Despite the discovery of animals in a Chinese live-meat market infected with a virus similar to the one that caused the global SARS outbeak, the source of the disease remains a mystery.

A team led by virologist Yi Guan of the University of Hong Kong reports today in the journal, Science, that it isolated and sequenced an animal coronavirus from two Himalayan palm civets on sale at a Guandong market.

They compared the genetic sequences of the isolates to five existing genomes of the coronavirus known to cause Severe Acute Respiratory Syndrome (SARS) in humans. The newly-emerged disease first appared in China and quickly spread across 30 countries, most of them in East Asia, killing 813 people from a total 8,437 infections before it was declared contained in July this year.

While the animal and human viruses were closely related, the genetic analysis showed clear differences. But the group said the detection of SARS-like viruses in small wild mammals in a live retail market does suggest an interspecies transmission as the most likely route for the SARS epidemic earlier this year.

A raccoon dog, a ferret badger and some human workers at the same market also showed evidence of infection with a coronavirus similar to the human SARS virus - but none of the people reported having SARS-like symptoms in the past six months.

In a related move, the Food and Agriculture Organisation (FAO), a United Nations body, has warned that a real threat of future SARS outbreaks exists while the source of the virus remains obscure.

Identifying the 'animal reservoir' - the animal species from which the infection of humans originated - would be of great importance for future prevention measures in China or elsewhere if such a link could be made.

"To date there is no evidence that farm animal species have been infected with SARS coronavirus found in humans," the agency said in a statement. It based this opinion on a report prepared by a Dr Laurie Gleeson, a senior veterinarian at CSIRO Australian Animal Health Laboratory, a specialist division of the country's Commonwealth Scientific & Industrial Research Organisation.

Gleeson recently returned from a three-week mission to China as part of a joint study by the FAO and the World Health Organisation (WHO), another United Nations body.

He said that Chinese and Canadian researchers have sampled or tested more than 600 farm animals - including chickens, ducks, pigs and rabbits - but have found no evidence of SARS coronavirus infection.

"Based on preliminary laboratory testing, a number of animal species is under investigation as a possible source for the virus, including the palm civet, racoon dog, a species of fruit bat, and one species of snake," Dr Gleeson said. "Yet we still don't know the original source as it is possible that these animals were exposed to the virus in the animal markets."

An expanded monitoring program for the live animal trade - at farms and slaughterhouses - and further searching for a SARS reservoir in wildlife has been recommended by an expert committee to help pinpoint the source and to provide early warning of viruses circulating in wild animals.

More work is needed as well to understand the biological significance of the differences between the animal coronavirus from the civet cats and the virus that causes SARS in humans, the Hong Kong team said.

In an accompanying report in the journal, Dr Alan Schnur, WHO's team leader for communicable disease control in Beijing, urges a speedy follow-up to the latest research findings: "We have got to be quick, because we want to get some of these answers before winter," when many experts fear that the SARS virus could re-emerge, he said.

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