Are bats a danger to humans?

Why are bat viruses so dangerous?

Coronavirus, SARS or Ebola - some particularly problematic viruses originally developed in bats. But why in these animals of all places? A study provides an explanation: According to this, bats breed sharp viruses, as it were. They have an immune system that keeps the pathogens in check - and in return, the viruses arm themselves. With these aggravated pathogens, the comparatively weak immune defense of humans is then rather overwhelmed, so the explanation.

It is not unusual for viruses to sometimes cross species boundaries: most of the newly emerging viral diseases in humans originally come from animals. In the case of influenza, birds, pigs or rodents are often the source of pathogens. But studies show that many particularly dangerous viruses such as Ebola, Marburg, Sars or the new coronavirus Sars-CoV-2 originally developed in bats as reservoir hosts. Through contact between humans and bats or through another animal as an intermediate carrier, these pathogens managed to penetrate humans.

Infection test on bat cells

Researchers working with Cara Brook from the University of California at Berkeley have investigated why the bat viruses are so particularly aggressive and reproductive. "Bats can harbor viruses that are highly virulent to non-flying mammals for a long time without showing any obvious symptoms of the disease," says Brook. What makes the animals so resistant and how this could influence the aggressiveness of the pathogens, she and her colleagues have investigated using cell cultures of the black flying fox (Pteropus alecto) and the Egyptian bat (Rousettus aegyptiacus). Both are known to be natural reservoirs for aggressive viruses. In the experiment, the researchers infected the cell cultures and control cultures from green monkey cells with various Marburg and Ebola-like viruses.

It turned out that the viruses grabbed the monkey cells within a few days. In the bat cell cultures, however, this was not the case: With them, the progression of the infection slowed down significantly. Although many cells were infected, some seemed to be able to defend themselves successfully against the attack: They remained healthy and not infected even after several days. “It's like a fire burning through a forest,” explains Brook. “Some trees - here bat cells - have protective covers so that the fire rushes by without harming them.” At the same time, there are still glowing coals lying around in the forest, from which a new fire can start at any time. In cell culture, this corresponds to bat cells that continue to carry active viruses.

Sharp defense breeds "turbo viruses"

A model of the bat's immune system, with which the researchers simulated the infection, then made it clear what the bat cells' protective strategy is based on: When they first come into contact with the virus, the entire system is flooded with the messenger substance interferon-alpha. This activates the cellular defense and causes the cells to seal themselves off from the pathogen. At the same time, the messenger substance prevents an excessive inflammatory reaction and thus dampens the symptoms of the disease. “If our immune system tried the same antiviral strategy, it would trigger system-wide inflammation,” explains Brook. Because we humans and most other mammals lack the strong interferon alpha release and the strong anti-inflammatory immune response. Ultimately, the bats' immune system is upregulated and accordingly acts effectively against pathogens, the researchers sum up.

The results explain why bat viruses in particular can be so dangerous for us. “If you have a strong immune response and some cells are protected from infection, then the virus can upregulate its reproduction without dying its host,” explains Brook. This makes the viruses more pathogenic, but still maintains their reservoir hosts. “Our study thus demonstrates how the immune system of bats can boost the virulence of such pathogens,” says Brook. If such a virus spreads to humans or other mammals, it can develop a threatening potential. Because their far weaker immune system can then do little to counteract this “turbo virus”.

Source: University of California - Berkeley, technical article: eLife, doi: 10.7554 / eLife.48401

March 27, 2020

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