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Coronavirus: how bats evolved to become the perfect disease host
Experts believe a unique genetic quirk means bats can tolerate more viruses than other mammals
Bats may be linked to good fortune in traditional Chinese culture, but as the most likely source of the fast-moving coronavirus outbreak sweeping the country and globe they have triggered havoc, not prosperity.
Already some 28,000 people have been infected with the virus – known as 2019-nCoV – and at least 565 have died, while cruise ships have been quarantined, international borders closed and China’s economy left in flux.
Initially rumours swirled that the new disease came from seafood or snakes, but genetic sequencing has since found that bats are almost certainly the original host: 2019-nCoV was 96 per cent identical to a bat-borne virus found in Yunnan province in southern China.
For virus hunters like Dr Peter Daszak the research only confirmed existing suspicions.
Dr Daszak and his colleagues have spent years exploring vast networks of limestone caves that are home to millions of bats in southern China – one of 10 countries they work in. Wearing hazmat protective suits the teams have collected faeces, urine and blood samples or oral swabs from some 10,000 bats in their hunt for new pathogens.
“We have found some 500 coronaviruses in China now that come from bats,” said Dr Daszak, disease ecologist and president of research organisation EcoHealth Alliance. “But there are likely more – and as of yet we have no way to know which will become a human pandemic before a spillover event [takes place].”
It’s not just coronaviruses that jump from bats to humans – they have also been identified as the natural reservoir for deadly diseases including rabies and the Ebola and Marburg viruses.
This is perhaps unsurprising when you consider that bats make up roughly 20 per cent of all species of mammals – there are more than 1,200 types – and they have extraordinarily long lifespans, with some bats known to reach 40 years old.
What's more, they are found on every continent except Antarctica, their ability to fly means they can transmit viruses far and wide, and they live in dense, crowded colonies.
“So if you think about how they live their lives, you can understand why there are a lot of viruses circulating among bats,” said Jonathan Ball, professor of molecular virology at the University of Nottingham.
But experts believe that they have also evolved to tolerate more viruses than other mammals. According to a 2017 study in Nature analysing animal disease reservoirs, bats carry a “significantly higher proportion of zoonoses” (diseases that can transmit from animals to humans) than other mammals.
“In a sample of mice, you might see one per cent carrying viruses,” said Dr Daszak, a lead author of the report. “In the same sized sample of bats this figure would be more like five, ten per cent.”
It’s likely that this comes down to a unique genetic quirk. Bats are the only mammal capable of flying, putting their bodies under huge physiological stress.
There is a theory that, to cope, bats have suppressed their immune system – thus allowing them to tolerate more viruses.
“There is evidence that a branch of the immune system called innate immunity works slightly differently in bats,” said Prof Ball. “This is what we first produce when our bodies detect a viral infection – it’s a broad-acting signal within cells that sets up an antiviral state.”
If this early warning system takes longer to kick in then viruses can circulate undeterred by their natural defences for longer in bats than other mammals, Prof Ball added. But for the most part, these viruses do little damage to the flying mammals.
“These viruses have been in bats for millions of years and evolved with them,” said Dr Daszak. “They’ve become mild infections, a bat equivalent of the common cold – it’s not to a virus’s advantage to kill their host as then they have nowhere to go.”
But when these viruses spill over into humans they can have a devastating effect, as we have seen in the last month – although it is still not entirely clear how 2019-nCoV jumped from bats to people.
Experts believe that an intermediary animal was involved, which then passed the virus to a human at the Huanan seafood market in Wuhan, though some caution about jumping to conclusions.
“As scientists we need to keep an open mind,” said Diana Bell, professor of conservation biology at the University of East Anglia. “I will only be convinced that bats are the source once other animals have been as extensively studied.”
But whether bats are the original culprit or not, it is human behaviour that has accelerated the transfer of new pathogens from wild animals to people.
“Bats have a bad reputation, yet these viruses are not new in the mammals,” said Dr Daszak. “But around the world we’re encroaching on new areas constantly – we’re building more roads and mines, cutting down trees, hunting wildlife and trading them in livestock markets.
“At the same time our population is expanding and increased travel means diseases can spread across the globe quicker and quicker.
“We know with mathematical certainty that what we’re doing on the planet right now is driving an exponential rise in the frequency of spillover events,” Dr Daszak added. “And we are not doing enough to prevent a pandemic.”
Numerous reports over the years have said as much – including a paper published in the journal Viruses last year which warned that “China is a likely hotspot” for bat-borne coronaviruses to emerge.
“Therefore, the investigation of bat coronaviruses becomes an urgent issue,” the study said – a statement now more pertinent than ever.
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