By Rob Stein
Washington Post Staff Writer
Friday, May 16, 2003; Page A16

Toronto hospital workers apparently caught the SARS virus despite intense precautions because they were exposed to an invisible mist of droplets containing the microbe while helping a very sick patient breathe, experts reported yesterday.

An investigation into a case in which nine health care workers were infected by a single patient concluded that the most likely cause was a virus-laden aerosol that the patient coughed out during a procedure to insert a breathing tube down his throat, the U.S. Centers for Disease Control and Prevention in Atlanta reported.

The procedure caused the very sick 54-year-old patient to cough heavily, producing an aerosol containing heavy concentrations of virus that enveloped the health care workers close to him, the CDC said. That could explain how the health care workers became infected even though they were taking every recommended precaution, including wearing protective gowns, gloves and masks.

The findings could also explain similar baffling incidents elsewhere in Toronto and in Hong Kong in which hospital workers became infected with severe acute respiratory syndrome even though they were being very careful to protect themselves. The investigation also did find, however, that some workers did not fully understand the safest way to remove their protective equipment, and in some cases their masks did not fit snugly.

"We are still learning about what's working and what's not working for infection control," Julie L. Gerberding, director of the CDC, said in a telephone briefing. "These experiences in Canada have taught us some lessons that we will transmit here in the United States."

The CDC also removed Vietnam from its list of countries that travelers should avoid, because no new cases have been reported there for more than 30 days. In addition, the agency said the number of probable SARS cases in the United States remains at 64, with only one new case reported so far this month.

"This indicates that international travel advisories and other steps being taken in countries where SARS has been reported are working," Gerberding said.

Meanwhile, World Health Organization researchers in Hong Kong reported that intensive testing found no more evidence of the virus at the Amoy Gardens apartment tower, where hundreds of residents were infected, or in the Metropole Hotel, where the outbreak began in Hong Kong. The findings indicate that contaminated buildings can be effectively cleaned and safely inhabited.

It is still unclear why the virus spread so readily in the Metropole, but the WHO team confirmed local health officials' conclusion that the virus probably spread through the apartment building in part through leaky plumbing, said Klaus Stohr, WHO's top SARS scientist.

In yet another finding, scientists in Canada said a series of experiments has shown that pigs and chickens cannot be infected with the SARS virus. That indicates the animals were probably not the original source of the virus and would not provide a "reservoir" in which the virus could hide and reemerge if public health measures succeed in stamping out the epidemic in humans, Stohr said.

Chickens and pigs were considered the most likely animals to play a role in the epidemic. Both species can be infected by related viruses, and people live in close proximity to both species in southern China, where the epidemic began.

"This is an important finding," Stohr said. "We were all speculating about animal reservoirs. These tests show that these animals, pigs and chickens, do not appear to play a role in any stage of the evolution of this virus. The virus does not like them. That makes them a very inefficient host for the virus."

Researchers are testing other species, such as cattle, as they continue to hunt for animals that can be infected, Stohr said.

Also yesterday, WHO recommended that people who have had SARS or may have been exposed to it be barred temporarily from donating organs or blood. Although there is no evidence anyone has become infected this way, WHO took the step as a precaution. The U.S. and Canadian governments have issued similar recommendations.

Under WHO's guidelines, people with probable SARS cases should not donate blood for three months after they recover, while those with suspected cases should not donate for a month after recovering.

Anyone who has either recently returned from an area where the virus is spreading, or who develops symptoms after having had close contact with a patient, should not donate blood for three weeks after their return or their symptoms subside.

Correspondent Ellen Nakashima contributed to this report from Hong Kong.