http://www.biomedcentral.com/news/20030520/03

Many Beijing SARS cases can't be traced back

WHA told 60 to 80% of Beijing SARS cases can't be traced back and existing PCR diagnostic test of limited use

By Robert Walgate
 

GENEVA—The World Health Assembly (WHA) has been told that 60 to 80% of severe acute respiratory syndrome (SARS) cases in Beijing, diagnosed by the symptomatic case definition of the disease, can't be traced back to a known SARS case, and other viruses could be involved. So said David Heymann, executive director of Communicable Diseases for the World Health Organization (WHO), speaking in Geneva yesterday (May 19).

"Our case definition has been highly specific," said Heymann. "Almost every case of SARS in the world today can be traced back to a previous case of SARS that fits the definition. But we understand there are other diseases that fit the case definition, and that's not surprising. What's important now is to figure out how many of those diseases that were called SARS might not have been SARS."

Hong Kong had been doing a "tremendous job" in looking at the blood of people who've recovered from SARS to see if the virus was present, said Heymann, and those data are now becoming available. But in Beijing, "…60 to 80% of cases can't be traced back to another case. The question is whether some of the cases fitting the case definition are caused by a different virus—a parainfluenza virus or some other virus."

The SARS test is a nested polymerase chain reaction (PCR) that runs twice. "Unfortunately, the diagnostic tests [for the coronavirus that's thought to cause SARS] are not yet perfect, especially the ones that can tell if someone's infected with disease at the time they are sick, so more work is being done," said Heymann.

But Klaus Stöhr, from WHO's Communicable Disease Surveillance and Response team investigating SARS, told The Scientist, "The fact that the number of cases in China that can be traced back is small could be due to difficulties in tracing. If you see cases coming up like mushrooms here and there, that would discomfort you, as it would mean you have an out-of-control situation. And that's what's happening in Beijing now."

As for diagnostics and the possible involvement of another virus, Stöhr claimed the issue was with the diagnostics rather than a second virus. "The reason why you don't find the [corona]virus in every SARS patient is that the tests we have are very specific but only 20 to 25% sensitive," Stöhr said, referring to the nested PCR test. "But still, if you look at feces, for example, where there's a lot of contamination, you have to filter, clean, and so on; so normally you repeat the test on different samples—on saliva, on conjunctival fluid, and on blood. But you can still miss the virus. The sensitivity of this [SARS] PCR is not very high."

Nevertheless, Stöhr claimed the SARS cases were caused by the SARS-associated coronavirus. "There will be co-infections, without any doubt, but what we are seeing is SARS caused by this coronavirus," he said definitively.

Antibody tests, which show the patient's response to infection, are under development, but they provide a diagnosis only 10 to 20 days into the disease. "We have an immunofluorescence assay that detects antibodies reliably from day 10 after symptoms first appear. But by then a patient will either be in intensive care and intubated or already getting better. So from a case-management or control point of view these antibody tests are not very useful," according to Stöhr.

But the existing diagnostic tests are useful in understanding the disease, said Stöhr. "Without them, we would not know the excretion pattern, the environmental studies would be impossible, and you wouldn't know which disinfectants would work. But they're of limited use for infection control."