An Ontario veterinarian’s January warning that COVID-19’s could jump back to animal populations has gained increasing attention recently.
More than 500,000 minks were ordered to be culled in early June from 13 infected fur companies in the Netherlands after the animals caught the novel coronavirus from human handlers and then passed it back to two handlers in the world’s first report of animal-to-human transmission since the pandemic began.
In his January blog post, Dr. Scott Weese, an infectious disease specialist at the Ontario Veterinary College, urged the public to assume animal populations could be infected with the virus. “We need to be proactive,” he wrote. “For containment measures for SARS, this new coronavirus or any other new disease, we need to assume that multiple species can be affected until proven otherwise and we need to act accordingly.”
The warning fell mostly on deaf ears. “What I got back was an ‘that’s interesting, thanks’ and they never really got back to it,” says Weese, “It was one of those things that is distant enough on the horizon to not pay much attention to.”
That initial lack of attention was in part due to a statement from the U.S. Centers for Disease Control that there was no evidence to support the idea that animals could play a role in transmission. However, as Weese points out: “You don’t know until you look.”
Undiscovered coronaviruses exist in many animal species. An undiscovered coronavirus reservoir in bat populations is widely considered to be the source of COVID-19, though it’s unclear whether transmission was direct or via a third-party species. Previous outbreaks – namely SARS (Severe Acute Respiratory Syndrome) and MERS (Middle East Respiratory Syndrome) – also began with major zoonotic jumps.
Zoonotic jumps happen in largely the same way human-to-human jumps happen, according to Dr. Sumon Chakrabarti, an infectious diseases specialist at Trillium Health Partners. The virus can be transmitted via droplets when humans are in close contact with animals.
However, human-to-animal-and-back transmission poses a unique risk as the virus can mutate in animal species. According to Chakrabarti: “There’s different pressures that are put on the virus itself (in an animal host). Occasionally a different type of mutation happens that makes the virus more transmissible and more virulent. This kind of thing doesn’t happen in humans but it does happen in, say, a bat.”
Dr. Craig Jenne, Canada Research Chair at Imaging Approaches Towards Studying Infection, says increased interactions between humans and animals cause viruses to mutate proteins to jump to new hosts.
An infection of, for example, 500,000 minks creates a reservoir of potential interactions that can become fatal when the virus mutates to jump back to human hosts. Since the beginning of the pandemic, there have been reported cases of human transmission to dogs, cats, tigers and lions, but the case in the Netherlands is the first report of transmission back to humans.
Weese says it was well known that SARS could infect cats and ferrets and that animals with SARS could infect other animals.
“What I’ve been saying for the past 15 years is that we got a little lucky with SARS because there wasn’t an animal component after that first outbreak. But we need to have a system in place so that when the next one hits, we have a way to address it,” says Weese.
Cats at one of the farms in the Netherlands also tested positive for the virus, echoing Weese’s concerns about the potential spread of COVID-19 in companion animal populations. Farm cats as well as indoor/outdoor cats pose a greater risk because of their exposure to wildlife. From there, the virus can become incredibly difficult to control.
“There’s enough wildlife around to move it around and to be a reservoir,” says Weese. “What we want to do is get rid of this virus and the more species you have carrying it, the harder it is to do.”
Of the seven cats that tested positive at the farm, only one had traces of the actual virus and all 24 on the farm have been quarantined.
Weese recommends treating cross-species interactions with pets and farm animals the same way we treat human-to-human interactions. “If your cat gets infected from you, and your cat stays in the house, it doesn’t infect anyone else. They’ll burn the virus off with you.”
As long as the infected animal is contained, the worst-case scenario, according to Chakrabarti, would be for the virus to “ping-pong” around a house.
However, it is more complicated with livestock.
“If you have a farm with a thousand pigs, you want to make sure nobody sick is handling those pigs,” says Weese. “It’s easier to prevent a problem rather than deal with people freaking out about food safety – which isn’t really a concern – and we have to figure out what to do with the manure and what to do with the pigs. Anyone going in the barn has to worry about being exposed.”
Researchers at University College London are calling for widespread surveillance of animal populations, citing similar reasoning to Weese: that there is no consensus yet on human-animal-human transmission and that the cost of taking extra precautions is far outweighed by the risk of another major zoonotic jump.
“Once SARS-CoV-2 circulates more widely beyond humans,” they write, “it will be challenging to trace natural transmission between species because the viral genome is essentially identical in humans and existing epidemiological methods of contact tracing are equipped to identify transmission between humans to interrupt it.”
Weese is working with Dr. Dorothee Bienzle and the University of Guelph Department of Pathobiology on a human-animal transmission study.
“We already know it can happen; we’ve seen it in various instances. Now we’re trying to find how common it is,” Weese stated in a release about the study.
“There are still many things we want to know about the virus and the interaction between humans and pets. But we need to have an idea of how often human-to-pet transmission occurs first. This study will lay the groundwork for that future research.”
The main takeaway from the Netherlands case, according to Chakrabarti, is more evidence of the potential for animals to become infected. “Theoretically, in areas where large groups of animals are living close to humans, it could be another source of back-and-forth mutation.”
However, adds Chakrabarti, “Just because we found out that minks can get (COVID-19) and potentially spread it doesn’t mean the next coming of a new outbreak or that we’re going to have a new, more powerful virus.”