I mean from what I’ve read, they’re basically the only carrier of ebola, they can carry rabies, there’s the COVID one obviously, a whole bunch of parasites, I think they carry nipah virus, and the list goes on and on.
How do they not die from all the diseases they carry, and why are they able to carry so many?
Comments
Bats are old. Like evolutionarily old. And they reproduce often. Thousands of generations of bats.
They become sick, a few are immune, and their offspring are also immune. They repopulate and now they can carry disease and not get sick.
Over and over and over until they’ve become immune to most everything.
It’s also why rats are big disease carriers.
There are a few reasons.
The first is that they have unusually robust immune systems, which biologists believe evolved as part of their adaptation to combating the biological stress of flying. Having robust immune systems mean they can host a lot of pathogens without actually getting sick themselves. In other words, their immune systems can keep the pathogens from making them sick while the pathogens just exist in their bodies.
They also have surprisingly long lifespans, which, combined with their strong immune systems, means pathogens can live in them and mutate over long periods of time into various new strains.
They also live basically on top of one another in massive colonies of up to hundreds of thousands, which is just a breeding ground for pathogens to spread from one bat to another. I mean, imagine 100,000 humans crammed into a cave for a week and 1000 of them have the flu. By the end of the week, everyone in that cave will have the flu.
They’re also highly mobile, with colonies flying long distances and encountering other colonies (and thereby exchanging pathogens).
All of the above factors are what make bats (sometime cute) but absolutely nasty petri dishes brimming with all kinds of diseases.
I actually have a partial answer. Most bats have constantly active interferon responses which makes them resistant to viruses but they also roost together in cramped quarters so they end up being petri dishes that don’t die from the viruses they transmit. The unique bat response to viral infection typically drives the viruses to be more virulent through evolutionary pressures.
Due to the way bats fly, by beating their wings pretty hard, their body temperature has evolved pretty high due to the inflammation this causes. Inflammation is the body’s main response when affected by a pathogen, that’s how we smoke em out. Since a bat is basically in a constant state of ‘fever’, they can acquire lots of diseases, coronaviruses being a big one, but not really be affected by them, since their immune system is naturally already protecting them. Since these diseases don’t affect/kill them, aka, prevent bats from living and reproducing, they’re more common in bats.
Why they carry so many: bats like to stay/roost in huge numbers, so they can spread a lot of disease amongst themselves. They’re also old species-wise, so they’ve had time to mingle with viruses.
As to why they don’t die: they have VERY good immune systems. This is partly related to the fact that they are the only mammal that flies. Flying is really hard, takes a lot of energy, causes inflammation/physical stress, and needs a high metabolism. Bats evolved to deal with these drawbacks, and it turns out they make super-immune systems.
So there’s 5 comments and 5 wildly different answers…
There are thousands of different viruses that cause colds in humans. We fight off the majority of them with little disruption to our lives.
Same deal with bats. They encounter lots of viruses that don’t do much to them.
But bats are really different from humans, so viruses can be benign in bats and deadly in humans, or vice versa.
They unfortunately have the perfect combination of unique traits to generate pathogens that can spread to us/our livestock. I’m not an expert, but here’s the ones I do know:
They’re mammals, making them more physiologically similar to us. Pathogens that infect them become suited to survive in the mammalian body specifically.
They fly, an energy-intensive process that makes their body temperature very high. This creates selective pressure – pathogens that can’t take the heat inside their body die, pathogens that can survive and reproduce. After a while, you have a strain of pathogens that can handle heat very well, enough that a human fever can’t wipe them out.
They’re very social, and often live in packed-together groups numbering hundreds to thousands. These are perfect conditions for pathogens to spread and mutate to become stronger.
Due to points 2 and 3, they have extremely hardy immune systems.
They live quite a long time for animals their size, again, giving the pathogens more time to evolve.
Taken altogether, they’re basically the perfect breeding grounds for pathogens that can cause us a hard time, through no fault of their own.
To be clear, it’s not that those pathogens are intentionally mutating to be able to infect humans, but rather that any germ that can survive and reproduce inside a bat can easily survive and reproduce inside a human. Once one of those battle-hardened pathogens make the jump to us, it’s free real estate.
>Why are they able to carry so many?
It’s the utility belts.
Their immune systems are ‘switched on’ all the time, so to speak. It has to do with the presence of interferon (chemical signal which incites an immune response), which is different for them. In humans, an overactive immune system would cause excessive inflammation; bats’ ability to fly is thought to suppress the inflammation this would cause. It is not well understood. The viruses become adapted to their reservoir host, so they are very destructive when they get into hosts with less powerful immune systems.
What bats cannot carry is rabies. By carry, I mean they cannot maintain a symptomless infection. They may be able to live longer with the infection than other mammals, but rabies is neurotropic and encephalitic in them as well, and they eventually die. This brings me to the question of whether rabies actually has a true reservoir host, but that’s another matter.
I might not be able to fight a gorilla, but I can carry more diseases than a bat.
Bat biologist here. I just want to add one more fact that hasn’t been mentioned that’s more related to the one health problem:
Deforestation means that a lot of these viruses that are not meant to leave the bat ecosystem gets crossed over to humans and other animals, thus posing a huge zoonotic/spillover risk.
Oh and another fact: yes bats have super immune system but only against viruses.
They commonly die from bacterial and fungal infections. You can look up white nose disease.
Happy to answer anything else that I could from my expertise!