Understanding SARS: One Year Later
U.S. suffered far fewer cases than thought
MONDAY, Jan. 26, 2004 (HealthDayNews) -- The first SARS epidemic rocked the world's consciousness one year ago, killing 813 people and sickening 8,437 others in 32 countries before running its course late last spring.
It proved particularly dangerous in Asian countries such as China, Singapore and Thailand, and to a lesser extent, western nations such as Canada. And the disease has not disappeared entirely, with three suspected cases reported recently in Guangdong, a southern province of China.
In the United States, health officials had reported 409 suspected cases of sudden acute respiratory syndrome by last June. Now, those estimates have been revised downward dramatically, to eight confirmed cases, 19 "probable" ones and 134 "suspected" cases.
What happened to the original high number? For the most part, it had to do with the definitions used by doctors struggling to contain a new and dangerous disease.
The initial case definition for SARS was purposely broad so investigators worldwide would not miss any cases, explains Dr. Gio Baracco, an assistant professor of infectious diseases at the University of Miami School of Medicine. This sweeping definition was very useful in areas where there actually was an outbreak, such as China and Toronto.
"We were not really having an outbreak of SARS in the U.S., so when we applied the case definition, the majority of cases that met it did not have SARS," Baracco says. "The more of the disease you have around, the better your case definition."
Doctors now know that SARS usually begins with a high fever -- a temperature greater than 100.4 degrees. Other symptoms may include headache, an overall feeling of discomfort and body aches. Some people also have mild breathing problems at the start, and about 10 percent to 20 percent of patients have diarrhea. Most victims develop pneumonia, federal health officials say.
The number of U.S. cases shrank even further once a test for the coronavirus that causes SARS became available.
"As you might expect, you've got lots of respiratory illness around, and SARS is just a small portion of the total," says Dr. Larry Anderson, chief of the respiratory and enteric viruses branch at the U.S. Centers for Disease Control and Prevention's National Center for Infectious Diseases.
So why was there no outbreak to speak of in the United States? Probably a combination of good luck and good infection control, Anderson says.
An article in the February issue of Emerging Infectious Diseases, which is devoted to SARS, found virtually no secondary transmission among the eight confirmed cases in the United States. One SARS patient was the wife of a California man who also came down with the disease, and had traveled with him to Hong Kong, a SARS hotspot. So it's not clear whether she acquired the infection from her husband or while in Hong Kong.
Even in high-risk areas such as China and Singapore, transmission patterns varied greatly.
Several of the papers in the journal remark on how the disease spread less rapidly in households than it did in hospitals. One study in Singapore found an infection rate of only 6.2 percent in households but more than 50 percent in some hospitals. The study also concluded the virus was less likely to spread in a household if the patient was a health-care worker and if the patient was young. The reasons for this aren't entirely clear.
"We suspect that there is a difference in social behavior between health-care workers and non-health-care workers. And this could possibly explain the difference in risks of household transmission," says Dr. Denise Li-Meng Goh, an assistant professor of pediatrics at the National University of Singapore and co-author of one of the studies.
"For example, the health-care worker may be more acutely aware of his/her risk of acquiring and transmitting SARS, and hence implement hygiene practices at home. In addition, their better health and disease prevention knowledge may influence the efficacy of such practices," Goh says.
Such studies highlight the importance of tracking patients' movements and their contacts with other people.
"Early and complete tracing was important," says Joseph T.F. Lau, director of the Centre for Epidemiology and Biostatistics of the School of Public Health of the Chinese University of Hong Kong.
"International transmission across the border is also very important in our current scenario as there are a number of suspected cases reported in Guangdong, China, a common destination for Hong Kong travelers," adds Lau, a co-author of one of the papers. "Some measures had been implemented, but I think that they may not stand against the test... Education for international travelers [is] required."
There was a time when SARS was unable to infect humans, or managed to infect them but didn't make them sick. Related viruses have been circulating in wild animals for hundreds and perhaps thousands of years, reports another paper in the journal.
The stage was set for human infection about 15 years ago, when wild animal markets started springing up in China's Guangdong province. At first, the virus spread only to closely related animals -- from civet cat to cat, for instance. But this sped up the virus' mutation rate, making it just a matter of time before the germ had adapted for human infection.
One paper in the journal found evidence that people had been infected with SARS in 2001, two years before the world had ever heard of the disease. Antibodies to SARS-associated coronavirus (human and/or animal) were detected in 17 of 938 adults tested in Hong Kong.
"The infected persons might have no symptoms, or mild symptoms that they had not been aware of, because the virus had not adapted well in the human hosts," says Dr. Bojian Zheng, principal investigator of this study and an associate professor of microbiology at the University of Hong Kong.
"The extensive exposure of the viruses and rapid virus evolution would then facilitate human-to-human transmission," Zheng adds. "Once some mutants adapted to grow in particular persons, the SARS outbreak occurred, which was what happened last year."