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TORONTO (AP) - The number of probable SARS cases in Canada's largest city could be 60 or more, a leading figure in Toronto's efforts to contain the disease said Thursday.

Dr. Donald Low said health officials were likely to designate more patients as probable cases of severe acute respiratory syndrome when they apply a broader definition of the diagnosis to a new outbreak first noticed last week.

Officials worry the World Health Organization could issue another warning against travel to the city, like one on April 23 that was lifted a week later. Dr. Colin D'Cunha, the Ontario commissioner of public health, said the criteria for such a warning are 60 or more probable cases, five new probable cases a day and proof the illness was being exported to other countries.

Low, a microbiologist and key figure of the anti-SARS team dealing with the Toronto-area outbreak, said he expected the total to include about 40 probable cases at North York General Hospital, 10 to 15 at Scarborough Grace Hospital and seven at St. John's Rehabilitation Hospital.

"We're talking numbers at least in the 60s or 70s," Low said.

Ontario and Toronto health officials have told more than 5,000 people to go into home quarantine because of the latest SARS cluster, the biggest outbreak of the illness outside of Asia.

They put the number of known cases Wednesday at 11 probable and 23 suspected, with another 50 people under investigation for possible SARS. They also announced two more deaths of elderly patients, raising the overall death toll in the Toronto area to 29.

Before the new cluster became known, authorities thought the city's initial outbreak of SARS in March and April had been brought under control.

The new cases have put Canada's largest city back on a World Health Organization list of SARS-affected areas. Four more probable cases from the initial Toronto outbreak in March and April also remained hospitalized.

On Wednesday, the World Health Organization advised Canada to broaden its definition of SARS following Low's concerns that the current one provided an incomplete accounting of the situation.

The Health Canada Web site defined a probable case as showing a severe progressive respiratory ailment. WHO has a less restrictive definition on its Web site, requiring a respiratory illness to be visible on chest X-rays.

Canadian federal health official Dr. Paul Gully said revising the case definition was under consideration. He conceded that changing Canada's definition would cause some suspected SARS patients to be classified as probable.

The quarantines include 1,700 students and staff at Father Michael McGivney Catholic Academy in Markham, a northern suburb of Toronto, where a student showed symptoms of SARS while going to classes for three days last week.

Health officials closed the school until June 3.

The student, the son of a health care worker at North York General Hospital, is listed as a suspected SARS case, but Dr. Murray McQuigge, a York Region Public Health official, said there was no doubt he had the virus.

McQuigge provided guidelines for home quarantine, including no visitors, sleeping in a separate room from anyone else and wearing a respirator mask when in contact with others. He said health authorities will deliver the masks to those in quarantine.

The new SARS cases mean further harm to Toronto's crucial convention and tourism industry. Officials have started aggressive marketing campaigns to lure back visitors after the initial SARS outbreak, including packages of cheap prices for lodging, meal and tickets to theater, baseball games and other entertainment.

Prime Minister Jean Chretien promised his government would help fight SARS in Toronto, and insisted the city was safe to visit.

"We had a new case that appeared last week and it was confined in the hospital section of the city of Toronto … it is under control," Chretien said Wednesday. "This is a problem that is serious, but it is not dangerous at all to travel to Toronto."

In response to the new cases, health authorities reimposed strict controls on Toronto-area hospitals - closing those where the new cases were found to new patients and limiting access to emergency rooms in all others.

On May 14, Toronto was removed from the WHO list of SARS-affected areas after more than 20 days passed without a new case being reported. It was put back on the list on Monday.

The new cluster is believed to come from an elderly patient whose case dates from April 19 and went undiagnosed for weeks.

Life has continued as usual in the metropolitan area of more than 3 million people on the north shore of Lake Ontario. No one wears masks in the bustling downtown streets, and restaurants, theaters and other entertainment venues remain open despite complaints of decreased business because of SARS.

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