An Australian virus hunter tracking the source of Sars has ruled out farm animals as likely hosts, but he has found no clue yet as to which wild animal harboured the coronavirus that spilled over to humans. Hume Field - who spent six weeks in Malaysia looking for the host of the Nipah virus, which infected thousands of pigs and killed more than 110 people in 1999 - said the epidemiological picture for Sars did not fit that of an association with farm animals. 'If it were farm animals, it would be a substantial source and [the illness] would happen all the time. It better fits the picture of exposure to an unusual source - a wild animal that doesn't have contact with people very often,' said Dr Field, who has spent a week in Guangzhou with a World Health Organisation expert group looking for clues to the source of the Sars virus. Dr Field, from Queensland, also dismissed speculation that the virus was carried by pets. He said there were two broad possibilities: that the Sars virus was a previously non-virulent virus, or that it was a virulent virus that had been dormant until contact with the host animal that allowed it to spill over to humans. But since mainland laws have always allowed people to eat wild animals or use them for medicines, the experts want to know why it is happening now. If they continue to find no clues, the experts will have to pursue other lines, including inoculating a range of wild animals with viruses to find out which one allows reproduction of the agent, as happened with the Nipah virus, which was tested on Malaysian fruit bats. 'It is possible that we may never find the animal. [If it is not found] it doesn't give you confidence that it can't spill over again,' he said. Dr Field helped trace the Nipah virus' reservoir in Malaysian fruit bats. Contaminated fruit dropped into pig pens located near orchards is believed to have infected pigs. Thousands of pigs died and a million were culled as part of efforts to stop the spread to the human population. Another WHO expert, Meirion Evans, suggested that more laboratory tests could be undertaken when the epidemic waned to find out if there was a background of past infection, and whether people in the community, or people in a particular occupational group, had antibodies to Sars.