In Asia there is widespread concern about the continued spread of SARS. Credit: © AP

Scientists have worked out the genetic sequence of the virus that is thought to cause severe acute respiratory syndrome (SARS). The code supports the idea that the disease leapt from animals into humans - and should help to refine a diagnostic test.

Over the weekend, two research groups separately revealed the complete genetic make-up of the suspected SARS virus, called a coronavirus. The flu-like disease has infected an estimated 3,169 people and killed 144 since November last year.

The sequence suggests that the coronavirus is "far from anything known before", says Herbert Schmitz of the Bernhard Nocht Institute for Tropical Medicine in Hamburg, Germany. Although it is more than 75% identical to known animal and human viruses in some regions, it diverges widely in others.

Using this information, scientists are already devising a more accurate diagnostic test for SARS. Existing tests have limitations - for example, those that detect and amplify a single viral gene from blood or lung samples may miss trace infections in some patients.

Tests that amplify different stretches of the viral genetic sequence may be more sensitive or accurate, says Albert Osterhaus, who is studying SARS at Erasmus University in Rotterdam, the Netherlands. "There's room for improvement," he says.

Leap of faith

Despite the scientific advances, investigators remain worried about the spread of SARS, particularly in Asia. In some cases, the disease seems to spread to people who have not had close contact with the sick - and there is no known treatment for the virus.

The unusual genetic sequence suggests that the SARS coronavirus lurked undiscovered in animals before making the leap into humans. "It suggests that it's been out there in a different host for millennia," says coronavirus expert Kathryn Holmes of the University of Colorado Health Sciences Center in Denver.

This could have happened when people came into contact with infected animals to which they had rarely been exposed before. Alternatively, an innocuous animal virus might have mutated to penetrate or harm human cells.

A third hypothesis - that the new virus formed when two coronavirus strains met in an animal and swapped genes - now seems less likely, as the sequence contains nothing to support the idea, says coronavirus expert Michael Lai of the University of Southern California in Los Angeles.

Scientists plan to trace the origins of the virus by gathering samples from animals in Guangdong Province, China, where the disease probably arose. They could also assess whether changing patterns of animal movement or agriculture might have facilitated the jump.

Armed with the genome, researchers can also begin to investigate how the virus infects human cells - and how to block it with drugs or vaccines. Within days, Osterhaus also hopes to reveal the clinching evidence proving that coronavirus is the sole cause of SARS: whether primates injected with the virus fall sick.