how can bats be hosts to a range of deadly viruses without getting sick?
In: Biology
https://royalsocietypublishing.org/doi/10.1098/rsif.2021.0211
Bats have big variations in their body temperature which impedes the reproduction of viruses and other pathogens
As with many diseases, bacteria and virus alike, they are adapted to a specific host. When that disease hops species boundaries, they are not adapted to that species and cause a slew of issues.
Many of humanity’s most deadly diseases are not human diseases, but are from other animals. This is because we are so closely intertwined with animals such as cows and sheep it makes it easier for things normal to them to infect us.
It’s mind blowing but the answer boils down to the fact that bats are not really well adapted for flight.
When you learn birds, you learn a lot of stuff they have to adapt for flight: special white muscle (that’s why chicken breast is different from steak), big breast muscles etc. Bats don’t have those.
In fact bats are so poorly adapted to flight so they have continuos muscle pain, same what you get after a thorough training. But continuously. And that is in fact a muscle inflammation. So to keep things in bay, bats evolved another kind of response to this inflammation: they evolved to tune down their immune system to turn off the inflammation.
Another mind blowing fact is that many virus nowadays are kinda harmless and the disease and the harmful symptoms are often coming from the immune system. Why is so then? Well, there are still bad viruses and the immune system cannot decide which one would be a peaceful virus and which one would be harmful. So there’s a first shoot then ask policy, and over our evolutionary history this might have been the good strategy. But as a sad side effect, many times it’s not the virus that’s deadly but our immune reaction. Unfortunately we do not un-evolve or tune down our immune system just to test whether nowadays viruses are safe. But you see, bats did because they needed.
And so immune system is a package deal, you cannot tune it down just for muscle inflammation. That’s why bats have less symptoms from viruses and so they peacefully co-exist. (Until they find a deadly virus where the virus itself is deadly because then they have no defense and die.) Okay they don’t have entirely no-defense but way more lazy than ours. And so that allows for viruses that are deadly in us (often because of our reaction), to thrive in bats.
Bats are very social animals – they live in huge colonies so it’s easy for viruses to spread between them. Over a very long time, evolution has selected for the adaptations that lets them tolerate these viruses. They evolved a much less destructive immune system than humans and other mammals. In humans, when we encounter a virus, our immune system is what actually causes us to feel so crummy. It’s *our body* that raises our temperature, makes our noses run, gives us a horrid cough, etc.
Bats have an immune system that doesn’t respond as strongly when it sees a virus, and when it does respond, it’s limited to just destroying the things it needs to. It doesn’t trigger such extreme inflammation as you see in humans, and what inflammation does happen, the bat has some other adaptations that helps limit it.
Humans have a similar ability but only in specific parts of our bodies – our eyes for example. When you get an eye infection, unless its very severe, you have a special immune response that keeps everything under much more control so as to avoid you going blind.
It’s not without it’s downsides though. Bats, while very good at tolerating intracellular pathogens like viruses, don’t seem to be as good when it comes to extracellular pathogens like bacteria and fungi. Very sadly, many populations of bats have completely collapsed due to a fungus *Pseudogymnoascus destructans* which causes white nose disease.
Their immune system also isn’t completely immune to viruses. Rabies, for example, is very lethal in bats.
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