Virus detectives seek source of SARS in China's wild animals.

Tokyo and Munich

Researchers investigating the source of severe acute respiratory syndrome (SARS) have turned their attention to the wild-animal markets of southern China. The move follows reports that workers and animals at the markets show high rates of infection with coronaviruses, the family to which the virus believed to cause SARS belongs.

The possible link to wild animals emerged on 23 May, when a team from the University of Hong Kong revealed that a coronavirus resembling the SARS virus had been isolated from six masked palm civets (Paguma larvata) and a raccoon dog (Nyctereutes procyonoides) in a market in Shenzhen, in the southern Chinese province of Guangdong. Antibodies against the virus were also found in a Chinese ferret badger (Melogale moschata) from the same market.

Although the virus is not the same as that believed to cause SARS — a member of the Hong Kong team describes it as “genetically very close, but not identical” — five out of the ten civet handlers at the market had antibodies against the SARS virus in their blood. A Chinese government team has since released results showing that 66 out of 508 animal handlers tested at markets in Guangdong had antibodies against the SARS virus.

Microbiologist Kwok-yung Yuen, who led the Hong Kong team, notes that in the general population, the level of antibodies against the SARS virus is much lower. “This suggests that the virus is jumping from wild animals to humans,” he says.

Because the animal virus is similar to the SARS virus, but different from other coronaviruses, it is now a prime suspect in the hunt for the origins of SARS. The likelihood that the virus is moving the other way — from humans to animals — is diminished by ongoing work on genetic sequences of the two viruses. These analyses suggest that the animal version has an extra stretch of 29 nucleotide bases. “Viruses tend not to gain stretches of nucleotides when they jump across species,” says Klaus Stöhr, a SARS expert at the World Health Organization (WHO) in Geneva.

But uncertainties remain over the exact source of the virus. Only six civets were present in the market, and the fact that they all had the virus suggests that they were infected recently. “They could have got it from another animal during transport to the markets,” says Zhang-liang Chen, a molecular biologist and president of the China Agricultural University in Beijing. Chen's group did not find the SARS virus in samples from eight civets taken from other markets and in the wild in Guangdong.