SARS: Where Did It Come From?

Emerging viruses: Scientists scramble to find the natural hosts before they spawn another outbreak.

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"The fact that both sars and most flu viruses originated in southern china is no surprise," says virologist Michael Lai, who believes that the region's social customs of catching and eating wild game expose people to animal viruses.

In April, when Canadian scientists sequenced the genetic code of the SARS virus, they discovered a microbe unlike any other ever seen in humans or animals. On its genome "there is a long stretch of nucleotides and then one big piece that sticks out," says University of Hong Kong microbiologist Malik Peiris, who first linked SARS to a novel coronavirus. "When we then looked to see if antibodies for it exist in human blood samples, there were none."

So where did the SARS virus come from? At press time, eight months after the first case was diagnosed in a bird and snake merchant in the Chinese city of Shunde, the source of the virus was still unknown. But researchers are narrowing the suspects to animals found in southern China, where humans and critters often live cheek by jowl.

Michael Lai, a virologist at the University of Southern
California, says the virus's genome is similar to that of both a mouse and a bird virus, hinting that it may be a mix of the two. "My analysis suggests that it likely existed in a wild animal, probably a bird. It jumped species only recently when it came into contact with a human being," he says.

In theory, SARS leapt from
a wild beast to a human when it acquired the molecular "keys" to gain entry to our cells, explains Lai. To do that, it may have first mingled with a human virus brewing inside another species. A pig, for example, can serve as a genetic mixing bowl when co-infected with two viruses, allowing them to swap genes.

In a recent experiment to show how easily the coronavirus can morph and become harmful in a new species, Peter Rottier of Utrecht University in the Netherlands simulated a gene swap by taking a coronavirus that is lethal to cats and adding a single gene fragment from a mouse virus. The recombinant virus was lethal to both
animals. "Coronaviruses have a unique ability to mix with other viruses," Lai says.

Meanwhile, scientists from Lyon to Winnipeg are spraying, injecting, and orally feeding the coronavirus to monkeys, dogs, cats, mice and rabbits. Goats and sheep are next. "We want to see how they react to high doses of the virus, how susceptible they are, which replicate the virus, which excrete it, which show antibodies," says Klaus Stohr, chief SARS scientist for the World Health Organization's Animal Influenza Network.

Once the lab tests yield more specific clues, experts like Stohr will comb the ground in southern China to pinpoint the SARS animal hosts. Doing so will help
scientists develop strategies to intercept other emerging animal-borne viruses. The WHO already maintains an active surveillance of animal flu
viruses in the region, where both the 1957 Asian flu and
the 1968 Hong Kong flu, which together killed some 1.5 million people, originated. So did the 1997 avian flu and possibly the 1918 Spanish flu, which claimed 20 million lives. All have been linked to animal hosts. For this reason,
a similar surveillance system
is being established for corona-
viruses. In the end, says Stohr, "there's no point in conquering SARS as it exists now, only to have something similar or related swirling in an animal reservoir, waiting to spark the whole thing all over again."

BY THE NUMBERS

**Where SARS Hit Hardest

Number of deaths by area ***

China 267
Hong Kong 227
Taiwan 30
Singapore 28
Canada 23
Vietnam 5
Malaysia 2
Philippines 2
Thailand 2
South Africa 1
United States 0

* As of May 14, 2003

Source: WHO