Bats are responsible for Deadly Viruses - Seeker's Thoughts

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Bats are responsible for Deadly Viruses

Ebola, Nipah, Marburg, SARS, MERS and now the new Coronavirus Covid – 19, all share one thing in common – they are thought to have originated in bats. 


A new study, led by scientists at the University of California Berkeley, is suggesting the mammals’ uniquely fierce immune system encourages viruses to reproduce and when the viruses cross over into other animals or humans they can be incredibly fatal.


There are more than 1,400 individual species of bat, spanning almost every corner of the world and comprising around 20% of all mammals’ species. 

Bats have very few natural predators and live extraordinarily long in relation to their size. Some bats have been found to live up to 40 years.
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.
However, bats are outnumbered by rodents in terms of volume of species and sheer number, and while rats certainly spread number of diseases, they are not generally known for incubating entirely new viruses (rats may have traditionally been blamed for the black plague in medieval Europe by research has shown the real cause of that infamous epidemic was parasites such as fleas and tick, not rats that carried them).


What is it about bats that allow them to harbor such virulent viruses without actually getting sick?
According to the disease ecologist and co-author of the new study, Mike Boots, the bottom line is that bats are potentially special when it comes to hosting viruses. And 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 the, to host many human viruses. But this work demonstrated how to bat immune systems could drive the virulence that overcomes this.
In order to effectively evolved and spread, a virus can’t kill its host too quickly. The faster the virus replicates and infects a host, the quicker its host will die, so the most effective viruses are the one than can maintain that precarious balance.
To understand how viruses can evolve in the presence of different mammal immune systems, the new study exposed two different bat cell lines to hemorrhagic fever virus. A cell line from African green money was also exposed as a control.

                         
The Australian black flying fox cell line demonstrated the most effective immune response to the virus, rapidly producing molecules called interferon-alpha. These immune signaling molecules are released by cells when they are under attack from a foreign substance. They signal to other cells to heighten anti-viral defenses, and actively disrupt viral replication.
What the researchers observed was a distinct slowing of viral replication in the bat cell lines. However, these particular bat interferon responses also allowed viral infections to persist in the mammals for extended periods of time.
"Think of viruses on a cell monolayer like a fire burning through a forest. Some of the communities – cells – have emergency blankets, and the fire washes through without harming them, but at the end of the day you still have smoldering coals in the system – there are still some viral cells," explains Cara Brook, first author on the new study.
This means a virus can increase its replication rate inside a bat without killing its host, essentially enhancing its virulence to a level that would be profoundly destructive in other organisms.
“This suggests that having a really robust interferon system would help these viruses persist within the host,". "When you have a higher immune response, you get these cells that are protected from infection, so the virus can actually ramp up its replication rate without causing damage to its host. But when it spills over into something like a human, we don't have those same sorts of antiviral mechanisms, and we could experience a lot of pathologies."
Why do bats have such a fundamentally powerful immune system?
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 swift response that walls the virus out of cells. While this may protect that 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 them 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. According to the researchers, “Our immune system would generate widespread inflammation if attempting this same antiviral strategy. But bats appear uniquely suited to avoiding the threat of immunopathology.”
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," who works with a bat monitoring program funded by DARPA (the U.S. Defence 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 spill over of bat viruses into other animals and humans.




"The bottom line is that bats are potentially special when it comes to hosting viruses, 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 to bat immune systems could drive the virulence that overcomes this."
The new study by Brook, Boots and their colleagues was published in the journal eLife. In February 2020.
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 the U.S. and Chinese scientists who are focused on disease ecology and emerging infections.

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