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Health Officials Struggle to Understand SARS

Bijal P. Trivedi
National Geographic Today
April 22, 2003
 
Severe acute respiratory syndrome (SARS) has terrified the world. The question on everybody's mind is whether SARS will become a global epidemic.

As of today, the World Health Organization (WHO) reported 3,947 cases of SARS. Of these 229 people, mostly in China, Hong Kong, Singapore, and Canada, have died.

Researchers worldwide have galvanized to combat SARS. The challenge for public health is to determine the nature and potential of any viral threat and to frame a battle plan.

"We are in an evolutionary stage of this epidemic," said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases in Bethesda, Maryland. "It could plateau, go up and down, disappear or explode. This virus is highly virulent, potentially lethal and highly transmissible—it has the potential to cause a really bad epidemic."

Currently the death rate from SARS hovers around five and a half percent—which makes it about 50 times more deadly than the average annual flu. But as diagnostic tests for SARS improve, says Fauci, medics may find that many more people have been infected by the virus, with no serious consequences, and the death rate will drop.

The World Health Organization has identified the nature of SARS. The U.S. Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, and the British Columbia Cancer Agency in Vancouver, Canada, have cracked the genetic code of the virus.



"It's encouraging that we have the rapid identification of a completely novel virus, the proof of causality, the sequencing of its genetic code, the generation of preliminary diagnostic tests and initial drug screening," Fauci said.

A Deadly Combination

SARS is a so-called hybrid virus—the kind that always triggers a red alert. Traditionally a virus affects a single species. But sometimes two viruses combine their genetic material and form a new virus that "jumps" to another species altogether.

Hybrids are dangerous because the body has never encountered them before, and the immune system is unprepared.

"From studying the sequence we see that the SARS virus is derived from a mouse coronavirus and an avian coronavirus," said Michael Lai, a pioneer in coronavirus genetics at the University of Southern California in Los Angeles.

Until now, though, coronaviruses that infect humans have not been lethal. They inflict the minor misery of the common cold. But will SARS turn the coronas into mass killers?

If the virus exists in a wild widespread host—like the West Nile virus carried by mosquitoes—then SARS could be a perennially recurring problem, according to Lai.

If the host is a domestic animal—like the chicken responsible for the Avian Flu virus of 1997—then the source of the virus could be eliminated. The Avian Flu effectively stopped after the Hong Kong government ordered the slaughter of chickens in the area.

Lai suspects that the SARS virus lurks in a wild animal. "It probably doesn't bother domestic animals or we would have come across it before," he said.

If authorities cannot stem the spread of the disease in people, SARS could become endemic in humans—like the flu, said Alison Galvani, a research fellow at University of California, Berkeley, who models the evolution and epidemiology of infectious agents.

Conversely, if people are the only carriers of SARS, "honest reporting of SARS cases and stringent quarantines could [potentially] stop the virus in its tracks," Lai said.

Knowing a Virus, Predicting an Epidemic

"Making a vaccine could be quite straightforward, if it is needed," Lai said. Vaccines already exist for swine and chicken coronaviruses.

But it isn't easy to gauge whether SARS, or any virus, can trigger an epidemic.

"Generally, we can't predict virulence or even whether a virus can infect humans," said Stephen Morse, director of the Center for Public Health Preparedness at Columbia University in New York City and editor of the book Emerging Viruses.

"We've been working with influenza for over 50 years and it can still evade our best attempts."

"In many cases of the influenza virus we don't understand the molecular properties that cause high mortality," said Nancy Cox, chief of the influenza branch at the CDC.

Producing a new influenza vaccine every year requires year-round surveillance. Flu infects between 600 million and 1 billion people around the world every year. Disease centers from 83 countries collect about 175,000 virus samples.

From those the researchers pick the three worst threats as the basis for next year's vaccine. Scientists have correctly predicted the three predominant strains of influenza virus for eight of the past ten years.

"Outbreaks are like wildfires," Morse said. "Some fires smolder and are easily extinguished. Others rage out of control, destroying everything in their path. The WHO and the CDC are like the fire department and they need to investigate all outbreaks because you never know which way they could go."

Morse points out our long history with influenza. SARS is only a few weeks old. Tracking and containing viruses, and predicting what they will do, Morse said, "is as much an evolving science as an evolving art."



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