SARS: Understanding an epidemic
Part II: A daunting disease and its foes
•SARS infection, incubation, death data changing
The World Health Organization intially underestimated the death toll from SARS, especially in regard to older patients. Also, the estimated incubation period may be too short.
The estimated overall death rate for severe acute respiratory syndrome (SARS) is approximately 15 percent, announced officials with the World Health Organization (WHO), an agency of the United Nations. Earlier WHO estimates of the overall death rate were as low as 2 percent. WHO officials also announced that the SARS death rate for patients over 60 years old in Hong Kong hospitals runs as high as 55 percent. WHO maintained its estimate that the SARS incubation period is 10 days, but a study involving only 57 cases that was recently published in the British medical journal The Lancet led some researchers to conclude that the incubation period may be as long as 14 days. Incubation time has major implications for control of the epidemic, including longer isolation and quarantine periods. A longer incubation would also require public health officials to trace an additional four days of patient contacts.
Diarmid of the Anti-Colonial Agitator observes SARS could grow from an epidemic to a pandemic and directs us to the research that explains why and how.
In a separate development, two new epidemiological studies have concluded that SARS is contagious enough to cause a global pandemic if it was not controlled. The researchers tried to calculate how fast SARS spreads, and what might stop it, by analysing data from Hong Kong, Singapore and other outbreaks. Both studies found that in the absence of isolation and other control measures, each SARS case causes on average two to four more cases. In a commentary on the research, the WHO's Chris Dye and Nigel Gay note that a few people, dubbed super-spreaders, shed large amounts of virus and have been known to infect up to 300 people by themselves. That results in a different type of epidemic from one caused by people who infect only two to four others.
The sample populations for making decisions about SARS are still small, but this information could be useful in managing future outbreaks.
•Successfully probing SARS
Insights into SARS have occurred almost as rapidly as the disease has spread and killed. Denise Grady and Lawrence K. Altman of the New York Times have written an article tracing the emergence of the disease and the invesigation of it.
SARS first came to the world's attention in mid-March, and only a week later, scientists isolated the virus that appeared to be causing it. A few weeks after that, two teams decoded the viral genome, providing information that could help to develop diagnostic tests, vaccines and antiviral drugs and to find out where the virus came from. Last week, scientists pinpointed a possible source of SARS — civets, badgers and raccoon dogs being sold for meat in China's Guangdong Province — and were able to compare the gene sequence of the animal virus with the one found in people.
Medical sleuths had difficulty obtaining samples of the virus because of cultural traditions in the impacted areas. They worked with very small samples of tissue, sputum or blood. Still, on March 21, an electron microscope specialist at the CDC, Cynthia Goldsmith, made a surprising discovery -- SARS appeared to be a coronavirus.
Although coronaviruses made animals very sick, in people they were known to cause only colds and gut trouble, not serious diseases like pneumonia. They had not even been mentioned as a possible culprit in SARS. And most did not even grow in Vero cells.
The clearest match via antibodies was to cats -- felines, that is. But, additional research revealed other animals had similar antibodies to the coronavirus.
By mid-April, two laboratories had mapped the genome of the SARS virus. First to finish was the British Columbia Cancer Agency in Vancouver followed by the C.D.C. The findings confirmed what initial studies had suggested: the virus was different from any known coronavirus, different enough, in fact, to become the first member of a new grouping of coronaviruses.
. . .Before SARS, few researchers studied coronaviruses. Known for making chickens cough and giving pigs diarrhea, coronaviruses were not seen as a path to scientific glory. In people they were thought to cause only mild diseases, and many researchers found them unexciting, difficult to grow and generally not worth the bother. Before SARS, one scientist said, coronaviruses were a "sleepy little corner of virology."
The leading researcher of coronaviruses, Dr. Kathryn V. Holmes, a professor of microbiology at the University of Colorado Health Sciences Center in Denver is focusing her attention on several inquiries, as are other experts in the field:
•How do coronaviruses get into cells?
•"Why is there a coronavirus of chickens, and one of dogs, and one of rats, and why don't they infect each other?"
•The virus to lock on to a molecule on the cell surface called a receptor. What is the receptor for the SARS virus?
•Is the burglary tool of the coronavirus, a protein spike, a mutation? (If so, that would explain the transfer to another species.)
However, the implications of this research reach farther than SARS.
Finding the origin of SARS, whether it is the civets and other animals or some other host, may also help researchers figure out how the virus evolved and how it found its way into people, Dr. Holmes said. That question extends far beyond SARS, to the larger problem of emerging infectious diseases, a category that includes scores of infections like West Nile encephalitis, hantavirus, Lyme disease and AIDS.
A process that may have begun when someone in Asia ate civet cat meat for dinner could end with breakthroughs in understanding and treating several of the most resistant diseases known.