Chinese researchers announced today that they have found a virus similar to the coronavirus that causes severe acute respiratory syndrome (SARS) in several wild animal species for sale at a market in southern China, providing the first clues to the origins of the SARS outbreak. But researchers say it's too soon to say whether any of the species is actually the virus's natural reservoir. At the same time, studies published online today by Science show that the SARS virus has the potential to spread widely, but that rigorous public health interventions can stem the epidemic, as it appears to have done in Hong Kong and Vietnam.
At a news conference today, a research team led by Kwok-Yung Yuen, head of microbiology at the University of Hong Kong, said they had isolated coronaviruses or found traces of the virus in six masked palm civet cats and two raccoon dogs; one Chinese ferret badger was also found to have antibodies against the virus, another indication that it may have carried the virus. The genetic sequence of the isolated virus showed that it was almost identical to the virus that causes SARS in humans, according to the World Health Organization (WHO) in Geneva.
WHO officials were cautious in their response to the news. It has yet to be shown that the animals are at the root of the human outbreak, WHO said today in a statement; they could have picked up the virus from prey, or even from humans. Moreover, it's unclear whether the animals can secrete the virus in sufficient amounts in their feces or respiratory fluids to infect humans. However, WHO does recommend that anyone handling or slaughtering the animals take precautions.
Meanwhile, modelers are trying to get a handle on what's in store for SARS (Science, 25 April, p. 558). Today, Science published two papers, both by international research groups, describing a model of SARS based on data from Hong Kong and Singapore (see M. Lipsitch et al. and S. Riley et al.). Both papers show that, without public health interventions, every SARS patient would infect between two and four people. Although that number is lower than for many other respiratory diseases, it does mean the virus can infect many people wherever it is introduced. But, the modelers say, rigorous surveillance, isolation of patients, and aggressive tracing of their contacts can eventually stop the spread.
Ira Longini, who models diseases at Emory University in Atlanta, says the studies are “both good beginnings.” But what current models can't take into account, Longini says, are the many things still unknown about SARS--for instance, its capacity to flare up again in the fall like most respiratory viruses do. Another key question, he says, is how many people are infected with SARS but show few symptoms or none at all. If these people can infect others, it would severely complicate efforts to stamp out the disease. In Hong Kong, about one on 12 cases cannot be traced to a known SARS patient.
A troubling indication that SARS may indeed be difficult to control came from Toronto, where four new probable cases were announced today--the first ones since mid-April. If they are confirmed to be SARS, there may have been an undetected chain of transmission, Longini says--for instance, by people who have only mild symptoms--or the virus may have persisted somewhere in the environment. Either way, “this doesn't look good,” Longini says.