“Western cultures have generally regarded bats with superstition and fear; but in China, expressed in art and handicrafts, the bat has achieved respectability as a symbol of happiness and good luck. Too often, popular misconceptions have labeled bats as “dirty,” “disease carriers,” or “blood suckers,” an unenviable—and unjust—reputation to be sure. In reality, the more than 1,300 bat species are vitally important to ecosystems and economies around the world: they perform pest control, they pollinate, and they disperse seeds.”

There is more to bats—bats are the only flying mammals and live very long lives. They do host viruses, though, and some of these viruses are extremely harmful when they infect humans and other animals, as for example Ebola virus, Nipah virus, severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS) coronaviruses. But how can bats carry so many viruses without suffering from the diseases that these pathogens cause in humans?

Results from a new study (Dampened NLRP3-mediated inflammation in bats and implications for a special viral reservoir host) published in the journal Nature Microbiology (February 25, 2019) show that bats are able to limit inflammation. In humans, when properly controlled, the inflammatory response helps fight infection. However, a poorly controlled inflammatory response may contribute to the damage caused by infectious diseases, as well as to aging and age-related diseases. Bats do not react to infection with the typical inflammatory response that often leads to damage.

For the study, researchers compared the responses of immune cells from bats, mice and humans to three different RNA viruses—influenza A virus, MERS coronavirus, and Melaka virus. They found that, when compared to mice and humans, inflammation mediated by the protein NLRP3 (NLR family pyrin domain containing 3) is significantly reduced in bats, even in presence of high viral loads. NLRP3 is the inflammation sensor that normally triggers the body’s response to fight off stress and infection, and has been linked to both viral-induced and age-related inflammation.

More specifically, the researchers found that “transcriptional priming”—a key step necessary for the up-regulation of NLRP3 protein—is reduced in bats when compared to mice and humans. They also found unique, less active variants of NLRP3 only present in bats. These variants are present in two different bat species—Pteropus alecto, a large fruit bat known as the Black Flying Fox, and Myotis davadii, a tiny vesper bat from China. Thus, the variants seem to have been genetically conserved through evolution. Comparison of NLRP3 gene sequences from 10 bat and 17 non-bat mammals confirmed that these variants appear to be bat-specific.

The researchers explain that, in bats, the reduction of inflammation appears to have no impact on the overall viral loads. Rather than being better able to fight infection, bats have a much higher tolerance for it—the dampening of the inflammatory response actually enables them to survive.

Wang Lin-Fa, senior author of the study, said in a press release: “Bats appear to be capable of limiting excessive or inappropriate virus-induced inflammation, which often leads to severe diseases in other infected animals and people. Our finding may provide lessons for controlling human infectious diseases by shifting the focus from the traditional specific anti-pathogen approach to the broader anti-disease approach successfully adopted by bats.”