It's no coincidence that some of the worst viral disease outbreaks in recent years -- SARS, MERS, Ebola, Marburg and likely the newly arrived 2019-nCoV virus -- originated in bats.
A new University of California, Berkeley, study finds that bats' fierce immune response to viruses could drive viruses to replicate faster, so that when they jump to mammals with average immune systems, such as humans, the viruses wreak deadly havoc.
Some bats -- including those known to be the original source of human infections -- have been shown to host immune systems that are perpetually primed to mount defenses against viruses. Viral infection in these bats leads to a swift response that walls the virus out of cells. While this may protect the bats from getting infected with high viral loads, it encourages these viruses to reproduce more quickly within a host before a defense can be mounted.
This makes bats a unique reservoir of rapidly reproducing and highly transmissible viruses. While the bats can tolerate viruses like these, when these bat viruses then move into animals that lack a fast-response immune system, the viruses quickly overwhelm their new hosts, leading to high fatality rates.
Some bats are able to mount this robust antiviral response, but also balance it with an anti-inflammation response. Our immune system would generate widespread inflammation if attempting this same antiviral strategy. But bats appear uniquely suited to avoiding the threat of immunopathology." Cara Brook, postdoctoral Miller Fellow at UC Berkeley and the first author of the study
The researchers note that disrupting bat habitat appears to stress the animals and makes them shed even more virus in their saliva, urine and feces that can infect other animals.
"Heightened environmental threats to bats may add to the threat of zoonosis," said Brook, who works with a bat monitoring program funded by DARPA (the U.S. Defense Advanced Research Projects Agency) that is currently underway in Madagascar, Bangladesh, Ghana and Australia. The project, Bat One Health, explores the link between loss of bat habitat and the spillover of bat viruses into other animals and humans.
"The bottom line is that bats are potentially special when it comes to hosting viruses," said Mike Boots, a disease ecologist and UC Berkeley professor of integrative biology. "It is not random that a lot of these viruses are coming from bats. Bats are not even that closely related to us, so we would not expect them to host many human viruses. But this work demonstrates how bat immune systems could drive the virulence that overcomes this."
The new study by Brook, Boots and their colleagues was published this month in the journal eLife .
Boots and UC Berkeley colleague Wayne Getz are among 23 Chinese and American co-authors of a paper published last week in the journal EcoHealth that argues for better collaboration between U.S. and Chinese scientists who are focused on disease ecology and emerging infections. Vigorous flight leads to longer lifespan - and perhaps viral tolerance
As the only flying mammal, bats elevate their metabolic rates in flight to a level that doubles that achieved by similarly sized rodents when running.
Generally, vigorous physical activity and high metabolic rates lead to higher tissue damage due to an accumulation of reactive molecules, primarily free radicals. But to enable flight, bats seem to have developed physiological mechanisms to efficiently mop up these destructive molecules.
This has the side benefit of efficiently mopping up damaging molecules produced by inflammation of any cause, which may explain bats' uniquely long lifespans. Smaller animals with faster heart rates and metabolism typically have shorter lifespans than larger animals with slower heartbeats and slower metabolism, presumably because high metabolism leads to more destructive free radicals. But bats are unique in having far longer lifespans than other mammals of the same size: Some bats can live 40 years, whereas a rodent of the same size may live two years. Related Stories
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