Scouring the Market for SARS

The

civet cat, a nocturnal mammal similar to the weasel, is served in southern China in a variety of ways: roasted whole, braised in brown sauce or standing in for tiger flesh in the classic Dragon, Tiger and Phoenix Soup. (The dragon is snake meat, the phoenix ordinary chicken.) Some diners believe cooked civet has medicinal properties, such as a warming effect during the winter months.

A team of Chinese microbiologists last week confirmed that the civet could indeed produce a unique effect on the human body: it might cause SARS. They've also extracted the virus from a species of wild dog and found antibodies—evidence of an earlier infection—in a Chinese badger. Those results probably confirm the long-dreaded notion that overly close cohabitation of man and animal is brewing up new, fatal plagues. Hong Kong's bird flu of 1997 was just such a creation: a virus harmless in waterfowl that jumped species to infect chickens and then mutated again, killing six people before authorities got it under control by wiping out 1.4 million chickens.

It's a positive development that scientists have managed to identify animals that may be the hosts for SARS. The potentially awful news: SARS might be present in a range of creatures too wide to be culled or controlled easily. "I am not happy (about this development)," says Dr. Yi Guan, a University of Hong Kong (HKU) microbiologist and co-leader of the study team. "I am very worried."

The SARS coronavirus is the 14th known member of a family of viruses named for their distinctive, crown-like shape. Eleven exist in animals—dogs, cats, rats, mice, pigs, cows, rabbits and turkeys—and two infect the human race, in which they produce that most familiar of all ailments: the common cold. Scientists, who have long suspected that humans were originally infected with common-cold coronaviruses by contact with an unknown animal many centuries ago, had already posited a possible animal connection in the current outbreak. The fact that many of the initial victims in China's southern province of Guangdong worked in livestock markets and restaurants was also a promising indicator. After the virus's genome was decoded in mid-April, that assumption looked increasingly likely: SARS was fundamentally different from human-cold viruses and therefore couldn't be a mere mutation. Farmers in Guangdong, who often test positive for antibodies against a wide range of flus, had none for SARS—it was a totally new disease in humans.

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In early May, scientists from HKU and Shenzhen's Center for Disease Control decided to test animals sold in one large food market in Shenzhen, including house cats, hares, beavers and the Chinese muntjac, a small deer. When they examined sputum, feces and blood of the masked palm civet, they hit pay dirt. All six of the civets tested, according to Professor K.Y. Yuen, head of the microbiology department at HKU, carried huge amounts of a coronavirus strikingly similar to the SARS agent. The scientists sequenced its genome and found the two viruses to be nearly identical. The World Health Organization points out that the results don't definitively prove that civets or other animals gave humans SARS. But theoretically, a civet bite or sneeze could infect its owner—so could the handling of a butchered carcass. (The virus is unlikely to survive cooking, so it probably wasn't contracted by a diner in a restaurant.)

The coronaviruses from the civets weren't a precise match for the sars virus: they had 29 nucleotides, the building blocks of the viral RNA, that the human viruses lacked, making them only 99.8% similar. A 0.2% variance, however, could be enough to constitute a significant mutation. In addition, their S genes were different from those of the SARS virus; that gene contains the blueprint for the virus's distinctive spike protein, which interacts with the immune system of the host. Knowing the genetic differences in the two viruses could help scientists develop treatments.

The findings of the Hong Kong and Shenzhen microbiologists are just a start, admits Dr. Zheng Bojian, the other leader of the team. Many more animals need to be studied. No house cats proved positive for the virus in the small-scale study, which was a relief. But, Zheng adds, his team wasn't screening for infected pets. Zheng speculates that SARS could have reached humans from civets through an intermediary host, such as a domesticated animal species with even closer contact with humans.

To solve this mystery and discover the real origins of SARS, scientists like Dr. Yi, the energetic microbiologist who plucked some of the civets from Shenzhen food markets himself by grabbing the slippery cats by the feet, will continue testing animals in the province, widening the net to other species. "The sampling work is very hard and difficult," Yi says. "This could take years and years." One animal they may have trouble finding is the civet. Chinese police have cracked down, and the civet cages were all empty at Guangzhou's sprawling Sinyuan market over the weekend. "You can't buy a civet now for $10,000," marveled one shopkeeper. Bad news for gourmands—but good tidings for the race.