Why bats don't get sick - Arinjay Banerjee
If this bat were a human,
she'd be in deep trouble.
She’s infected with several
deadly viruses,
including ones that cause rabies,
SARS, and Ebola.
But while her diagnosis would be lethal
for other mammals,
this winged wonder is totally unfazed.
In fact, she may even spend
the next 30 years living
as if this were totally normal–
because for bats, it is.
So what’s protecting her
from these dangerous infections?
To answer this question,
we first need to understand
the relationship between
viruses and their hosts.
Every virus has evolved to infect specific
species within a class of creatures.
This is why humans are unlikely
to be infected by plant viruses,
and why bees don’t catch the flu.
However, viruses do sometimes jump
across closely related species
And because the new host
has no established immune defenses,
the unknown virus presents
a potentially lethal challenge.
This is actually bad news
for the virus as well.
Their ideal host provides a steady stream
of resources
and comes into contact with new parties
to infect—
two criteria that are best met
by living hosts.
All this to say that successful viruses
don’t typically evolve adaptations
that kill their hosts—
including the viruses that have infected
our flying friend.
The deadly effects of these viruses
aren’t caused by the pathogens directly,
but rather, by their host’s uncontrolled
immune response.
Infections like Ebola
or certain types of flu have evolved
to strain the immune system
of their mammalian host
by sending it into overdrive.
The body sends hordes
of white blood cells,
antibodies and inflammatory molecules
to kill the foreign invader.
But if the infection has progressed
to high enough levels,
an assault by the immune system
can lead to serious tissue damage.
In particularly virulent cases,
this damage can be lethal.
And even when it’s not,
the site is left vulnerable
to secondary infection.
But unlike other mammals,
bats have been in an evolutionary arms
race with these viruses for millennia,
and they’ve adapted to limit
this kind of self-damage.
Their immune system
has a very low inflammatory response;
an adaptation likely developed
alongside the other trait
that sets them apart from other mammals:
self-powered flight.
This energy-intensive process can raise
a bat’s body temperature to over 40ºC.
Such a high metabolic rate
comes at a cost;
flight produces waste molecules
called Reactive Oxygen Species
that damage and break off
fragments of DNA.
In other mammals, this loose DNA
would be attacked by the immune system
as a foreign invader.
But if bats produce these molecules
as often as researchers believe,
they may have evolved
a dampened immune response
to their own damaged DNA.
In fact, certain genes associated
with sensing broken DNA
and deploying inflammatory molecules
are absent from the bat genome.
The result is a controlled
low-level inflammatory response
that allows bats to coexist
with the viruses in their systems.
Even more impressive,
bats are able to host these viruses
for decades
without any negative health consequences.
According to a 2013 study, bats
have evolved efficient repair genes
to counteract the frequent
DNA damage they sustain.
These repair genes may also contribute
to their long lives.
Animal chromosomes end with a DNA sequence
called a telomere.
These sequences shorten over time
in a process
that many believe contributes
to cell aging.
But bat telomeres shorten much more slowly
than their mammalian cousins—
granting them lifespans
as long as 41 years.
Of course, bats aren’t totally
invincible to disease,
whether caused by bacteria,
unfamiliar viruses, or even fungi.
Bat populations have been ravaged
by a fungal infection
called white-nose syndrome,
which can fatally disrupt hibernation
and deteriorate wing tissue.
These conditions prevent bats
from performing critical roles
in their ecosystems, like helping
with pollination and seed dispersal,
and consuming pests and insects.
To protect these animals from harm,
and ourselves from infection,
humans need to stop encroaching
on bat habitats and ecosystems.
Hopefully, preserving these populations
will allow scientists to better understand
bats’ unique antiviral defense systems.
And maybe one day, this research will
help our own viral immunity take flight.
