Researchers believe that the coronavirus that causes severe acute
respiratory syndrome (SARS) in humans probably originated in an animal in
southern China. China.
| Figure 1 Possible hosts for the SARS virus include bears, monkeys, and the
endangered pangolin (above) |
That was the conclusion drawn by a group of experts meeting at the New York
Academy of Sciences to discuss the origins of SARS and the state of
development of a vaccine or antiviral drug to help quell the virus.
SARS has killed 642 people and infected 7860 across the world. No one
understands why SARS makes some people sicker than others and why some people
seem to be more likely to spread it to others.
It could be linked to stress or to infection with something else as well as
SARS, such as influenza or hepatitis, researchers said. Co-infection with
other infections is known to worsen coronavirus infection in farm animals. The
disease's long incubation period—estimated to be two to 10 days, or even
14 days—makes the virus perfect for travel. It gives infected
individuals enough time to go elsewhere before they get sick.
Experts who examined the genetic map of the SARS virus say that although it
is related to the three families of coronaviruses that cause respiratory and
gastrointestinal disease in animals, it is different enough to make up its
own, fourth family.
“I think it jumped from an animal, but we don't know which
one,” said Kathryn Holmes, a molecular biologist at the University of
Colorado Health Sciences Center who has studied coronaviruses extensively.
“The question is how much change does there need to be for a virus to
jump to a new host,” she said.
Dr Holmes expressed optimism that the carrier animal could be found in
Guangdong province, where the first SARS cases occurred last November. Animals
such as bears, monkeys, and the endangered pangolin are sold in the markets
there for food. Investigators will take blood from all the animals in that
region and test the blood for the virus, she said. Dr Holmes said there are
many ways the virus could have leapt from the animal population to humans,
including handling or eating the animal or contamination from faecal
waste.
As the SARS virus does not resemble any known coronaviruses, the virus is
unlikely to have its origins in a domestic animal or any animals that have
become important parts of commerce, such as cows, chickens, or pigs. Two human
coronaviruses cause about 30% of common colds, but the viruses cause more
important diseases in pigs, chickens, and other livestock. Many of the common
coronaviruses found in these animals have already been identified.
Researchers believe that many coronaviruses exist in wild species of
animals but remain unknown to scientists because nobody has had a reason to
look for them.
Linda Saif, a researcher at Ohio State University's Agricultural Research
and Development Center, agreed, saying that Chinese scientists should test
wild animals as this particular coronavirus has not been seen in more common
household animals such as cats, chickens, or pigs. Although scientists have
not yet found the SARS virus in a particular animal, Professor Saif said they
have derived valuable clues from studying other animal
coronaviruses—some encouraging data and some more worrisome.
“Shipping fever,” an illness that affects cattle when they
travel from farm to feed lot, shows some parallels to patients with SARS whose
illness may have been exacerbated by the stress of travel, she noted. In some
animals, the coronavirus reappears and reinfects them, raising concerns about
the future health of survivors of SARS.
Perhaps most alarming, Professor Saif said, was one recent experiment
showing that pigs infected with both the coronavirus and flu became much
sicker than those infected with just the coronavirus.
With reference to the recent cases of avian flu in people in the
Netherlands, she said: “You can imagine what would happen if both these
viruses infected people together.”
“The question remains, what was its host?” Dr Holmes said.
“Was it a virus in a host that can affect both the host and humans, or
was it a mutation that caused it to jump to humans and it's no longer able to
infect its host?” she said. “In the first scenario, you still have
a reservoir for the virus,” Dr Holmes said. “In the latter
scenario, you can contain it. But we just don't know.”
The SARS virus comes from a family of viruses called RNA viruses, which are
known to naturally undergo a high degree of mutation. Some researchers believe
that as the SARS virus doesn't resemble any known coronavirus, it could have
evolved separately.
Although researchers agree that finding the origin of SARS could hasten the
development of an effective vaccine or antiviral drug, they say that promising
treatments are already in development. Dr David Ho, president of the Aaron
Diamond AIDS Cancer Center in New York, described his early success using
fusion inhibitors, a drug technique commonly associated with AIDS treatments,
for targeting the SARS virus.
“It's pretty clear from our initial set of experiments that this
concept is as valid for SARS as has been noted for HIV,” Dr Ho said.
“It's something that could potentially be developed quickly,” he
said.
Dr Frederick Hayden of the University of Virginia School of Medicine in
Charlottesville said that antivirals could provide an important adjunct to the
vaccines that researchers are trying to develop. “Protracted viral
replication tells us that an antiviral can have considerable good in this
case,” he said.
On Saturday, another group of more than 40 experts convened by the World
Health Organization ended the meeting with a message of hope, saying that
early detection and isolation of cases had already brought outbreaks in Hanoi,
Toronto, Singapore, and Hong Kong under control and should eventually contain
the disease in China and Taiwan.