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Bats and Coronavirus

These bat-watchers in Austin, Texas likely do not suspect that all those bats are vectors of many viruses.  Congress Avenue Bridge at dusk.  Photo:  Peter Potrowl.
If previous outbreaks of coronavirus are any indication, the Wuhan strain that is now spreading may eventually be traced back to bats.  China, with over 1.4 billion people and 100+ species of bats in an area roughly the size of the continental USA (excluding Alaska) is emerging as a lethal threat to world health with new viruses emerging every few years.  

Imagine how crowded you'd feel if there were 1.1 billion additional Americans crammed into the lower 48.  Perhaps the bats of China feel the same way.  Nature has a way of evening out the odds for species survival despite modern human's ability to adapt to emerging threats.  So how do bats survive viruses?

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Red-flying Fox bats (Pteropus scapulatus)
Photo:  Bob Stefko.
The Cost of Flying:
How do Bats Survive Viruses?
Bats are considered the probable source of the coronavirus outbreak spreading from China. How do they survive?  Bats may have an immune system that lets them coexist with many disease-causing viruses.

Dr. Peter Daszak, president of EcoHealth Alliance, who has been working in China for 15 years studying diseases that jump from animals to people, said, “We don’t know the source yet, but there’s pretty strong evidence that this is a bat origin coronavirus.” He said, “It’s probably going to be the Chinese horseshoe bat,” a common species that weighs up to an ounce.

If he’s right, this strain will join many other viruses that bats carry. SARS and MERS epidemics were caused by bat coronaviruses, as was a highly destructive viral epidemic in pigs.

One bat can host many different viruses without getting sick. They are the natural reservoir for the Marburg virus, and Nipah and Hendra viruses, which have caused human disease and outbreaks in Africa, Malaysia, Bangladesh and Australia. They are thought to be the natural reservoir for the Ebola virus. They also carry the rabies virus, but in that case the bats are affected by the disease.

What Makes Bats the Perfect Hosts for Viruses?
Bats have been known to host up to 137 different kinds of viruses.
What makes bats the perfect hosts for so many viruses?
Their tolerance of viruses, which surpasses that of other mammals, is one of their many distinctive qualities. They are the only flying mammals, they devour disease-carrying insects by the ton, and they are essential in the pollination of many fruits, like bananas, avocados and mangoes. They are also an incredibly diverse group, making up about a quarter of all mammalian species.

But their ability to coexist with viruses that can spill over to other animals, in particular humans, can have devastating consequences when we eat them, trade them in livestock markets and invade their territory.

Learning how they carry and survive so many viruses has been a deep question for science, and new research suggests that the answer may be how the bats’ evolutionary adaptations to flight changed their immune systems.
Bats soaring across a dusk sky.
Photo:  Ann Froschauer/USFWS.

In Dampened STING-Dependent Interferon Activation in Bats, Xie et al., scientists in China and Singapore, reported their investigation of how bats handle something called DNA sensing. The energy demands of flight are so great that cells in the body break down and release bits of DNA that are then floating around where they shouldn’t be. 

Mammals, including bats, have ways to identify and respond to such bits of DNA, which might indicate an invasion of a disease-causing organism. But in bats, they found, evolution has weakened that system, which would normally cause inflammation as it fought the viruses.

Bats have lost some genes involved in that response, which makes sense because the inflammation itself can be very damaging to the body. They have a weakened response but it is still there. Thus, the researchers write, this weakened response may allow them to maintain a “balanced state of ‘effective response’ but not ‘over response’ against viruses.”
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