SARS Epidemic Caused By Evolution Of Single Coronavirus Lineage

NEW YORK (Reuters Health) - During the 2003 SARS epidemic in China, the SARS coronavirus (SARS-CoV) isolated during different phases appears to represent different evolutional stages of the same viral lineage.

To investigate SARS-CoV adaptation to humans, Dr. Guo-Ping Zhao and members of the Chinese SARS Molecular Epidemiology Consortium analyzed viral genomic sequences of isolates obtained throughout the epidemic. Their findings were published January 29th on the Sciencexpress Web site.

The early phase of the epidemic involved 11 individuals from different geographical locations within Guangdong Province, where exotic animals are part of the diet. Seven of these cases had documented contact with wild animals. Two major genotypes predominated, whose sequences were similar to those of coronavirus infecting other mammalian hosts.

The second phase started with the first "super-spreader event (SSE)," associated with more than 130 primary and secondary infections in the city of Guangzhou.

The late phase involved an outbreak in Hong Kong, traced to a physician who worked with patients in Guangzhou who visited Hotel M.

During the second phase, the authors found that the SARS-CoV sequences contained a new 29-nucleotide deletion that dominated the viral population for the rest of the epidemic. The non-synonymous mutation affected the S protein, which is responsible for virus-host interactions, the report indicates.

"We observed that the epidemic started and ended with deletion events," Dr. Zhou's group writes, "together with a progressive slowing of the non-synonymous mutation rates and a common genotype that predominated during the latter part of the epidemic."

Dr. Earl G. Brown, a microbiologist at the University of Ottawa whose research encompasses viral evolution, virulence and virus-host interactions, discussed the report with Reuters Health.

"Their findings point to the fact that SARS was a focal event, something that happened once," he said. The observed genome deletions "can only go one direction," thus proving that all the isolates evolved from the same viral lineage.

He noted that the single case of SARS reported in China last December was associated with a virus whose genome sequence was distinct from that seen in the epidemic, and more like that of virus isolated from civets. "This suggests a new host interaction, that it was not the old virus that was still kicking around."

It probably would not have been recognized as SARS if healthcare providers had not been sensitized to recognize the syndrome, he suggested. "It would have been seen as just another pneumonia."

In fact, this last case may represent a common spontaneous event, Dr. Brown said, where isolated cases come from virus that is not well adapted for human hosts, resulting in "a flare of cases followed by burnout."

On the other hand, the possibility exists that a new SARS coronavirus could adapt to humans so well "that it would cause a sustained epidemic in human populations that would persist for perpetuity."

Dr. Brown believes that this paper highlights "the big frontier for microbiologists, which is to interpret genotypes of viruses [as they are identified] so we can say where they come from and what their properties are."

www.sciencexpress.org/29 January 2004.