When scientists published a letter in early August announcing a little-understood virus was infecting dozens of people in China, it echoed headlines from more than two-and-a-half years earlier.
A novel pathogen. People falling ill with fever, cough, fatigue. Both humans and animals getting infected.
Sound familiar?
In this case, the letter was about the arrival of the Langya henipavirus, and its likely links to animal populations, which might call to mind the early days of COVID-19 — or, as the virus behind it was known then, the "novel coronavirus."
"There is a bit of dj vu here," said Dr. Ari Bernstein, interim director of the Center for Climate, Health, and the Global Environment at the Harvard T.H. Chan School of Public Health.
"This is yet another example of a pathogen moving from an animal to a person and, as we know, that is the root cause of most of the emerging infections in the world."
But when it comes to newly discovered viruses, some cause more concern than others, and not all instances of animal-to-human transmission will spark massive outbreaks or trigger years-long pandemics. Many, however, are worth keeping an eye on in the scientific world — since more hosts means more opportunities for a virus to mutate, potentially unlocking new ways to infect, transmit, and spread.
And, as several scientists who spoke to CBC News agreed, the world needs to brace for more viruses like this spilling over into human populations in the years ahead, with Langya just the latest example.
"The fact that there is a virus that has made the jump into people, and that has evidence of exposure in other species, is already sufficient information for us to say we should be keeping a close eye on it," said Simon Anthony, a researcher and associate professor in the department of pathology, microbiology and immunology at the U.C. Davis School of Veterinary Medicine.
No transmission between humans reported yet
In the case of Langya, the virus was identified through surveillance of people with fevers and recent animal exposure in eastern China.
Scientists based in China published their findings in a letter in the New England Journal of Medicine in early August, noting that between April 2018 and August 2021, 35 patients infected with the virus were reported in China's Shandong and Henan provinces.
The vast majority of cases were farmers, who faced a range of symptoms, including fever, fatigue, cough, nausea, headaches, and vomiting.
But there are no reports of human-to-human transmission — at least not yet.
There was no close contact or common exposure history among the patients, "which suggests that the infection in the human population may be sporadic," the researchers wrote.
"Fortunately, in this case, it seems that this virus — at this time — is not readily transmissible between people," Bernstein said. "So I think we can take a breath here."
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As the world fights the latest phase of the COVID-19 pandemic, Canadian researchers are analyzing hundreds of bat specimens in order to identify new viruses and get a head start on future vaccines. Langya does appear to have multiple animal hosts, primarily shrews. That's still concerning, said Anthony, who studies emerging viruses and viral evolution.
"The more species that they affect, the more chance there is for adaptations to occur. You can't predict what those adaptations will do."
Ideally, any random mutations a virus develops as it bounces between species wouldn't cause harm to humans. Others could make the pathogen more contagious, or capable of evading the human immune system — as the world has seen through the evolution of SARS-CoV-2 — or even more lethal.
When it comes to Langya, the number of human cases right now isn't as concerning as the virus's apparent ability to spread to different species, Bernstein stressed.
"There's only a few dozen human cases," he said. "We don't know if this might get into some sort of domesticated animal facility and cause a lot of animals to get sick… we're playing roulette here."
More animal-to-human spillovers expected
The most likely scenario for Langya, Anthony said, is that the case count remains small, human transmission fizzles out, and it doesn't spark a COVID-level pandemic.
"But also, why be silly? Why ignore the information that we have in front of us?" he questioned. "And so at the very least, more research is needed to understand the potential risks that it poses."
Langya falls under the family of paramyxoviruses, a group of RNA-based viruses which are known for mutating fairly quickly.
There have been a handful of other documented zoonotic transmissions of paramyxoviruses to people in recent years, said Ryan Troyer, a virologist at Western University in London, Ont.
That's on top of more regular spillovers and outbreaks linked to Nipah virus, a rare, brain-damaging virus in the same family which has a death rate of up to 75 per cent.
"The others that happened before COVID-19 didn't get the same level of attention," Troyer said. "It's definitely disconcerting that this regularly occurs, but I suppose comforting that it almost never results in substantial spread."
Under the right conditions, various types of zoonotic viruses have exploded worldwide, from deadly avian flu that's so far staying primarily among bird populations, to the current global human outbreak of monkeypox that is spreading mainly through sexual networks.
Researchers expect more animal-to-human virus transmission in the years ahead, thanks to factors such as globalization, widespread development, and ongoing encroachment on animal habitats.
A Harvard Global Health Institute review of 40,000 species around the world also found roughly half were already on the move due to changing climate conditions.
Bernstein said it's critical to curb climate change and preserve ecosystems in order to prevent viruses from reaching humans in the first place.
"You prevent spillover, you protect habitats … it doesn't matter what the pathogen is, it's a uniformly effective approach," he said.