China Is Perfect Breeding Ground for Viruses Like SARS, Expert Says
for National Geographic News
|May 6, 2003|
For a virulent virus seeking to jump from animals to humans China's Guangdong province may be the perfect habitat.
Along the highways, ubiquitous farms are lined up next to each other, with farmers tending their ducks, chickens, and pigs in teeming and cramped quarters. In the city's food stalls, meanwhile, vendors keep their meatalive and deadin cages and baskets stacked on top of each other. Customers can choose from a menu of rats, cats, dogs, frogs, snakes, and exotic birds.
In such unhygienic conditions, scientists have long warned, it's just a matter of time before a hidden and potentially lethal virus would make the leap from animal to man. It should come as no surprise, then, that one virusthe enigmatic coronavirus that causes severe acute respiratory syndrome, or SARSin all likelihood did.
Among the first people to die from SARS, when the epidemic began in the southern Guangdong province late last year, were food handlers. Since then, at least 6,583 people in 30 countries have been infected with SARS. As of yesterday, the disease has claimed at least 461 lives.
China has an unfortunate history of producing new viral strains. Two devastating influenza pandemics, in 1957 and 1968, both originated in China, each killing more than a million people. Experts maintain that outdated farming practices, overpopulation, and even political secrecy may be to blame.
"China is the perfect breeding ground for new viruses," said Christoph Scholtissek, a German virologist whose research has linked China's traditional farming system with the emergence of lethal human viruses.
Although researchers havein record timemanaged to crack the genetic make-up of SARS, the mysterious virus doesn't match any known animal or human viruses. In other words, scientists still don't know the source of SARS.
Many emerging infections already exist in nature. They may enter humans as a result of changed ecological or environmental conditions that place humans in contact with previously inaccessible pathogens or the natural hosts that carry them.
"The most likely scenario is that [SARS] has been circulating in another species in southern China, and human beings came in contact with it this past autumn," said Stephen Morse, an epidemiologist at Columbia University in New York and author of Emerging Viruses.
Known as "zoonosis", the virus transfer from animals to humans has long been established. Foreign microbes can be lethal because the new host may not have the immunity that has built up over time in the original species.
Some virologists believe traditional farming practices in China help spread new viruses. Chinese farmers raise ducks, pigs, and fish in one integrated system, and the animals may exchange viruses through their feces.
Ducks can pass avian flu viruses to pigs, where two viruses can mingle and form a new strain that is passed on to the farmers. The pigs, which have a genetic make-up similar to humans, act as the "mixing vessels."
Eurasian avian influenza viruses have been recognized as the precursors to the influenza virus genes that re-assorted with human strains to generate the 1957 and 1968 flu pandemics.
However, Scholtissek, who linked China's agricultural practices with those influenza pandemics, said it's unlikely the same thing happened with SARS. While the flu virus' genome is divided into eight separate strands, which can break up and easily mix and match individual pieces to come up with brand new variants, the SARS virus consists of a solid genome.
But very little can be said with certainty about the SARS virus, partly because it's believed to be mutating all the time. Hong Kong scientists this weekend said the SARS virus is mutating rapidly into at least two forms.
"This rapid evolution is like that of a murderer who's trying to change his fingerprints or even his appearance to escape detection," said Dennis Lo, a chemical pathologist at the Chinese University of Hong Kong.
There is little doubt that crowded conditions, like those in China, play a major part in the spread of SARS. Shoppers in Guangdong province, which has a population of 76 million, throng the busy markets for fresh meat. The need to feed a growing population has greatly increased the number of animals raised.
"Viruses transfer when you have a density of population of people and animals," said Ian Jones, a professor at the School of Animal and Microbial Sciences at the University of Reading in England. "Given that about a quarter of the world's population is living in China, things are just bound to pop up there."
As people expand into ever-wilder areas, humans are coming in closer contact with wild animals. In southern China, wild and exotic animals like snakes and birds are treated as delicacies with many people believing they are good for one's health.
Michael Lai, a professor at the University of Southern California in Los Angeles and an expert on coronaviruses, believes that SARS existed in a wild animal, most likely a wild bird, before transferring to humans.
"The SARS viral sequence is different from any known human or animal," said Lai. "The sequence and genome structure of the SARS virus most closely resemble those of an avian coronavirus, [though] it also has a hint of a murine [or rodent] coronavirus."
Add to the mix the Chinese government's capricious handling of the SARS outbreak. Although doctors in southern China were aware of the epidemic as early as January, information about the disease was kept secret by the government for months. It only began to emerge at the beginning of April, under intense international pressure.
For months, doctors in places as far-flung as Canada scratched their heads as new cases of a mysterious pneumonia-like disease arrived at their hospitals. Health officials now charge that the epidemic could have been contained sooner if the Chinese government had been more forthcoming with information about the epidemic.
In a rare public disclosure of failure, Chinese officials late last month admitted that infection rates were many times higher than initially reported, and the government fired both the health minister and the mayor of Beijing.
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