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SARS Lessons Unlearned

Will SARS hit hard again this year or in the future? Experts go over what happened and what may be next.

Stopping SARS

What ended the SARS epidemic? Klaus Stöhr, PhD, director of the WHO's global SARS laboratory network, credits early identification and isolation of SARS patients. It took heroic efforts from health officials in Hong Kong and elsewhere, who refused to allow anyone with a fever to board any form of transportation. Moreover, air travel to cities with ongoing SARS outbreaks virtually ceased.

"Most countries did temperature screening," Stöhr tells WebMD. "In Hong Kong, every day there were 750,000 people screened at airports, seaports, and land ports. Every day, several hundred people were found to be feverish, and quite a number turned out to be suspected cases of SARS. That is one measure that worked to limit the number of cases. Also helpful was the recommendation to the public to suspend air travel to countries where SARS cases were occurring in the community. These are two measures that we considered successful."

As it turned out, SARS wasn't as easily spread as it first seemed. Most cases could be traced to "superspreaders" -- a few people who became especially ill with especially large doses of especially infectious virus.

"People who were relatively close to the original source of infection obtained a larger dose of SARS virus, were more severely ill, and secreted a large amount of virus," Stöhr says. "With each link in the chain of transmission, the virus excretion rate changed. Those first in the chain were most severely infected. But the super spreading was mostly seen in the initial phase of the outbreak when people did not understand the measures that needed to be taken."

Where Did SARS Come From -- and Where Is It Now?

Foshan, China, is in Guangdong province. As elsewhere in southern China, Guangdong markets feature exotic "game food." These live, exotic animals of nearly every imaginable kind are caged very close to one another. They're butchered and eaten as culinary delicacies.

Some of the earliest SARS cases seem to have been in people whose jobs involved dealing with these animals. Blood from people who handle these animals are more likely to have antibodies against the SARS virus than other people working in the same markets. And health authorities have isolated SARS virus from at least two kinds of these animals -- the Himalayan palm civet and the raccoon dog.

This doesn't necessarily mean the animals are the source of SARS. It's possible that the animals caught the virus from humans, and not the other way around. Pets owned by SARS patients in Hong Kong -- cats and dogs -- have been infected with the virus.

Stöhr says it's clear that no humans now have SARS disease. This means there are only five ways the disease could make a comeback:

The SARS virus is hiding in humans. These people would be infected but without symptoms. Stöhr finds this unlikely. Intense, ongoing screening of blood donors and health-care workers in Hong Kong finds no trace of active SARS infection. This fits with the idea that SARS can only be spread by people who are severely ill. "Asymptomatic carriage, if it happens, plays a small role," Stöhr says.

  • Silent transmission. If some people got infected but never had an immune response to the SARS virus, they couldn't be detected by SARS screening tests. "This has not been seen at all," Stöhr says.
  • The virus might get away from a laboratory where it's being studied. Labs studying the virus might store it unsafely. This happened twice. In the first incident, a lab worker in Singapore became infected. He did not spread the SARS virus, even though he came into close contact with 25 other people. More recently, a worker in a Taiwan military lab was accidentally infected in December 2003. This case is more troubling, as the worker traveled to Singapore after becoming infected. A WHO investigation -- including tracing of all contacts -- is under way.
  • A more sinister possibility is the intentional release of the virus. "We do have to be concerned about this virus as it sits in refrigerators around the world," Low says. "I am worried about SARS as a bioterror weapon. It's already been shown to be very effective in bringing health care to its knees."
  • If SARS came from animals the first time, it could happen again. "If the original animal reservoir is not detected, one cannot rule out re-emergence, especially as in China there has been no attempt to segregate exotic animals in the marketplace," Stöhr says. "These animals have been allowed back into the markets and are still a threat." But Low sees this as a sign that the original emergence of SARS from animals was a one-time event. "There is no evidence it ever happened before and -- even though the same circumstances remain in place -- it hasn't happened since," he says.

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