Genetic testing of animals sold as delicacies in a southern Chinese market confirms suspicions that the deadly SARS virus jumped from animals to people, Chinese researchers said on Thursday. The researchers found clear differences between the animal and human versions of the virus, but said they were minor enough to show that SARS jumped from animals, as influenza and other viruses have done.
“It's a landmark discovery,” said Kathryn Holmes of the University of Colorado Health Sciences Center, a leading expert on the coronavirus family that SARS belong to.
“The question that everyone has had is, ‘how did this virus appear in the human population?”’ Holmes added in a telephone interview. “This is the first major clue about where the contact might have come from.”
Severe acute respiratory syndrome, or SARS, emerged in southern China last November. Its symptoms include severe pneumonia.
Between November and May SARS infected an estimated 8,500 people in 30 countries, killing more than 800. It spread as far as Toronto, killing 41 people in Canada.
SARS had never been seen before and it took several months to determine that it was caused by a coronavirus, a member of a family of viruses that causes cold-like symptoms in people and a range of diseases in animals.
But the SARS coronavirus is genetically unique and the search was on to find its source.
Yi Guan and colleagues at the University of Hong Kong tested eight animals taken from a market in Shenzhen. They also tested 95 workers at the market and patients hospitalized for diseases other than SARS.
The lead suspect was a palm civet, a raccoon-like animal valued as a delicacy. They also tested other animals including a raccoon dog and a ferret badger.
SIMILAR VIRUS FOUND IN ANIMALS
In the animals they found antibodies to a coronavirus similar to SARS and the virus itself.
“Since we detected SARS-like coronavirus in several different species of animals from the market, it suggests that markets may be an important source of human SARS,” Guan said in an interview conducted by e-mail.
Guan said it is too early to say whether the civet or some other animal was the primary source of the epidemic.
“This paper doesn’t prove that the animal host is a palm civet. It says they also could be infected with related virus. They could have gotten it from eating a mouse or something like that,” said Holmes, who helped confirm the findings.
Guan’s team also found that many of the workers had antibodies to the virus but had never been sick. None of the hospital patients tested for SARS had any evidence of exposure to the virus.
Having antibodies to a virus suggests that a person was exposed but the immune system fought it off.
Holmes said an important question is whether the antibodies were to human SARS or to the slightly different animal virus. “Was it mutated or not mutated?” she asked. “Had it received the mutations that allowed it to transmit to humans or not?”
Writing in the journal Science, Guan and colleagues said they found an important clue. The human version of the coronavirus was missing a genetic sequence — a long one —that suggest what changes were needed to make the virus infect humans.
One human patient was infected with an intact virus, but most of the others were missing this sequence, they reported.
Guan said it was not clear what this genetic sequence might do — whether it affected the ability of the virus to infect, or its virulence.
New cases of SARS have not been reported for several months. But health experts fear it could return in the autumn because other known coronaviruses are seasonal, as are numerous cold viruses and the influenza virus.