The virus last week confirmed as causing severe acute respiratory syndrome (SARS) is transmitted almost entirely by large droplets reaching the mucous membranes of nose and lungs and not by feces or urine reaching the mouth, and the epidemic is coming under control, a meeting of SARS epidemiologists concluded Saturday (May 17) at the World Health Organization (WHO) in Geneva. But two mysteries remain to be solved in China: the reservoir and a possible alternative transmission route.

The meeting of 16 laboratories and groups concerned with SARS largely confirmed the existing paradigm of infection and control, according to a press conference. The sense of relief that the outbreak was coming under control was palpable. According to Michael Ryan, coordinator of WHO's Global Alert and Response Programme, "The control measures we designed at the beginning of the epidemic have worked, and in country after country we've been able to break the cycle of transmission… and we've seen the number of secondary cases per case dropping dramatically in so in many places we can see the epidemic coming to an end."

Margaret Chan, director of Hong Kong's Department of Health, said, "We see very clearly that the mode of transmission is droplets as opposed to airborne." And, she told The Scientist, a Health Canada study will confirm the department's findings on transmission in Amoy Gardens, the apartment block where 300 persons were infected. There, feces and urine from SARS patients with diarrhea were aerosolized through a faulty sewage system and emerged into apartments—very special circumstances, said Chan.

Arlene King, director of the Division of Immunization and Respiratory Diseases, Health Canada, confirmed that "Amoy Gardens shows that [apart from respiratory droplets] you can aerosolize other sorts of infectious materials, like feces or urine that can be inhaled and have contact with mucous membranes." But this is not what is normally understood as the "oral–fecal" transmission route typical of enteroviruses and bacteria, like salmonella, Ryan explained, which requires, for example, the soiling of food with feces.

A worrying secondary transmission issue remains open, however, in China. According to Ryan, "The Chinese authorities are making great strides in identifying contacts of cases, but they can't always identify the chain of transmission by which that case was infected. And that is an issue because there are many factors that might explain that, such as that there might be another infectious process going on as well as the respiratory route."

The possible original animal source and reservoir of the virus in China has also still not been identified, and little research on the issue is in progress. But, said King, "Although the virus can be sometimes found on animals or in animals, there is no evidence that animals or insects are spreading the infection in any country…" She did not rule out other transmission routes, adding, "There are certainly things that can be learned about other [transmission] routes like contaminated articles."

In a separate development, the author of the monkey study that led WHO to announce on April 16 that coronavirus causes SARS told The Scientist that the study did not rule out the possibility that SARS is exacerbated by other infections.

Albert Osterhaus, head of the Institute of Virology at Erasmus University and lead author of the paper published last week in Nature (Nature, 423:240, May 15, 2003), said that although there were only four macaque monkeys in the experiment, the pathology in the monkeys infected with coronavirus was "indistinguishable from what we see in SARS cases in humans." Osterhaus added, "We cannot exclude that the whole thing might not be exacerbated by other infections, but we have no indication of that from our limited experiments."