Bat Diseases May Be the Most Deadly to Humans

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We know that diseases can occasionally jump from species to species, including from other species to humans, but scientists have found why diseases from bats are more deadly to humans than any other.
The researchers also figured out that the reason for bat diseases being so deadly is largely due to them having evolved how to fly, according to a new study in the journal PLOS Biology.
“Viruses derived from bat hosts result in higher case-fatality rates following spillover to humans than do zoonotic viruses derived from any other mammal or bird host. These include Ebola and Marburg filoviruses, Hendra and Nipah henipaviruses, and SARS and MERS coronaviruses,” Cara Brook, an assistant professor of ecology and evolution at the University of Chicago and lead author of the paper, told Newsweek.
Stock image of a common pipistrelle bat. Viruses from bats are likely to be more deadly to humans than viruses from other animals due to bats’ resistance to inflammation. ISTOCK / GETTY IMAGES PLUS
The paper reveals that the reasons for this are because of bats’ innate tolerance of inflammation, which evolved in tandem with their ability to fly.
“We use a mathematical model, supported by empirical data, to provide a mechanism for this pattern: essentially, in this model, we demonstrate how a virus can be expected to optimize its within-host virus growth rate by balancing the gains in transmission that result from high virus growth rates against the elevated virulence (disease incurred on the host) that these growth rates incur,” Brook explained.
“We demonstrate mathematically how a few key features of bat physiology and life history that are thought, in part from the evolution of flight—chiefly tolerance of immunopathology/inflammation and robust constitutive immune systems—should select for high growth rate viruses that can achieve gains in transmission without causing extreme virulence to bat hosts.”
Stock image of a wildlife biologist checking the wings of a bat for signs of white-nose syndrome. ISTOCK / GETTY IMAGES PLUS
Therefore, due to bats’ resistance to inflammation, they are also tolerant of the immune responses of their bodies when they are infected by more virulent viruses with faster growth rates.
The researchers used their model to “spillover,” or infect, bat-optimized viruses into a hypothetical human immune system, and showed how much more deadly they could be compared to viruses from other mammals.
“We then allow these viruses to demonstrate how high virus growth rates optimized on an animal reservoir could result in significant pathology in a different (e.g. human) immune system,” Brook explained.
“The study is not just limited to bats—we use the life history traits of 19 different mammalian orders to ‘predict’ optimal virus growth rates and the ensuing zoonotic virulence following spillover to humans for a virus evolved on these diverse reservoir hosts.”
These findings don’t necessarily mean that a bat virus will wipe out the human species one day, however.
“With regard to dangers posed by bat viruses, it’s important to remember that transmission and virulence tradeoff, so viruses that cause the highest case fatality rates are not likely to result in the highest number of human infections, or, therefore, the highest burden of human mortality.”
Stock illustration of Ebola. ISTOCK / GETTY IMAGES PLUS
Viruses therefore need to find a balance between keeping their host alive long enough to spread to further hosts. One example of this is seen in Ebola, a highly deadly virus with a fatality rate of around 50 percent, which often kills its host before they have a chance to transmit.
“Thus, while our paper suggests that bats—and a few other mammalian orders—are likely to continue to host and source viruses with high growth rates that could be virulent following spillover to humans, these viruses are not predicted to be the most transmissible, either across species (from bat to human) or within the human population following the spillover event,” Brook said.
“Additionally, there’s no evidence or mechanism we’ve yet discovered to suggest that bat viruses might be evolving higher virulence or would do so in the future. Indeed, one of the best ways to prevent spillover would be to undertake conservation projects to restore wild bat habitats and reduce bat-human contact rates in the future.”
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