Inter Press Service - April 23, 2003
Stephen Leahy

David Heymann, executive director of the communicable diseases section at the World Health Organization (WHO), says SARS could pose a more serious global health threat than any other new disease in the past 20 years, with the sole exception of HIV, which causes AIDS.

The epidemic had infected 4,000 people and killed at least 228 in 25 countries as of Apr. 22, according to WHO. Roughly 90 percent of the cases were in China and Hong Kong and overall numbers were expected to continue to rise. Thousands of people are currently in quarantine worldwide, and that number could grow dramatically as China struggles with the disease.

In 1918 a new influenza virus killed at least 20 million people before it disappeared. While the worst human epidemic on record, that flu virus was not particularly lethal, killing only three to five percent of those infected. The SARS death rate is estimated at four percent. The Ebola virus is much more deadly, killing half its victims, while HIV kills virtually everyone infected.

In an unprecedented world-wide collaboration, scientists earlier this month identified SARS as a new Coronavirus. "It took three years to find the cause of AIDS and HIV," said WHO spokesman Dick Thompson. "It took eight days to find the cause of this disease."

On the other hand, SARS is operating in a much more fast-paced, interlinked world than that which spawned HIV, Heymann pointed out. Its apparent incubation period of two to 10 days is long enough for infected people who are asymptomatic to travel "from one city in the world to any other city having an international airport".

SARS can also be spread via droplets, excreted when we sneeze or cough, and the initial symptoms - dry cough, headache, fever - are non-specific and common, he noted.

The global cost of the disease is already estimated at 30 billion U.S. dollars, Heymann said.

The family of Coronaviruses, so named because of the distinctive crown that encircles the viral particle, cause 20 to 30 percent of common colds in humans as well as diseases in many domestic animals, including pigs, cats, dogs, cattle, chickens, turkeys, rats and mice.

Viruses are simply small packages of genetic material - DNA or RNA - plus a few enzymes surrounded by a protective coat of protein or fat. On their own, they are relatively inert - unable to move, grow or reproduce.

But what viruses do exceedingly well is hijack the machinery of living cells they come into contact with to make more viruses. Chemicals in their "coat" act as a key to open the protective lock of a cell membrane, giving the virus access to the genetic material.

Once inside, the virus takes over the cell so it can make copies of itself. As soon as enough are created, the new viruses either leak out through the cell membrane or cause the cell to self-destruct, bursting open to release the viral progeny that move on to colonize other cells.

Viruses easily swap genetic information with other viruses or host cells. The constant reshuffling enables them to jump from one species to another. HIV, for example, is believed to have transferred from wild animals in Africa to humans.

Animals are hosts to many kinds of viruses, most of them benign, but a single gene change can make them highly dangerous to other animals or humans. Many flu viruses start in pigs or chickens, undergo mutation and start infecting humans. The SARS virus may be another example.

Controlling a highly contagious virus is much more difficult without a diagnostic test. One that would identify SARS in a person's blood or saliva could be just weeks away after Canada's Michael Smith Genome Sciences Centre in Vancouver recently announced the virus's genetic sequence.

Drug companies "are madly testing all of their drugs that work against AIDS and hepatitis C right now to see if they work on the Coronavirus," said Kathryn Holmes, a Coronavirus expert at the University of Colorado Health Sciences Center in Denver.

But Heymann cautioned that companies' eagerness to develop a vaccine will hinge on how likely it is that SARS will be here for the long-term. The ability to recoup research costs and make a profit will drive their decisions.

"Can we prevent a global pandemic of SARS?" asked U.S. Centres for Disease Control Director Julie Gerberding writing in the Apr. 2 issue of the 'New England Journal of Medicine'.

If civilization is lucky, she said, diagnostic tests and treatment will be found to curtail the epidemic. A seasonal pattern will evolve, allowing scientists to contain infections within regions, and the infection rate will slow.

But if the virus escapes the noose of public health control strategies, the world will be in for a long, difficult struggle, she concluded.

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+WHO (http://www.who.int/csr/sars/en/)

+U.S. Centers for Disease Control (http://www.cdc.gov/ncidod/sars/)

+Michael Smith Genome Sciences Centre (http://www.bcgsc.ca/bioinfo/SARS/)

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