Mar 10, 2005 (CIDRAP News) – The strain of SARS (severe acute respiratory syndrome) that erupted from obscurity to kill 774 people worldwide has gone quiet, at least for now. Yet its lessons will keep benefiting people even if it never reappears.
Some researchers say that the killer strain may now exist only in laboratories. It hasn't been found outside laboratories since July 2003, said Kathryn V. Holmes, PhD, a professor of microbiology at the University of Colorado Health Center at Fitzsimmons in Aurora, Colo.
Holmes recently conducted an extensive literature review on SARS for an overview during an emerging diseases symposium at the American Association for the Advancement of Science's annual meeting on Feb 19.
"The good news is there are good markers for the strains that evolved for human-to-human passage, the epidemic strains," she told CIDRAP News. That's how researchers can identify or rule out that strain.
SARS first emerged in southern China in November 2002 and later spread to Hong Kong and on to many other countries. During the epidemic, which lasted until July 2003, more than 8,000 people contracted the illness worldwide; just under 10% of them died.
Four non-laboratory SARS cases have been identified since the epidemic. An article in the February Proceedings of the National Academy of Science said the four patients in Guangzhou, China, in December 2003 and January 2004 probably acquired the virus through contact with wild animals. None of the patients had a history of contact with other SARS patients, and their viral strains were consistent with strains found in palm civets at the time, Holmes said.
In addition, three laboratory-associated SARS outbreaks occurred from Sept 2003 to April 2004, with nine cases, according to the World Health Organization (WHO) SARS data. As the WHO predicted last fall, things are quiet on the SARS front. Scientists even sound cautiously optimistic about the future.
"The series of events that led up to SARS being readily transmissible is a rare thing," said Frank Plummer, MD, FRCPC, scientific director of the National Microbiology Laboratory in the Public Health Agency of Canada.
The events included a mutation in the virus that apparently allowed it to spread easily from person to person, he said. Also part of the circumstances that probably contributed to the epidemic was the use of civets for food in southern China; the animals were found to be frequent carriers of the virus. Since then, China has worked hard to eradicate the civet meat industry, Plummer said.
Holmes said SARS has likely infected humans sporadically in the past. Thanks to a rare mutation, the virus spread from an animal host to a human, and then survived long enough to develop the ability to spread among people, she said.
Although the events that triggered the SARS epidemic were unusual, it could happen again.
"We don't completely understand SARS. We don’t really have good animal models," said Plummer. "Exactly why some people got so ill and others didn't, I don't think we know."
However, SARS has offered lasting lessons for the public health community on how to address emerging infections. If the same disease re-emerged, the response would be very different, scientists told CIDRAP News.
"I don't think the situations and events that happened during SARS would be repeated," Plummer said. "We've learned those lessons."
For example, Plummer said, China is more open about reporting, many countries have developed surveillance, and scientists have created tools to diagnose and respond to SARS. (The disease first emerged in southern China in November 2002, but the Chinese government was secretive about it for several months.)
Umesh Parashar, MD, MPH, is the lead medical epidemiologist for the SARS task force of the US Centers for Disease Control and Prevention (CDC) in Atlanta. He noted other improvements, including better infection control and training guidelines, improved availability of diagnostic tests and treatment protocols, and new systems for Internet-based reporting.
"We have learned a lot about SARS," Parashar said. "The biggest challenge and uncertainty is around the likelihood of resurgence."
"The wild card in all this is certainly that the virus can evolve or change," he said, adding that there's no clear evidence as to which animal is the virus's primary reservoir. "The data do not convincingly demonstrate that civets are the natural reservoir."
If SARS mimics other zoonotic outbreaks, such as Ebola and Nipah virus, it may disappear and then re-emerge, Parashar said. However, it would require perfect circumstances for the disease to re-emerge in the same form, he added.
Meanwhile, though, SARS has slipped down the threat list. Plummer's team, for example, once had about 30 scientists working on SARS. Most of them have returned to their usual research areas, he said. Five are now studying avian influenza.
"In some ways it's good that [the SARS outbreak] happened," Plummer said. "A fairly mild event taught us a lot about how to prepare for new infectious agents and for pandemic influenza."
Plumbing the secrets of SARS will help in handling other zoonoses. Scientists still have many important questions left, said Holmes.
"Why was it killing people? We have no idea," she said. "SARS has many lessons to teach medical science.
Huai-Dong S, Chang-Chun T, Guo-Wei Z, et al. Cross-host evolution of severe acute respiratory syndrome coronavirus in palm civet and human. Proc Natl Acad Sci 2005;102(7):2430-5 [Abstract]