SARS  pg. 3

Confocal immunofluorescence image of cultured mouse brain tumor cells that have been infected with the mouse hepatitis virus, a coronavirus. The culture was mixed with a fluorescence-labeled antibody that specifically attaches to a viral enzyme. Confocal microscopy visualizes infected cells and virus-induced syncytia, or cell fusion, which enables the virus to slip from one cell to another.
Sarah Brockway
Besides the inherently interesting biological features of the coronaviruses, Denison says, they are important and widespread pathogens. Coronaviruses have long been known to cause severe disease in animals, particularly pigs, calves, and chickens. Two human coronaviruses are responsible for between 20 and 30 percent of cases of the common cold.

“They were important human pathogens; they just weren’t severe or critical human pathogens,” Denison says of coronaviruses before SARS.

But coronavirologists like Denison recognized the capacity of these viruses for trans-species adaptation. Over the last decade, he says, accumulating evidence has shown that coronaviruses can move between species “without too much fuss.”

So when an ordinary coronavirus took a leap to human beings—most likely from a still unidentified animal source—and caused SARS, “I think coronavirologists were amazed, but not surprised,” Denison says.

The leap appears to have happened in the southern Chinese province of Guangdong, where SARS-like illnesses occurred before the epidemic was acknowledged. Retrospective studies of patient records by Chinese and WHO epidemiologists have identified independent clusters of cases in seven Guangdong municipalities between November 2002 and January 2003.

The absence of a link between these clusters adds weight to theories that the virus jumped to human beings from an animal species or other environmental reservoir in southern China, according to WHO.

The civet connection

In Guangdong China, wild animal markets and restaurants cater to the population’s penchant for exotic fare—a made-to-order situation for putting people in contact with unusual animal viruses. Suspect animals in the SARS jump include the masked palm civet, a relative of the mongoose, and the raccoon dog, which are both consumed as delicacies in southern China and have been confirmed to be infected with the SARS coronavirus. In fact, the genetic sequence of the virus isolated from captive civets was nearly identical to that from the first confirmed SARS patient this year, prompting Chinese officials to order the killing of all civets—estimated at 10,000 animals—in the region to protect against further SARS cases.

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