LONDON – Researchers in Hong Kong have found evidence of the SARS (search) virus in three small mammals, including a civet cat that is eaten as a delicacy by some Chinese.
But the World Health Organization (search) said Friday it cannot rule out the possibility the animals acquired the virus from humans, or that the virus jumped to humans from another animal altogether.
"It's certainly too early to draw final conclusions on those findings, but they are clearly quite exciting," said Dr. Francois Meslin, a WHO expert on diseases acquired from animals.
Meanwhile, health officials in Canada announced a new cluster of 20 or more possible SARS cases in Toronto little more than a week after the WHO said the biggest outbreak of the illness outside of Asia appeared to have been snuffed out.
The U.S. Centers for Disease Control (search) reinstated a travel alert that warns Americans to take precautions when traveling to the city but stops short of advising against trips to Toronto. The WHO said it would await further information before deciding whether to put Toronto back on its list of SARS-affected areas.
The Hong Kong findings were announced hours after WHO lifted its travel warning for Hong Kong and the southern Chinese province of Guangdong, saying outbreaks of severe acute respiratory syndrome in those areas are now under control.
Researchers from the University of Hong Kong examined 25 animals belonging to eight species in a live animal market in southern China that supplies restaurants in Guangdong province, where the SARS outbreak is believed to have started.
Six of the animals tested were masked palm civets, which look like long-nosed cats but are related to the mongoose. All the civets, which came from several different owners and appeared healthy, tested positive for a SARS-like virus, said Dr. Klaus Stohr, WHO's chief SARS virologist.
One raccoon dog — a member of the dog family native to eastern Asia — was tested and found to have the virus in its feces. Antibodies against the virus were also found in the dog.
Antibodies were also found in the lone badger that was examined, Stohr said.
The results from the civets were considered the strongest, Stohr said, because the virus itself was found in their bodies, they carried antibodies and they all tested positive despite having different owners and being kept on different sides of the market.
"That's a relatively strong indication that these animals may play a particular role," Stohr said.
The scientists unraveled the genetic code of two of the virus samples from the civets to see how closely they matched the human SARS virus.
"These viruses were almost identical to the human virus. The only difference is that they have 29 extra amino acids," Stohr said.
The scientists also found that blood from the animals inhibited the growth of the human SARS virus in a laboratory. Similarly, blood from people who had recovered from SARS hindered the growth of the virus from the civets.
"This cross reaction is another very strong indication that these viruses are very, very, very close. They are practically identical," Stohr said.
Although experts are confident the animals had the SARS virus, it's unclear whether the animals are the source of the human outbreak.
One of the study's leaders, microbiologist Yuen Kwok-yung, told reporters in Hong Kong he believed the virus jumped from the civets to humans, but WHO experts were not convinced.
A recent study from Guangdong indicates more than 30 percent of the early SARS cases in Guangdong occurred among food handlers. Stohr said the latest findings could provide evidence of a possible link between wildlife and the emergence of SARS.
Experts say that if humans caught SARS from an animal, it probably came from preparing a carcass for eating — not from eating it since cooking would likely kill the virus.
However, the market study could not determine whether civets are so-called "reservoirs" — the species that maintain the SARS virus in nature. It's possible they play some other role in the chain of transmission from animals to humans, Meslin told reporters in a telephone briefing.
Some viruses that have jumped from animals to humans involved multiple animals playing different roles. While one species may be the reservoir, others can be what is known as "amplification hosts," which become infected by the reservoir species and then pass the virus on to humans.
"We cannot say those animals are the source of the initial cases of SARS," Meslin said. "We really need to investigate more the relationship between the different animal species found to have the virus, and maybe others."
"These animals can consume small mammals, particularly rats and mice, and those could be the source of the infection, so for the time being it's all conjecture," he said.
Stohr said it's possible the animals could have all become infected through feed at the market, or that they infected each other. It's also theoretically possible the animals contracted SARS from humans, perhaps by a sick person coughing into the cages, he said.
"We have to narrow down the animal reservoir. These animal species should be tested in different markets, that's the first thing," Stohr said. "In addition, other animal species should also be tested."
"Until the role of these animal species is better defined ... owners and others who might come into contact with these animals or their secretions should be aware of the possible health risk," he said.
In Hong Kong, Yuen said it was important the civet trade be closely watched.
"If you cannot control further jumping of such viruses from animals to humans, the same epidemic can occur again," he said. "So it's very important that we have ways of controlling the rearing, the slaughtering and the selling of these wild game animals so that such an epidemic will not occur again."