TORONTO — Their immune systems weakened by anti-rejection drugs, people who have had organ transplants seem at particular risk from SARS and are more likely to become virus "super-spreaders," a new study by Canadian scientists suggests.

Two such patients in Toronto, including a lung recipient who was still recovering in hospital, contracted the disease and died after infecting numerous others, said one of the study's authors. At least one of those infected by the recipients also died.

In response, hospitals have developed a new screening system they hope will prevent transplant patients from getting the virus from an infected organ if there is another outbreak of SARS.

"If they are infected, they seem to have more severe disease, harder to control disease," said Dr. Deepali Kumar of Toronto's University Health Network, lead author of the paper in the August issue of the American Journal of Transplantation.

"Because they are (theoretically) shedding a lot more virus, they can infect a lot more people."

Even with the screening system in place, SARS could have a significant impact on the transplant world, which already suffers from a shortage of suitable organs, experts say.

Restrictions on transferring patients between hospitals in the Toronto area meant losing 10 available donors who would have provided up to 30 donor organs, said Dr. Cameron Guest, head of the Trillium Gift of Life Network, the Ontario agency that oversees organ transplants.

Having to screen out potentially infected donors in future would again curb the supply of organs, he said.

"If there was another outbreak of SARS... it would likely have a major impact on organ donation in that region," he said.

The paper focuses on a 74-year-old man who had received a liver transplant about 10 years ago because of alcohol-related cirrhosis. In March, he visited Scarborough Grace hospital at the heart of Toronto's first SARS outbreak for a foot-doctor appointment.

About five days later, he showed up again at the hospital with high fever, chills, dry cough and achey limbs, classic SARS symptoms. Not knowing yet what they were dealing with, staff took no special precautions, either there or at Mount Sinai Hospital, to which he was transferred. Only when he ended up at Toronto General were full respiratory safeguards put in place around him.

It is believed he infected at least 10 people, including his wife, family physician, two visitors and six other health care workers, the paper says. One of those people died.