Chinese researchers have found a series of new coronaviruses in bats, including one that's the second-closest genetically to Covid-19.
The discoveries were made in a small region of the Yunnan province in China's southwest, and show just how many coronaviruses – defined as a "large family of viruses that usually cause mild to moderate upper-respiratory tract illnesses, like the common cold" – there are in bats and how many have the potential to spread to people, researchers said.
Three new coronaviruses have emerged from animal reservoirs over the past two decades and caused widespread illness and death: severe acute respiratory syndrome (SARS) in 2002; Middle East respiratory syndrome (MERS) in 2012 and, of course, SARS-CoV-2 – better known as Covid-19, which emerged in China in December 2019.
The University of Shandong's Weifeng Shi and colleagues collected samples from small, forest-dwelling bats between May 2019 and last November, testing the animals' urine and faeces as well as taking swabs from their mouths.
"In total, we assembled 24 novel coronavirus genomes from different bat species, including four SARS-CoV-2 like coronaviruses," the researchers wrote in the report, released on Thursday, in the journal Cell.
One of the 24 – viral sample RpYNO6 – was the closest strain yet to Covid-19, although it had genetic differences on the spike protein, the knob-like structure that the virus uses when attaching to cells, the researchers said.
"Together with the SARS-CoV-2 related virus collected from Thailand in June 2020, these results clearly demonstrate that viruses closely related to SARS-CoV-2 continue to circulate in bat populations, and in some regions might occur at a relatively high frequency," they wrote.
While a bat is still the likeliest source of Covid-19, it's possible the virus infected an intermediary animal, a World Health Organisation (WHO) investigation in Wuhan earlier this year concluded.
Dr Peter Ben Embarek, the WHO's food safety and animal disease specialist and chairman of the investigation team, told reporters at the time an intermediary host species is "the most likely pathway [for crossover into humans] and one that will require more studies and more specific targeted research".
The panel said it was likely other animals may serve as reservoirs, including those from the "feline family", given the high susceptibility of minks and cats to the virus.
The SARS virus that caused an outbreak between 2002 and 2004, for instance, was tracked to an animal called a civet cat.
"Bats are well known reservoir hosts for a variety of viruses that cause severe diseases in humans and have been associated with the spillovers of Hendra virus, Marburg virus, Ebola virus and, most notably, coronaviruses," the University of Shandong researchers wrote.
"Aside from bats and humans, coronaviruses can infect a wide range of domestic and wild animals, including pigs, cattle, mice, cats, dogs, chickens, deer and hedgehogs."
The most recent coronavirus samples all came from horseshoe bats.
Speculation over the origins of the Covid-19 pandemic has mounted in recent months after the Biden administration ordered US intelligence officials to investigate the possibility the virus leaked from a research laboratory in Wuhan.
Republicans, including former President Donald Trump, have promoted the theory that the virus emerged from a laboratory accident rather than naturally through human contact with an infected animal in Wuhan.
Biden said the majority of the intelligence community had "coalesced" around those two scenarios but "do not believe there is sufficient information to assess one to be more likely than the other".