Submitted by JustJustinInTime t3_118pnjj in askscience
I was reading about how bats can carry a lot of diseases that are transferrable to humans and it made me wonder why. I was curious in general what features made a species a good disease vector relative to another species, especially when thinking of non-pest animals, e.g. armadillos with Leprosy, as opposed to animals like fleas that were able to transfer the Bubonic Plague because they were on rats that were near humans. Thanks!
It depends on what species of bat you're referring to but as a blanket statement, it seems bats have the ability to limit certain responses to diseases like inflammation. It's not that bats are immune to viruses or anything, they simply have a much higher tolerance for diseases. A lot of the damage caused by illnesses is our body's reaction and how it fights it off. Things like fever which is designed to kill off infections also significantly disrupts the body and in some cases can cause as much if not more damage than the actual infection or virus.
Bats have the ability to control their body's response to diseases much more than other animals. This may also explain their relatively long lifespans. So they can act normally despite carrying a viral load that would make other animals like humans go nuts trying to get rid of it.
[removed]
Makes me think the fact that many bats eat mosquitoes would expose them to many diseases from a variaty of other animals. Also, bats are often communal, so pathogens that can spread among the bats are selected for.
Insectivoir bats can't eat if they are weak so the bats' immune response has evolved to best handle frequent outbreaks of all sorts.
[removed]
Bats are a species that live in extremely close quarters. Say a viral disease is using them as a reservoir, this greatly increases its ability to transmit itself to other bats via aerosol or guano. This allows the virus to go through mutagenic changes in its genome to overcome host defense or increase transmittance. As mutations in the genome are driven by pressure or random chance. Now, the virus has both what it needs which is reproduction due to the large number of bats but also since bats are mammals it puts them a little bit closer in terms of protein expression compared to humans. Both bats and humans have similar protein receptors which viruses exploit to gain access to the cell. So, its easier for a virus to make the jump from a bat to a human. Bats are also ubiquitous among landscapes. This tends to give them proximity to humans. Like your rats in your example of bubonic plague. Therefore, contact between bats and humans is unavoidable.
I hope I was able to answer your question!
[removed]
So viruses hoping from bats to humans have a greater potential to be deadly then viruses which hop from human to dogs. Wouldn't this create a bias because... viruses hoping from bats would get much more attention, and viruses hoping from dogs, pigs, cows would be mostly benign and remain undetected.
Or in other words, bats are not a good virus vector at all, instead they are a vector for deadly viruses.
[removed]
When normal animal gets sick, it dies. The body dies, so the viruses in the body can’t go anywhere and die also. Disease not spread
When bats get sick, it doesn’t hurt them. They survive and the virus in them gets to grow and mutate and eventually evolve to jump species and infect humans because the bat is alive flying around humans or being ate by humans
Where other animals would get sick and die, ending the viruses spread, bats easily survive virus’s thus are able to incubate them and spread them around
Something not mentioned yet -
Bats have a larger variation in body temperature (10 c to 40 c) and so pathogens which evolve to survive in bats are able to cope better with the much smaller temperature variations in humans. If you have a fever of 39c that's not going to bother a pathogen that's at home in a 40c bat.
https://royalsocietypublishing.org/doi/10.1098/rsif.2021.0211
Yes if you think about it communal bats living in a cave are like humans living crammed in a city, the absolute worst case scenario for disease transmission in a population. Maybe even worse because they don’t have houses they retreat to and are constantly exposed to everyone crawling around.
[removed]
[removed]
[removed]
bats also have fairly high body temperatures.
Meaning that anything they have is unlikely to be susceptible to fever and will be fine in the high temp of a human body with a fever.
[removed]
[removed]
[removed]
Bats are mammals and they have a big overlap with humans regarding virus affinity. They get the viruses because they eat parasites and insects carrying these viruses. I saw a post yesterday where they were talking about some cave in Africa that people just cant go in because there are too many crazy viruses in bats in there.
Also as mentioned in other comments. Bats can just host a virus without getting sick from it. Except for rabies. Bats can still give you rabies, but they cant host that without being affected by it.
[removed]
[removed]
As far as why they're a danger to humans, they're mammals. They also tend to be susceptible to disease because they are very social mammals that live in large groups with minimal... social distancing. They and their bodily excretions are in relatively frequent contact with humans. This is why the diseases they do incubate as others have said are more readily spread to humans.
[removed]
Also, they can fly (enabling long distance transmission) and critically, they are mammals
Also, bats are very social, and sleep in clusters. If one gets something, it will spread rapidly if it is spreadable...
[removed]
[removed]
[removed]
[removed]
I've heard it's because of their rare status as a flying mammal. In short, keeping their weight aloft when they aren't as well adapted to flying as, say, birds which have hollow bones, means they have to be exceptionally tough, and this includes having a highly robust immune system.
Robust immune system means they are ideal carriers because they can contact loads of viruses without becoming incapacitated, and coupled with their ability to cover reasonable distances they have the opportunity to spread them.
Iirc bats also manufacture large quantities of their own vitamin C in a symbiotic relationship with the viruses living mostly in their hair follicles and triggering vit C production as viral loads increase. Humans either don't make their own endogenous VitC or only make a miniscule amount
[removed]
They are mammals (so, somewhat similar to humans in some ways.....ie they can carry viruses that can also infect other mammals like humans), they fly long distances (they can spread things far and wide), they come into contact with humans (anecdotally, but I remember being in Sydney Australia at night and seeing a ton of bats flying around above). All three factors together = they are good at spreading disease.
[removed]
[removed]
[removed]
Any animal that lives away from humans is a good scapegoat to blame laboratory created viruses on. Imagine in resident evil if Umbrella could just be like, Oh monkeys did it! Wasnt me! Bats did it! Wasnt william birkin in the lab
[removed]
Not to mention, and I'm speculating bathed in ignorance here, you don't ever have one bat. If a bat colony is hosting a particular virus, and 50% of the colony are so affected by disease that they can't function well enough to infect you, that still leaves a lot of bats that presumably have been exposed and carry the infectious agent and are function well enough to get you sick, too.
It‘s the other way round, they sound like a perfect reservoir for viruses. They might have benign viruses on top of the deadly ones in their system. Why should there be a selection for deadly ones if they are a great reservoir for virus amplification?
[removed]
By eating mosquitoes and slowing the spread of malaria and yellow fever, bats prevent more diseases than they spread. As long as you don't have bats living in your house and you're not farming and eating bats, having bats around turns out to be good for you.
When you say "fairly high" how does that compare to like cats and dogs? I only ask because they also have a warmer body temperature and was wondering if we could see a similar, if downplayed, effect
The problem isn't that bats are a good disease vector. The problem is that any virus that is well adapted to infect bats is capable of operating in higher temperatures than humans can survive.
One of humans' best defenses against a virus is a fever, which limits a viruses ability to fold proteins and replicate itself. But, bats' normal body temp during activity often reaches or exceeds 40C (104F). Normal human body temperature is 36C (98.6F). Thus, the human immune system isn't well adapted to combat viruses that are found in bats. By the time our vodies are hot enough to combat the virus, our brains are cooked.
Viruses sometimes jump from animals to people but are unable to transmit further. But, sometimes, they also mutate and become transmissible from human to human, and that's a problem. The more time humans spend in close proximity to large quantities of non-domestic animals. The more likely it is to see an infection jump across species.
I am out of my field of expertise. So take this with a grain of salt. I also imagine that there are plenty of bat species to whom this does not apply
I believe that during flight and other activities like social crowding bat body temps reach 40 or so
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4012789/
There is a whole review paper on this if you are interested
[removed]
In general the smaller the mammal the faster it's metabolic rate (with some exceptions). A faster metabolic rate generates more heat. Flight also requires a lot of effort and the metabolic rate goes quite high while animals are flying. In birds we see a typical core temperature of 102-109F and bats 99-106F.
It's because they're mammals that fly. They need super-powered mitochondria to create enough energy to fly, but there's some sort of trade-off to that which involves the immune system. The solution to this trade-off is for the immune system to function while harboring all these viruses in a way that's harmless to the bats. If you want to know all the specific sciency details involved, there's videos about it on YouTube.
It's because they're mammals that fly. They need super-powered mitochondria to create enough energy to fly, but there's some sort of trade-off to that which involves the immune system. The solution to this trade-off is for the immune system to function while harboring all these viruses in a way that's harmless to the bats. If you want to know all the specific sciency details involved, there's videos about it on YouTube.
This is the answer in relation to why bats are a disease vector for humans. Other animals may may be more prone to viruses, but their body temp differences mean that the virus can’t survive in humans.
[removed]
[removed]
Bats don't respond too much to many viruses etc. They're also tiny living instinctually in hard to find, out of the way, dark places away from predators in massively dense populations. Sometimes just a crack in the rock is enough for a huge bat population.
So because of their low response/death from viruses entire populations in a cave will simply serve as a petire dish while viruses pass freely mutating away as they evolve. Then bats will fly KMs away from there looking for insects etc. Some bats will migrate 100s of miles as well. Huge intense breeding ground combined with wide spread area of infection to spread in makes them super dangerous.
There's a cave in uh, Africa, I believe that is considered one of the most dangerous places on earth. They've traced Ebola among other diseases from that location. It's kind of terrifying.
[removed]
[removed]
The exceptionally high metabolism of bats has resulted in the evolution of an exceptionally effective immune system to counter the stress of that metabolism, inflammation, and DNA damage. The downside? That means viruses get into bats and mutate to variants that are stronger and stronger as they try to evade the immune system of these animals. When they hit on a mutation that enables them to jump to a new host species and out of bats, they have become quite nasty.
There are many reasons, some of which have already been mentioned:
There are also unknown mechanisms, for instance bats do not really get much cancer and maybe there is an overlap in the immune response that protects them from cancer and allows them to become very successful primary hosts for pathogens. This is a large area in research and I am sure has more updated knowledge since I stopped studying the field.
For example: recently identified areas for research - viruses housed in vesicles of stem cells of bats, which suggests there's a lot more going on to immunotolerance of bats than we currently know.
[removed]
[removed]
And since bats are the only mammals that fly it would be a good guess to say they might have the highest body temperature of all mammals.
[deleted]
Just the fact that they are mammals and can cover large distances make them pretty effective diisease vectors, it would seem.
Bats are mammals, we are mammals. Bats come into contact with other vectors, such as mosquitos. They sleep in huge dog piles with barely an inch or two ever separating one bat from its neighbor.
Plus they poop, a lot, all over the place. If we were closer to birds, evolutionarily speaking, we'd get even more diseases.
The reason this seems counter-intuitive to me is, if our immune system becomes dysfunctional as in AIDS/HIV then a simple cold becomes life threatening.
How do bats not die of infection if their body doesn’t have an immune response.
[removed]
[removed]
[removed]
[removed]
Bats are considered good disease vectors for several reasons. One of the main reasons is that they are able to host a large number of viruses and other pathogens without becoming sick themselves, due to their unique immune system. This means that they can serve as a reservoir for many diseases, and can transmit them to other animals or humans that come into contact with their bodily fluids, such as saliva, urine, and feces.
Additionally, bats are highly mobile and can travel long distances, which allows them to spread diseases across large areas. They also tend to live in large groups in close proximity to each other, which can facilitate the spread of diseases among their populations.
Finally, many species of bats are known to have close contact with humans, either because they roost in human-made structures or because they are hunted or consumed as food. This can increase the risk of disease transmission from bats to humans.
[removed]
[removed]
[removed]
[removed]
[removed]
[removed]
It's important to note that viruses don't actually want to kill their host. A dead or excessively sick host is less likely to spread the virus to the next person. So viruses generally evolve to make its host sick without killing them.
The issue is that bats have a much heartier immune system than humans do. So when a virus adapted to living in bats jumps to humans, it's waaaay too virulent for the human body. It's not that bat viruses are more likely to infect humans than a cow virus or some other animal. It's just a lot more likely to be deadly when it does.
One of the things that made human cities unhealthy until modern times was that they didn't have a good system for getting poop away from where people lived. Bats don't have sewers either, so their poop (and any pathogens it contains) stays where other bats can easily come in contact with it.
[deleted] t1_j9inouy wrote
[removed]