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The W.H.O. has organized a network of 11 laboratories around the world, including the Centers for Disease Control and Prevention in Atlanta, to seek the cause of SARS and help develop diagnostic tests. But in the absence of a diagnostic test, the network is working on the assumption that the coronavirus causes SARS. It is a gamble the researchers are unhappy to be taking. The C.D.C. has developed two promising but rudimentary tests. But the tests need substantial refinement before they can be used widely, in part because they cannot detect infection from the new coronavirus in its earliest stages. Also, the tests must be validated by tests on thousands of additional specimens from SARS patients and healthy people, the C.D.C. has said. Still, late last week C.D.C. began releasing to state health departments findings from experimental tests conducted among suspect cases in this country. The tests measure the amount of antibodies that these individuals' immune systems formed to fight the new coronavirus. But the C.D.C. cautioned against interpreting that the findings proved the virus caused SARS and that these individuals had it

While SARS has spread easily to health workers in China and Hong Kong, Dr. [Julie L. Gerberding] said the patterns have been different in the United States. In this country, only three health workers appear to have contracted SARS from patients. One important reason is that hospitals quickly put isolation precautions in place after the W.H.O. issued a global health alert on March 15. Dr. [David L. Heymann], the W.H.O. communicable disease expert, said an unusual cluster of 29 SARS cases in Singapore was ''particularly worrisome'' because its source was unknown. Singapore health officials classified 4 cases as probable and 25 as suspected; all were health workers from two wards of one hospital where there had been no known SARS cases. The clustering seems to have occurred on March 29. Dr. Heymann said his agency was concerned about that finding because it would be highly unusual for so many workers to have caught SARS from one patient on the same day. No SARS patient was transferred to the new hospital from another one that had cared for earlier cases, Dr. Heymann said

This article was reported by Elisabeth Rosenthal, Keith Bradsher, Lawrence K. Altman, Clifford Krauss, Donald G. McNeil Jr. and Amy Harmon and written by Denise Grady. *** Last November in Foshan, a small industrial city in Guangdong Province in southern China, a businessman became desperately ill with an unusual type of pneumonia. Doctors could not identify the germ that was making him sick. Ominously, although pneumonia is not usually very contagious, the four health workers who treated him also fell gravely ill with the same disease. Now, scientists say, the Foshan businessman appears to have had the earliest known case of severe acute respiratory syndrome, or SARS, which has since become an international epidemic. As of Saturday, 2,416 people in 20 countries were reported to have contracted the disease, and 89 had died. The overwhelming majority of cases were in mainland China, with 1,220, and Hong Kong, with 800. The cause of the disease is unknown, but scientists suspect it is a new coronavirus, from a family of highly changeable viruses that until now have been known to cause only more minor illnesses in people, like colds and diarrhea. SARS was brewing in Guangdong Province for months but was not revealed by the Chinese government until February, when Beijing began reporting cases to the World Health Organization. The Foshan businessman's case was disclosed just last week, when Chinese officials finally agreed to open their casebooks and hospitals to international specialists. The businessman recovered, but Chinese officials have not discussed what happened to those who took care of him, nor have they said where or how he might have contracted the disease. The rest of the world did not know much about SARS until March 15, when the WHO issued an alert calling the disease a 'worldwide health threat.' Since then, fear of SARS has led many countries and corporations to halt tourism and business travel to China, Hong Kong, Vietnam and Singapore, decisions that are expected to cost the region billions of dollars. Efforts to control the disease in Hong Kong appeared to be making headway last week, but the progress may have been short-lived, as cases jumped sharply over the weekend. There were 42 new cases reported Sunday, up from 39 on Saturday and 27 on Friday. The new cases included doctors and nurses. Two women, aged 68 and 71, died, bringing the number of deaths in Hong Kong to 22. Health experts say it is too soon to tell whether SARS will turn into a global wildfire or cool down. But scientists insist that the disease must be treated as an urgent public health threat. What worries epidemiologists is that SARS can spread rapidly through the air, via coughing and sneezing, and its death rate of 3 percent to 4 percent is significant, particularly because healthy people are among those who have died. People of all ages, from children to the elderly, have caught SARS. The illness typically starts like any other acute respiratory infection: with a fever, chills, headache, malaise and dry cough. Chest X-rays tend to show what doctors call 'atypical pneumonia' in a lower lobe of a lung. In the following days, a victim may develop difficulty breathing as the pneumonia spreads to another lobe. About five to seven days after onset, the symptoms improve in about 80 percent to 90 percent of patients and worsen in the remainder. Many of the sickest patients require intensive care, some to the point of being connected to a respirator. Why some people improve and others die is not known

Epidemiologists have traced most cases to close person-to-person contact. But they have been unable to trace a number of SARS outbreaks in hotels, hospitals and apartment complexes in Hong Kong, Singapore and China to personal contact. That has made health officials increasingly suspicious that the disease can also be spread through contaminated objects such as doorknobs, or through water and sewage. Hong Kong officials also have theorized that cockroaches could track contaminated sewage from apartment to apartment. In other developments, panicky residents and some medical personnel in Thailand blocked the cremation of a Hong Kong man who had died of SARS. They were afraid that the virus would spread through the smoke. With tens of thousands of soldiers based in Asia, the U.S. military is shoring up its defenses against SARS by restricting travel to affected areas and encouraging personnel to avoid crowds and wash their hands frequently. In Singapore, military personnel can get clinical masks to wear

In recent days epidemiologists have been unable to trace a number of SARS outbreaks in hotels, hospitals and apartment complexes in Hong Kong, Singapore and China to such person-to- person spread. Because of that, many health officials have become increasingly suspicious that the disease can be spread through contaminated objects such as door knobs, water and sewage, as well as by person to person contact. Hong Kong officials also have theorized that insects such as cockroaches could spread the disease, perhaps by tracking contaminated sewage from apartment to apartment

Epidemiologists have traced most SARS cases to close person-to- person contact. That finding initially led officials to believe that they could break the SARS chain by isolating patients and their contacts and by requiring health workers to use standard infection control measures in caring for patients. Such measures, including frequent handwashing and wearing masks, gloves, gowns and goggles, have worked in most places. If SARS can be spread by insects or objects or healthy human carriers, containing its spread would be much more difficult. The fear is that in short time SARS could become yet another on the long list of diseases that are a fixture in many areas. To help determine whether seemingly healthy people can spread the SARS agent, Hong Kong is conducting a number of studies involving the hundreds of people in isolation here because they had contact with a SARS patient

In recent days epidemiologists have been unable to trace a number of SARS outbreaks in hotels, hospitals and apartment complexes in Hong Kong, Singapore and China to such person-to-person spread. Because of that, many health officials have become increasingly suspicious that the disease can be spread through contaminated objects like door knobs, water and sewage, as well as by person-to-person contact. Hong Kong officials also have theorized that insects like cockroaches could spread the disease, perhaps by tracking contaminated sewage from apartment to apartment. Experts think that SARS is largely spread through droplets exhaled by dry coughs. There are two distinct ways in which coughing can spread viral diseases: droplet or airborne transmission. In droplet transmission, the infective material is coughed or sneezed out of a patient's airway surrounded by a bit of moisture. The particles are too large to travel more than about three feet, and so relatively close, face-to-face contact is required for transmission to take place through the air. But the viruses may also persist on inanimate objects, and people can become infected from touching contaminated surfaces. Masks, gloves and frequent hand washing can sharply reduce droplet transmission. To help determine whether seemingly healthy people can spread the SARS agent, Hong Kong is conducting a number of studies involving the hundreds of people in isolation here because they had contact with a SARS patient

Dr. David L. Heymann, executive director in charge of communicable diseases for W.H.O., said the agency ''is 99 percent sure'' that SARS is caused by the new coronavirus based on the monkey experiments in the Netherlands. Experiments on animals are necessary because the lack of an effective treatment for SARS and the relatively high death rate make it unethical to conduct such experiments on humans. The monkey experiments are essential in fulfilling the steps known as Koch's postulates that are needed to establish proof that a virus or other microbe causes a disease. Applying the postulates to SARS, scientists must determine whether injecting the coronavirus into animals causes similar symptoms to those that humans experience. A formal announcement that the likely cause of SARS has been found could come as early as Wednesday. The Chinese government's decision to allow the W.H.O. team to visit military hospitals ''is a welcome indication of China's willingness to come to terms with the SARS outbreak on the mainland,'' the organization stated. On Monday, China's president, Hu Jintao, said on state television that he was ''very worried'' about SARS

Several SARS patients have infected more than 30 people, according to the World Health Organization. The biggest reported superspreader is a 26-year-old airport worker admitted to Prince of Wales Hospital in Hong Kong in early March. He infected 112 people, including every doctor and nurse who treated him. Referring to a well-known study of a cold outbreak at the Eagle Heights Apartments in Madison, Wis., and to an early theory that the outbreak of more than 300 SARS cases in the Amoy Gardens apartment complex in Hong Kong was spread by cockroaches, he said: ''Don't blame the cockroaches. In Wisconsin, it wasn't the cockroaches, it was the kids.'' Whoever put SARS in the Amoy Gardens sewage pipes -- and one regular visitor was a dialysis patient at the Prince of Wales Hospital while the airport worker was on the nebulizer -- would be a superspreader, with the help of rusty pipes

The gene maps were computer generated from viruses isolated from SARS cases in Canada and Asia. The maps detail the genetic sequencing of the nucleic acids, or nucleotides, in the virus. The maps generated by scientists at the Centers for Disease Control and Prevention and in Vancouver showed only ''a trivial'' difference in 15 among the 29,727 nucleotides in the coronavirus, Dr. [Julie L. Gerberding] said. Known members of the coronavirus family typically contain from 29,000 to 31,000 nucleotides. Samples of the coronavirus used to generate the gene maps came from patients with SARS, and health officials say a new type of coronavirus remains the leading suspect of the respiratory ailment that has spread around the world. But to prove that this particular coronavirus causes SARS, scientists need also to see if animals that are infected with the virus become ill with the same disease. Officials of the Centers for Disease Control have said that they have isolated the new coronavirus from four suspected SARS cases in this country. Laboratories in the W.H.O. network have also isolated the coronavirus from SARS patients elsewhere. But the scientists have not reported the systematic studies of several laboratories testing the same samples using the same techniques. That will require testing thousands of specimens from SARS patients and healthy individuals for purposes of scientific comparison to prove that the coronavirus causes SARS