah and Wisconsin in the U.S.
In Utah, the virus killed thousands of minks in October much in the same way it has killed humans.
“It’s going through the breeding colonies and wiping out the older mink and leaving the younger mink unscathed,” said Dean Taylor, the state veterinarian.
Most of Canada’s mink farms are located in Nova Scotia where there have been no outbreaks to date. Nor have any been reported in B.C., which has about a dozen mink farms, most of them located in the Fraser Valley.
Minks seem to be highly susceptible to the virus because they contain a high number of ACE2 receptors, the part of a cell upon which the invading virus attaches itself in both humans and animals before converting the entire cell into a viral factory.
The proposed mink culling in Denmark highlights the hazards of intensive livestock operations because viruses typically mutate while jumping into other species as a form of adaptation.
Once established, a mutation can then spread rapidly as it explodes through genetically-uniform farmed populations and then spills back over into their human caretakers as well as wild animals or other nearby species, much the way avian flu has done in the past in Asia’s massive poultry farms.
A group of veterinarians wonders if COVID-19 outbreaks on mink farms could trigger a major panzootic (a global animal pandemic) in related wild animals around the world, thus creating new reservoirs in which the virus can spread back to humans again.
“If infection by SARS-CoV-2 spills into wild mustelids, these have the potential to become a permanent reservoir of infection for other animal species,” warned the researchers.
They added that “Several wild mustelids have become acclimated to urban areas — such as raccoons, otters and badgers, and some are raised in households as pets — such as ferrets.” Raccoon dogs are a major invasive species in much of Eastern Europe.
From China to Holland, a virus-vulnerable industry
China is a major promoter of intensive fur farming ranging from raccoon dogs to civet cats (they are also raised for meat) — all species susceptible to the virus. In the last decade the government has also encouraged poor rural dwellers to intensively farm wildlife (everything from peacocks to bears) as a means to get rich.
China is one of the world’s largest industrial mink farmers, producing 26 million pelts a year. The industry peaked in 2016 with the production of 60 million pelts.
Jonathan Epstein, a disease ecologist at the U.S.-based EcoHealth Alliance, told CBC’s The Current that it is conceivable the pandemic originated on an intensive fur farming operation in China.
Mink farming began 150 years ago in the United States and is a global industry that gases and then skins some 50 million animals a year for fur production, largely for Asian consumers.
After 100 generations of domestication (the equivalent of 2,000 human years of breeding), these animals are genetically uniform. Crowded conditions (as many as 7,000 minks in one building) combined with that uniformity makes ideal conditions for the rapid spread of a virus.
Earlier this year Dutch scientists documented the “first animal-to-human SARS-CoV-2 transmission events.”
That’s when mink farmers in the southern region of the Netherlands noticed an increase in mortality in their facilities. Many of the animals showed no symptoms, while others had runny noses. Those that died succumbed to pneumonia. Feral cats near the facilities also showed evidence of infection.
Upon further investigation, researchers at Wageningen University found the coronavirus clinging to inhalable dust particles in the air in two mink farms. They also documented that the virus was fully capable of bouncing from humans to mink and from mink to humans.
On two farms Dutch researchers tested farmworkers, their relatives and contacts and discovered more than two out of three of the group were infected with the virus. This indicated “the contact with SARS-CoV-2-infected mink represents a risk factor for contracting COVID-19,” they said.
Mutations similar to those now found on Danish farms occurred in the Netherlands, but authorities say the mink were destroyed and no cases spilled over into humans.
Another study on 16 infected mink farms in the Netherlands found that the virus spread among farms the same way it has spread around long-term facilities via the movement of temporary or part-time workers.
Both the Dutch and Danish outbreaks prove that the virus can take up long-time residence in farmed populations and thereby serve as a reservoir for persistent infection in humans and even spill over into wild populations.
“If that happened, then you have a reservoir in our local wildlife, and we could get reinfected before we even get a good-quality vaccine,” explained Dutch virologist Wim Van der Poel to the Guardian.
The Netherlands has ordered all mink farms to cease their operations by the end of this year as a public health precaution and because of concerns about animal welfare.
Disagreement over the threat level
Not all scientists believe the risk of COVID-19 spreading from industrial farms housing mustelids to free wild ranging species is likely at this time.
“Based on our current understanding of transmission pathways, the infection appears less likely to spread among animals with a solitary lifestyle (as occurs with many mustelids and felids) than among animals that live in social groups,” noted the U.S-based Association of Fish and Wildlife Agencies in a statement.
“There is no evidence at this time that such infections, if they occurred in individual wild mustelids, felids or canids, would necessarily be maintained in populations of these species, or that SARS-CoV-2 would cause significant disease if established in these species,” added the association.
Meanwhile, researchers aren’t just worried about farmed mink.
They predict that COVID-19, once confined to wild bat populations in China, could also spill over into one of 40 bat species in North and South America via the virus’s largest reservoir to date: millions of infected humans.
If SARS-CoV-2 became established in one or more North American bat species, it could create new reservoirs capable of causing ongoing human infections.
The researchers also warned “it could potentially evolve or recombine with endemic viruses to become more pathogenic or infectious to humans or other animals.”