SARS  pg. 5

They found multiple SARS coronavirus strains present during the early phase, with wide variation in the outer spike protein used for viral attachment to host cells. As the epidemic progressed, the spike protein sequence stabilized, presumably to the form with the greatest capacity for infecting human cells, according to the team led by Dr. Guoping Zhao of the Chinese National Human Genome Center in Shanghai.

The remarkable speed with which the SARS coronavirus adapted to human hosts underscores the importance of having robust public health systems in place that can recognize and defeat emerging viral threats before they sharpen their human attack skills.

Since the end of last year’s epidemic, there have been five confirmed SARS cases—two researchers handling laboratory samples and three members of the public in China’s Guangdong province. Quarantine measures have apparently been successful -- none of the cases became the focus of a new epidemic, and all of the victims recovered.

Much to learn

“We really don’t understand why SARS came up in the first place, or where it’s gone,” Denison says. “There is certainly still a risk that it will reemerge as a severe pandemic disease, and based on that, there’s a need to understand the virus and its emergence, biology, pathogenesis, treatment and prevention.”

SARS has been called a respiratory illness -- patients have usually presented with flu-like symptoms of fever, chills, aches, and coughing or breathing difficulty. Some developed hypoxia, with 10 to 20 percent of cases requiring mechanical ventilation. Most developed pneumonia. It appeared to spread by close person-to-person contact, probably involving respiratory droplets. But other features—a high incidence of diarrhea, the prolonged (seven- to 10-day) incubation period, and the mild disease in children—suggest to Denison that SARS might be a systemic disease, like measles, with a severe respiratory manifestation.

“We don’t fully understand the pathology of this disease,” he says.

So the world watches and waits. Surveillance programs, especially in regions that were hardest hit by SARS, aim to swiftly detect and isolate suspected SARS cases. In Hong Kong, for example, where SARS sickened 1,755 people and killed 299, every passenger entering or leaving the city—by any route—has been required since last summer to fill out health forms and pass in front of infrared cameras that measure the temperature of skin and clothing. Anyone with a fever must see a doctor.

But protracted surveillance of this level is an arduous prospect, and it may not catch the single case that starts a new epidemic.

“Surveillance is a difficult thing; formal surveillance programs are often not located in the right place at the right time,” says Larry Anderson, M.D., director of the Division of Viral Diseases at the U.S. Centers for Disease Control and Prevention. Instead, for SARS and other emerging infectious diseases, the CDC and WHO rely on what’s called the “astute clinician concept”—the idea that practicing physicians notice something odd, talk about it, pursue it and bring the information to the public health community.

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