http://bmj.com/cgi/content/full/326/7399/1109

BMJ  2003;326:1109 (24 May)
 

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News

Chinese scientists must test wild animals to find the host of SARS

Scott Gottlieb

New York

Researchers believe that the coronavirus that causes severe acute respiratory syndrome (SARS) in humans probably originated in an animal in southern China.

Possible hosts for the SARS virus include bears, monkeys, and the endangered pangolin (above) MICHAEL PITTS/NATUREPL

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.

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Collections under which this article appears: Other Epidemiology Other Public Health Other respiratory infections

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