Why Do Bats Carry So Many Dangerous Diseases?

Why Do Bats Carry So Many Dangerous Diseases?


[♪ INTRO] One fifth of all mammal species today are
bats. And that’s awesome, because they help us
out in all sorts of ways. Like, they pollinate a lot of plants, help
regrow forests and control pests, and their poop is pretty
excellent fertilizer. Plus, they’re just really cool. Some of them can sense magnetic fields, or
use sound to find their food—and, of course, there’s
the one thing they all have in common: they can truly fly. They’re the only mammals around capable
of powered flight… without the help of machines. But they’re also somewhat notorious for
something else: being flying sacks of germs. You might have noticed that part, what with
all the talk about zoonotic diseases that’s been happening
lately. Those are diseases that are passed to humans
from other animals. And while bats aren’t to blame for everything,
they have played a role in the transmission of at least 11
viruses —probably 12, counting SARS-CoV-2. These diseases aren’t the bats’ fault,
of course. If anything, they’re ours. Research shows that disturbing animal habitats
is usually what causes the transfer of a zoonotic disease
to humans. Still, bats in particular do carry a lot of
viruses. And that’s because they have unique immune
systems. Which means we can learn a lot about these
pathogens and their effects by studying bats. In fact, bat immune systems are so special that what we learn from them could someday
help us treat a wide variety of conditions, from cancer
to diabetes. And the kicker is: bats probably have weird immune
systems because they fly! Unlike gliding, flapping flight requires a
huge amount of energy. So, bats have evolved ways to kick their cellular
fuel production into high gear—mainly, by putting their
mitochondria into overdrive. Those are special compartments within cells
that turn food into fuel. But there’s a catch! When mitochondria convert nutrients into energy, they also create byproducts called reactive
oxygen species. So basically, mitochondrial exhaust fumes, in the form of really reactive molecules which
contain oxygen. Now, these aren’t all bad. The immune system uses them to rouse immune cells to action and kill bacterial
invaders. But, they can also cause a lot of damage. They can weaken cell membranes, mess with
proteins, and even break DNA, and because of that, they
play an important role in diseases like cancer
and arthritis. Cells can try to keep them in check with antioxidants — compounds that essentially neutralize
these overeager molecules. But those can only do so much, and when the
balance gets out of whack, cells experience a condition called oxidative
stress. This is when most of the DNA damage happens. So, bats’ supercharged mitochondria mean
extremely high levels of oxidative stress—which, in turn, means
constantly high levels of DNA damage. But since bats don’t immediately get super-cancer after their maiden flight, researchers have
long suspected they’ve evolved ways to protect themselves from all this flight-related damage. And a few years ago, genetics studies found
mutations which boost their ability to detect and repair
damaged DNA. Essentially, they’ve also turbocharged the
mechanisms that prevent genetically damaged cells from
replicating. Which may also explain why they don’t seem
to get cancers very often. So, bats produce tons of energy without damaging
their cells. Sounds pretty awesome, really. There’s just one small problem. DNA damage can also be a sign of a viral infection, because viruses need to hijack the cell’s
genetic machinery to reproduce, and that process usually involves
some strategic snipping. So, naturally, DNA damage triggers an immune
response: inflammation. Essentially, when cells detect DNA damage
or other signs of infection, they chemically call in white blood cells. These cells kill and destroy pathogens using
a variety of genetic and chemical tools. And they also help control how the inflammation
unfolds—like, by bringing in additional white blood cells
or switching some of them from germ-killing to tissue repair once the
invasion is over. This immediate or acute inflammatory response
helps to get rid of the invaders and promotes healing. But remember, thanks to their supercharged
mitochondria, bat cells experience constant DNA damage. They can repair this damage thanks to those
advanced DNA-repair tools. But the damaged DNA should still light an
immune flare before it’s fixed. So bats would experience super-inflammation
all the time! And prolonged, chronic, and systemic inflammation
isn’t so great. White blood cells and the processes they set
in motion can be really destructive to the body’s
own tissues. In short, too much inflammation can lead to
organ failure and even death. So flight should be a death sentence. Except, bats have evolved some neat ways to knock down inflammation, too. For one thing, they dampen the activity of
STING proteins. These proteins are one of the ways mammalian
cells trigger an inflammatory response when a virus
is detected. Also, a genetic analysis of multiple bat genomes showed that they’re the only mammals that
completely lack genes for PYHIN proteins—another set of inflammation-triggering sensors activated by damaged DNA. And those are just part of the story. It will still be a while before we completely
understand how bats prevent or dampen inflammation in
their bodies, because it seems like every time they look,
researchers keep finding more of these adaptations. So to recap: we know that to make sustained
flight possible, bats have ramped up fuel production and DNA
damage detection while dialing inflammation down to a 1. But we know that inflammation is one of the
big ways the immune system fends off intruders. So, doesn’t that leave them open to all
kinds of actual pathogens?! And… The answer is yes! Around the turn of the 21st century, scientists
discovered that bats act as a reservoir for a lot of
viruses that are extremely dangerous to humans. This includes filoviruses, which cause hemorrhagic
fevers, like Marburg or Ebola. Also, henipaviruses like Hendra and Nipah, both of which can cause fatal brain infections … and, of course, coronaviruses, like SARS,
MERS, and (most likely) the notorious new coronavirus that started
the COVID-19 epidemic. And there’s mounting evidence that bats
were involved in transmitting these diseases to humans, either
directly or indirectly, like by infecting farm animals. Bats are also suspected of having given us
other diseases in the past, like mumps, measles, and hepatitis B! But here comes another magic thing about bats: Even though they’re widely infected with
notoriously deadly viruses, they don’t actually seem to get sick from
them. The virus can be found in their bodies, but they don’t have any symptoms. As for how that’s possible? Well, we still have more questions than answers, but researchers have discovered a few evolutionary quirks about bat immune
systems that help make that happen. In part, that’s because active viral infections
in bats tend to be pretty short-lived, thanks to their hypervigilant interferon production systems. Remember how we said DNA damage is a signal
of infection? That’s because viruses try to reprogram
cells to create copies of themselves. But cells aren’t sitting ducks during this
process. In addition to calling out for help—which
is that whole inflammation bit we discussed—cells have an internal defense mechanism. They can make a protein called interferon
alpha, which activates genetic and chemical tools
that reduce the virus’ ability to multiply and spread. Every other mammal we know of switches their
interferon system on when an infection occurs. But genomic studies suggest bat cells always
have their interferon alpha genes activated! This drastically cuts down the time it takes
to react when a virus is present, so it allows bats to nip the infection
in the bud before it becomes a full-blown disease. And—of course—there’s more. All mammalian cells contain an enzyme called
ribonuclease L which, when activated, chops up viral RNA to stop
a pathogen from spreading. But, in us and most other mammals, activating
this enzyme takes a complex chain of steps—so it’s
not super quick. But bats, on the other hand, can activate
it directly with interferons, drastically speeding up infection
containment. Scientists also think this fast activation
of ribonuclease L helps bats outsmart viruses that have evolved
to inhibit the enzyme before it can be switched on—something
HIV does in humans, for example. So basically, when bats do get viruses, they’re
able to quickly clear them out… for the most part. They can still fall prey to at least a few,
like the rabies virus. And even quick suppression of a virus can
mean it stays around in a group of bats. Bats tend to huddle close together when they
roost. Plus, they fly around in the same areas as
bats from other colonies, and they’re constantly spraying snot and
saliva everywhere when they echolocate. So there’s a good chance that, during that
window when a bat has a virus that is actively replicating, it will
spread it to another bat. That means even if each bat only hosts a virus
for a short time, it can linger in the population. And even this probably isn’t the full picture. Mathematical models indicate that, by themselves,
bats’ social habits don’t completely explain why they host so
many viruses. Instead, research suggests that the viruses
themselves have figured out how to lie dormant and undiscovered—like
in the bats’ lungs, spleens, or intestines. And then, when the bat gets stressed—like
when it’s roused from hibernation—that stress temporarily
dampens its anti-virus systems, allowing any hidden
viruses to emerge. This leads to another period of increased viral replication and shedding. So again, the bat can transfer the infection
to other animals. Then, its antiviral systems get back up to
speed, and the viruses are eliminated or driven back
into hiding. Still, even when a virus is replicating and
being shed, bats generally don’t seem sick—not like
a person would with the same virus. And that’s likely because their inflammation-dampeners are still running. So, they’re still suppressing a lot of the
immune response that would make them noticeably sick. And that may also be why viruses that are
deadly to us aren’t lethal to them. It turns out that the most severe symptoms
of illnesses like MERS and Ebola, the symptoms usually
responsible for their lethality aren’t caused by what
the virus itself does to the body. Instead, they’re the result of the destructive, catastrophic inflammation the virus triggers. This includes an extreme systemic reaction
called a cytokine storm, where an over-release of pro-inflammatory
signaling proteins turns a person’s own immune system into
their worst enemy. And some researchers think that our aggravated
inflammatory response may actually be because these viruses have
evolved to dodge the super-refined immune systems
of bats. Basically, they evolved to survive in a host
that constantly and aggressively attacks their ability to
replicate. So when that assault is suddenly weaker in
a new host, they go wild and produce a lot of little virus
babies that send the host’s immune system into
panic mode. This kind of overreaction can also happen
to bats. It’s just not usually in response to viruses. Instead, it seems like their unique immune
system may leave them vulnerable to non-viral invaders. The most infamous example of this is White
Nose Syndrome, a fungal pathogen which has devastated bat
populations in North America. Some researchers think the bats’ dampening
of inflammation —especially during hibernation—makes it
easy for the fungus to infect the bat. Then, once the bat awakes, it’s immune system
does react—only, it goes too far, which can lead to the bat
developing a life-threatening form of systemic inflammation. So, basically, the reason they die from white
nose is similar to why we die from viruses like
MERS and Ebola. That may mean that the key to saving bats
from this fungus (and us, from the deadly viruses they carry)
may lie in further research on bats. And also, if we’re being selfish, we have
a lot of other diseases characterized by inflammation, like heart
disease and diabetes. So studying their inflammation system may
lead to treatments for our chronic conditions. And the same goes for studies on the ways
that bats keep viruses at bay. Right now, we only have effective antiviral
drugs for about 10 of the more than 200 viruses that can infect
humans, and very few broad-spectrum antivirals. If we can discover more of bats’ tricks,
we might be able to use them to develop therapies against the diseases
they host and other dangerous viruses. Plus, all this research might help us live
longer. Many researchers think bats’ immunological
adaptations are also behind some of their other superpowers,
like how they seem to rarely get cancer, or how they live
incredibly long lives for animals of their size. Usually, little animals live fast and die
young. But Brandt’s bats can live for over 40 years
even though they only weigh 4 to 8 grams! So instead of looking at bats as species zero, we should think of them as flying keys to
longevity and resilience. And in the end, they’re going to be our
allies in health, not our enemies. Speaking of allies, before we go, we’d like to thank our patrons on Patreon. It takes a lot of people to make long, complex
episodes like this come to life, and we can only bring all those
people together because of the support of our patron community. So, thank you! And if you want to learn more about joining this awesome group of people who help keep
SciShow running, you can learn more at Patreon.com/SciShow. [♪ OUTRO]