(CNN)Reclusive, nocturnal, numerous -- bats are a possible source of the coronavirus. Yet some scientists concur they are not to blame for the transfer of the disease that's changing daily life -- humans are.
Zoologists
and disease experts have told CNN that changes to human behavior -- the
destruction of natural habitats, coupled with the huge number of
fast-moving people now on Earth -- has enabled diseases that were once locked away in nature to cross into people fast.
Scientists
are still unsure where the virus originated, and will only be able to
prove its source if they isolate a live virus in a suspected species -- a
hard task.
But viruses that are
extremely similar to the one that causes Covid-19 have been seen in
Chinese horseshoe bats. That has led to urgent questions as to how the
disease moved from bat communities -- often untouched by humans -- to
spread across Earth. The answers suggest the need for a complete rethink
of how we treat the planet.
Bats
are the only mammal that can fly, allowing them to spread in large
numbers from one community over a wide area, scientists say. This means
they can harbor a large number of pathogens, or diseases. Flying also
requires a tremendous amount of activity for bats, which has caused
their immune systems to become very specialized.
"When
they fly they have a peak body temperature that mimics a fever," said
Andrew Cunningham, Professor of Wildlife Epidemiology at the Zoological
Society of London. "It happens at least twice a day with bats -- when
they fly out to feed and then they return to roost. And so the pathogens
that have evolved in bats have evolved to withstand these peaks of body
temperature."
Cunningham said this
poses a potential problem when these diseases cross into another
species. In humans, for example, a fever is a defense mechanism designed
to raise the body temperature to kill a virus. A virus that has evolved
in a bat will probably not be affected by a higher body temperature, he
warned.
But
why does the disease transfer in the first place? That answer seems
simpler, says Cunningham, and it involves an alien phrase that we will
have to get used to, as it is one that has changed our lives --
"zoonotic spillover" or transfer.
"The
underlying causes of zoonotic spillover from bats or from other wild
species have almost always -- always -- been shown to be human
behavior," said Cunningham. "Human activities are causing this."
When
a bat is stressed -- by being hunted, or having its habitat damaged by
deforestation -- its immune system is challenged and finds it harder to
cope with pathogens it otherwise took in its stride. "We believe that
the impact of stress on bats would be very much as it would be on
people," said Cunningham.
"It
would allow infections to increase and to be excreted -- to be shed. You
can think of it like if people are stressed and have the cold sore
virus, they will get a cold sore. That is the virus being 'expressed.'
This can happen in bats too."
In
the likely epicenter of the virus -- the so-called wet-markets of
Wuhan, China -- where wild animals are held captive together and sold as
delicacies or pets, a terrifying mix of viruses and species can occur.
"If
they are being shipped or held in markets, in close proximity to other
animals or humans," said Cunningham, "then there is a chance those
viruses are being shed in large numbers." He said the other animals in a
market like that are also more vulnerable to infection as they too are
stressed.
"We are increasing
transport of animals -- for medicine, for pets, for food -- at a scale
that we have never done before," said Kate Jones, Chair of Ecology and
Biodiversity at University College London.
"We
are also destroying their habitats into landscapes that are more
human-dominated. Animals are mixing in weird ways that have never
happened before. So in a wet market, you are going to have a load of
animals in cages on top of each other."
Cunningham
and Jones both pointed to one factor that means rare instances of
zoonotic spillover can turn into global problems in weeks. "Spillovers
from wild animals will have occurred historically, but the person who
would have been infected would probably have died or recovered before
coming into contact with a large number of other people in a town or in a
city," said Cunningham.
"These
days with motorized transport and planes you can be in a forest in
central Africa one day, and in a city like central London the next."
Jones
agreed. "Any spillover you might have had before is magnified by the
fact there is so many of us, and we are so well connected."
There are two simple lessons, they say, that humanity can learn, and must learn fast.
First,
bats are not to blame, and might actually help provide the solution.
"It's easy to point the finger at the host species," said Cunningham.
"But
actually it's the way we interact with them that has led to the
pandemic spread of the pathogen." He added that their immune systems are
poorly understood and may provide important clues. "Understanding how
bats cope with these pathogens can teach us how to deal with them, if
they spillover to people."
Ultimately
diseases like coronavirus could be here to stay, as humanity grows and
spreads into places where it's previously had no business. Cunningham
and Jones agree this will make changing human behavior an easier fix
than developing a vastly expensive vaccine for each new virus.
The
coronavirus is perhaps humanity's first clear, indisputable sign that
environmental damage can kill humans fast too. And it can also happen
again, for the same reasons.
"There
are tens of thousands [of viruses] waiting to be discovered,"
Cunningham said. "What we really need to do is understand where the
critical control points are for zoonotic spillover from wildlife are,
and to stop it happening at those places. That will be the most
cost-effective way to protect humans."
Jones
said viruses "are on the rise more because there are so many of us and
we are so connected. The chance of more [spillovers into humans]
happening is higher because we are degrading these landscapes.
Destroying habitats is the cause, so restoring habitats is a solution."
The
ultimate lesson is that damage to the planet can also damage people
more quickly and severely than the generational, gradual shifts of
climate change.
"It's not OK to
transform a forest into agriculture without understanding the impact
that has on climate, carbon storage, disease emergence and flood risk,"
said Jones. "You can't do those things in isolation without thinking
about what that does to humans."