Secrets of Sars virus's lethal mutation unlocked by scientists

Scientists have identified thestages in the evolution of the Sars virus that turned it from a simple animal infection into a lethal human disease.

A study of the genes of more than 60 Sars viruses has found that it took just three major changes to the virus's genetic material to transform it into a highly effective killer of people.

During the earliest phase of the epidemic the Sars virus infected about 3 per cent of those who came into direct contact with it, according to the study. Within a few months the proportion was 70 per cent.

Chinese scientists, working with a team at the University of Chicago, analysed the Sars virus from the early, middle and later phases of the 2002-03 epidemic. The researchers found that the first phase of the epidemic involved a virus virtually identical to that found in wild animals but it mutated to cause a second phase, during which the virus spread from human to human.

A final, third phase involved a further set of mutations that allowed the virus to stabilise and get accustomed to spreading still further within its new host species, said Chung-I Wu, professor of ecology and evolution at the University of Chicago. "What we see is the virus fine-tuning itself to enhance its access to a new host - humans. The virus improves itself under selective pressure, learning to spread from person to person, then sticking with the version that is most effective," Professor Wu said.

The findings, published in the journal Science, confirmed that Sars was an animal disease that had jumped the "species barrier" into man. But the study did not determine which animal was the natural "reservoir" for the virus.

In the early stages of the epidemic, in November 2002, there were about 11 seemingly independent cases of human Sars in the Pearl river delta area of Guangdong province in China. The scientists found that the rapid economic development of the area had led to "culinary habits involving exotic animals". Six of the 11 outbreaks involved people who had contact with wild animals.

During the middle phase of the epidemic the scientists discovered that there were changes to the Sars virus that enabled it to spread rapidly from person to person, accelerated by a "super-spreader" patient in a hospital in Guangzhou.

This phase, in January 2003, resulted in an outbreak of about 130 cases, including 106 patients who acquired the virus in hospital. A doctor at the hospital carried the virus to the Metropole Hotel in Hong Kong where the guests became infected and spread the virus across the world.