Hunt for SARS source far from over: researcher

Virologists have some suspects, but the animal that originally hosted the lethal coronavirus that causes severe acute respiratory syndrome (SARS) in humans remains unidentified.

And for as long as that continues to be the case, there is a real threat of further outbreaks of SARS, according to CSIRO veterinarian Dr Laurie Gleeson, of the Australian Animal Health Laboratory (AAHL) in Geelong.

Gleeson has just returned from a three-week mission to China, where, acting as a consultant for the Food and Agricultural Organisation, he reviewed field and laboratory data gathered by Chinese researchers in a massive and ultimately inconclusive attempt to track the origin of the epidemic.

The good news is that serum tests from more than 600 farm animals -- including pigs, chickens, ducks, and rabbits -- produced no evidence that the virus originated in any livestock species.

By excluding familiar livestock species, Chinese researchers have probably avoided the mass slaughter that followed the discovery in the late 1990s that chickens in Hong Kong's market were the source of a lethal new strain of influenza virus, the so-called 'bird flu'.

The bad news is that excluding livestock has only complicated the search. The virus could still be lurking in any one of the 114 animal species -- fish, amphibians, reptiles, birds and mammals -- sold for food in the Guangzou City market, in China's Guandong Province, where the epidemic began in November last year.

Gleeson said the market provided rich opportunities for 115 animal species -- counting humans -- to select from the smorgasbord of viruses harboured by species that, in many cases, do not come into contact in the wild, or on farms.

Identifying the natural host of the unusual coronavirus, which, according to genomic sequence data, is not closely related to coronaviruses that infect humans, their pets and livestock, is essential for preventing further SARS outbreaks.

Gleeson said two Chinese research groups -- one from the National Virology Institute in Hubei Province in northern China, the other from Hong Kong -- claim to have isolated SARS-like from the palm civet, an arboreal relative of the mongoose.

Anecdotal accounts after the SARS outbreak reached Hong Kong in January suggest the first victim in Guandong was a Chinese cook who had killed a palm civet for meat].

But Gleeson said two other teams -- one from China's Ministry of Agriculture, the other from the Chinese Agriculture University in Beijing -- were subsequently unable to detect the virus in serum samples from 75 palm civets.

He said some Chinese researchers favoured an alternate hypothesis: that the SARS virus originated in another species, and civet cat may have been an intermediate host, that amplified the virus and subsequently transmitted it to humans.

Gleeson said Chinese researchers had found SARS-like viruses in two other species: a fruit bat, and a water snake.

"Clearly, it would be a really unusual event to have virus go from a reptile to a mammal to man, but obviously the potential train of events needs to be properly investigated," he said.

The report of the virus in a water snake was intriguing, given that, during the 1990s, researchers from the Bureau of Animal Production and Animal Health in Chungchun had found coronaviruses in a number of other native creatures, and had implicated a coronavirus as the agent of a fish disease known as "mad eel swimming disease".

"No further research was done on the virus," Gleeson said. "Further studies are needed now. Lots of study [into coronaviruses] is needed. The story is getting bigger --- the SARS epidemic may have gone away in man, but the natural history of this virus needs to be unravelled if we are to avoid new epidemics"