Why bats are the perfect hosts for viruses

The common horseshoe bat, Rhinolophus affinis, is a feat of biological engineering. The size of a small pear, it spends most of the day hanging upside down in dark, dank caves. To keep itself warm it huddles in tight colonies, wrapping its wings — which can measure up to two inches — around itself like a blanket. Even so, in winter its body temperature can fall as low as 43°F (6°C).

Then, two hours before sunset, it emerges to forage for insects using a horseshoe-shaped sonar dish on its nose to find its bearings. But now it has a new problem: flying requires so much energy that its metabolic rate can spike as high as 34 times its resting level, and its core temperature can exceed 104°F, causing cells within its body to break down and release bits of DNA.

Ordinarily, these cell fragments would cause inflammation, but thanks to bats’ weakened immune systems, they don’t. Instead, R. affinis and other species of bats can tolerate a wide range of viruses that in other mammals would trigger aggressive and sometimes deadly immune responses.

That is good news for bats, but potentially very bad news for humans. Bats harbour thousands of viruses, including coronaviruses, any one of which, under the right ecological and epidemiological conditions, could be the spark for the next pandemic.

No one can yet be certain where Sars-CoV-2, the coronavirus that causes Covid-19, originated. But its nearest relative is a coronavirus isolated by Chinese scientists from an R. affinis bat that roosted in a cave in Yunnan, south-west China, in 2013. The virus, dubbed RaTG13, shares 96.2 per cent of its genome with Sars-CoV-2. Case closed, you might think.

However, the virus is missing a key gene that enables it to attach to cells in the human respiratory tract (interestingly, in the human virus, this section of the gene more closely resembles a virus found in pangolins smuggled to China from Malaysia).