Experiments in monkeys have confirmed the identity of the virus that causes severe acute respiratory syndrome, the World Health Organization announced Wednesday, an important step toward developing new drugs to combat the disease.
It will also help scientists trace the evolution of the virus and could help them determine whether it jumped from animals to humans, and if so, from which animals. Tests are under way in pigs and poultry to see how susceptible those animals are to SARS.
SARS, which emerged in China in November, has sickened 3,293 people in 22 countries and killed 161.
Scientists have now determined it is caused by a new member of the coronavirus family, so named because a crown shape is seen when the viruses are inspected under a microscope.
Scientists had been almost certain the new form of coronavirus first isolated from sick patients March 21 by the University of Hong Kong was the cause of SARS. But they could not say for sure until they had satisfied what is known as Koch's postulates -- four scientific tests that verify whether a virus causes a certain disease.
"The Koch's postulates have been fulfilled, so we can now say for certain that the new coronavirus is the cause of SARS," said Dr. Klaus Stohr, a World Health Organization virologist who is coordinating the scientists' work.
The first test requires that the virus be found in all the sick people, but not in healthy people. The second isolates the virus from a sick patient and shows that it multiplies in a lab dish.
The third step uses the virus from the petri dish to make a lab animal sick with the same disease as that seen in humans. The final step requires isolating the SARS virus from the sick lab animal and showing it can grow in a petri dish.
A team led by Dr. Albert Osterhaus, the director of virology at Erasmus Medical Centre in Rotterdam, Netherlands, carried out the final two verifying steps.
Early in the hunt for the cause of SARS, scientists found a virus belonging to the paramyxovirus family in some patients. It was later determined that this was the human metapneumovirus, which is known to cause respiratory problems in children, the elderly and people with weak immune systems.
A few days later, scientists in Hong Kong found the new coronavirus, providing a new track for researchers to pursue.
This prompted a theory that perhaps both viruses play a role, with one causing the disease and the other making it worse.
Osterhaus said they infected two groups of monkeys with either the coronavirus or with the human metapneumovirus. In a third group, the monkeys were infected first with coronavirus, then with the metapneumovirus.
"The animals infected with the coronavirus alone developed the full-blown disease. They developed clinical symptoms and the lesions that are identical to what we have seen in people who have died from SARS," he said. "The animals infected with human metapneumovirus developed only very mild symptoms and definitely not the typical SARS pattern."
The third group did not develop a more serious version of SARS, Osterhaus said.
"The conclusion today was that the coronavirus alone is capable of causing the typical symptoms," he said.
Symptoms of SARS include fever, shortness of breath, coughing, chills and body aches.
The findings were announced in the middle of a daylong meeting at the WHO's Geneva headquarters of scientists from laboratories around the world working together to find the cause of SARS and tests to diagnose it.
WHO said the scientists agreed Wednesday to name the new virus simply SARS virus, despite earlier proposals that it should bear the name of Dr. Carlo Urbani, the WHO doctor who first alerted the world to the existence of SARS in Hanoi, Vietnam, and who died from the disease on March 29.
The U.N. health agency said the lab network paid tribute to Urbani by dedicating to him its work in tracking down the virus.
The scientists stressed that although the SARS virus is part of the same family of viruses that cause the common cold, it is quite different from the common cold virus. Genetic studies have indicated that the SARS virus shares some similarities with the mouse hepatitis virus and the avian infectious bronchitis virus, which come from a different branch of the coronavirus family.
Experts said it is reasonable to imagine that the SARS virus came from animals, although its genetic code does not give any clear leads as to exactly where it came from. The genetic makeup is not very close to any of the known animal or human coronaviruses, they say.
Dr. Masato Tashiro, director of the National Institute of Infectious Diseases in Tokyo, said he believes the virus has probably existed for a long time in animals in the southern Chinese province of Guangdong, where SARS was first detected.
WHO experts in China said Wednesday they discovered unreported SARS cases in Beijing military hospitals but had been barred from giving details. Another WHO official estimated there had been 100 to 200 cases in Beijing since March; the official total is 37.