EMILY KWONG: Since the new coronavirus began infecting
people last winter in China, scientists have been working
nonstop to try to figure out how it spreads. They've learned a lot,
but one of the mysteries they're still trying to untangle is where
this virus came from in the first place. And for that, scientific
evidence points overwhelmingly to wildlife.
Zoonotic disease is on the rise as interactions between humans
and wildlife increase. Food and agriculture practices,
landscape development, wildlife and livestock trade — all these
things put humans in closer contact with animals, especially in
densely populated areas. And this creates the perfect conditions for
zoonotic spillover, for animal-borne diseases to jump to humans.
The CDC, the leading public health organization in the United
States, estimates that 6 out of 10 infectious diseases in people
come from animals. And with this coronavirus, the most likely
origin point is bats.
These winged mammals have been natural reservoirs for the
rabies virus, the Marburg virus, the Hendra and Nipah viruses.
The Ebola virus outbreak in West Africa was traced to a bat colony.
The 2003 SARS outbreak originated in bats, as do other
coronaviruses. Now, with this coronavirus, scientists can't say
for sure that it came from bats alone. The virus shares a 96%
genetic similarity to coronaviruses that have been found in bats.
Ninety six percent sounds close, but...
ROBERT GARRY: That 4% difference is actually a pretty
wide distance in evolutionary time. It could be even decades.
KWONG: That extra 4% suggests the virus may not have evolved
from bats alone, but has some viral material from another animal
mixed in which acted as an intermediate host, ultimately
transmitting the virus to humans through feces, an animal bite
or wild meat consumed by a human. We just don't know for sure.
One possible intermediate host could be pangolins.
Scientists have found a genetic similarity between this coronavirus
and coronviruses found in these scaly anteaters, which are
vulnerable to illegal wildlife trade. But until scientists have more
genetic evidence, this theory about the role of an intermediate
host is all hypothetical.
Initially, researchers looked for clues about the virus's origin
at the Hunan Seafood wholesale market in Wuhan.
The market was connected to many of the early infections
in China, but there is not enough evidence to prove that is where
the virus jumped from animals to humans. One thing scientists
do agree on is that there is little chance this virus was
released as the result of a laboratory accident in China or
elsewhere, unless evidence comes out. So we don't have a
definitive answer on the chain of transmission from bats to
humans. And it may take years to figure out.
In fact, it took virus hunters over a decade to track down the
true origin of the 2003 outbreak of SARS to horseshoe bats
in a cave in the Yunnan province of China. Virus hunters had to
sample feces, urine and blood from thousands of bats across
the country to find a genetic match. Hunting viruses is a time
consuming and expensive endeavor. And while technology has
improved in the last 17 years, there still aren't many shortcuts
to the detective work of tracing a virus to its source.
