A study conducted by researchers at UC Berkeley and other institutions found that pathogens increase in severity when they pass through a bat’s immune system, leading to the spread of virulent pathogens, including coronavirus.
The faster a virus can replicate inside a host cell, the more virulent, or severe, its results become, according to study co-author Cara Brook, a postdoctoral fellow at UC Berkeley’s Miller Institute for Basic Research in Science. She added that since bats have a perpetually strong immune system, viruses can replicate quickly without killing the host. When these viruses are passed on to species with weaker immune systems, they are much more pathogenic, according to the study.
“A pathogen’s goal is to infect as many new hosts as possible. That’s how it reproduces,” Brook said. “But at the same time, there’s trade-off in that it typically causes damage to its host. It causes disease when it replicates faster.”
In evolutionary theory, the need for a virus to reproduce quickly without harming the host is referred to as the transmission-virulence trade-off. A bat’s immune system allows a virus to reproduce faster because the system’s defenses allow it to incur less harm from the virus.
One of the defenses of a bat is its release of interferon-alpha, an antiviral protein, according to a Berkeley News article on the study. An interferon defense protects the bat against the infection but allows it to last longer in the bat’s body. Bats are more resistant to oxidative damage, which also allows them to sustain a virus for a longer period of time than other species, according to Brook.
“Bats appear to be really resilient to oxidative damage,” Brook said, adding that their resilience helps mitigate the virulence caused by viruses that can replicate more quickly.
When viruses spill over to a species that does not have the same immune response as bats, it can cause a lot more damage, Brook added.
Brook hypothesizes that bat immune systems may be stronger because bats are the only mammals that fly, which is a metabolically demanding activity. Usually, mammals with such high metabolism live much shorter lifespans, according to Brook.
“They’re the only flying mammal, so we think they’ve evolved really efficient ways of mitigating that damage,” Brook said.
The study was conducted by infecting bat tissue with viruses and then tracking their spread over time by taking photos of the cells.
According to the Berkeley News article, two bat species were studied: the Egyptian fruit bat and the Australian black flying fox. The cells of these species were compared with monkey cells, which were used as a control group. This data was then fitted to a model.
Brook said the Mills Institute will continue to study evolutionary models.
“We’re still working on these topics,” Brook said. “I’m working on the evolutionary model right now and a follow-up on some of these findings.”