SINGAPORE (BLOOMBERG) - Hundreds of thousands of people may be infected annually by animals carrying coronaviruses related to the one that causes Covid-19 each year in China and South-east Asia, according to a study emphasising the ongoing pandemic threat from spillover events.
An average of 400,000 such infections occur annually, most going unrecognised because they cause mild or no symptoms and are not easily transmitted between people, researchers of the EcoHealth Alliance and Singapore's Duke-NUS Medical School said in a study released on Thursday (Sept 9) before peer review and publication.
Still, each spillover represents an opportunity for viral adaptation that could lead to a Covid-like outbreak. The question of where and how the virus that causes Covid-19 emerged has become particularly contentious, with some leaders blaming a hypothetical leak from a lab in Wuhan, China that studies the pathogens.
The research, supported by the US National Institute of Allergy and Infectious Diseases, builds on evidence that bats are the main host-animals for viruses like Sars-CoV-2 and that people living near their roosts are especially vulnerable.
"This is probably the first attempt to estimate how often people are infected with Sars-related coronaviruses from bats," said Mr Edward Holmes, an evolutionary biologist at the University of Sydney who was not involved in the research.
Humans are continually exposed to bat coronaviruses, he said. "Given the right set of circumstances, one of these could eventually lead to a disease outbreak."
Almost two dozen bat species that can be infected by coronaviruses dwell in Asia, with southern China and parts of Myanmar, Laos, Vietnam and Indonesia deemed the riskiest for spillovers. Mr Peter Daszak and his colleagues at the New York-based EcoHealth Alliance used bat distribution modelling and ecological and epidemiological data to estimate the risk of exposure to Sars-related coronaviruses, and the rate of unreported bat-to-human infections in China, South Asia and South-east Asia.
The approach provides proof of concept for a systematic risk assessment of wildlife-to-human spillover events and a strategy to identify key geographic areas that can be prioritised for targeted surveillance of wildlife, livestock and humans, the researchers said.
"Given the challenges of identifying the origins of Covid-19 and pathways by which Sars-CoV-2 spilled over to people, this approach may also aid efforts to identify the geographic sites where spillover first occurred," they said in the study.
Almost two years since Covid-19 began infecting people in the central Chinese city of Wuhan, scientists have yet to determine the pandemic's genesis. Mr Daszak, who supports the theory of wildlife source, has been criticised for collaborating in National Institutes of Health-funded research at the Wuhan Institute of Virology. Some scientists say the lab studies in the research may have led to the creation of a progenitor virus.
No evidence supporting the lab-leak theory has emerged. Last month (August), the US intelligence community ruled out the possibility that Sars-CoV-2 was developed by China as a biological weapon, but no consensus has been reached on its origin.
Mr Daszak's study estimated that a median of 50,000 bat-to-human spillover events occur in South-east Asia annually and said the number could run into the millions. Animal intermediaries make the risk of exposure to animal viruses in nature "far, far greater than any possible exposure in a lab", Mr Holmes said. "And this is just bats. The risk of exposure is even higher when you factor in all the possible intermediate animal species."
These include minks, civets, raccoon dogs and other mammals commonly farmed and traded for food and fur in Asia, according to the research. It said 14 million people were employed in wildlife farming in China alone in 2016 - an industry worth US$77 billion (S$103 billion) annually.
In Asia, about 478 million people live in an area inhabited by coronavirus-carrying bats, covering most of Laos, Cambodia, Thailand, Vietnam, Nepal, Bhutan, peninsular Malaysia, Myanmar, south-east China, and the western islands of Indonesia, the study said.