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A measles virus is seen through an electron micrograph in a file photoCynthia Goldsmith/The Canadian Press

The number of known measles cases in Southern Ontario doubled over the Family Day weekend, as officials took the extraordinary step of publicly warning hundreds of patrons of a Christian youth gathering in Toronto to watch out.

"We are up to 16 now," said Robin Williams, the province's acting chief medical officer, in an interview. She said there are now nine active cases in Toronto, one in each of the city's York and Halton suburbs, and another five in Niagara Falls.

Past measles scares in the province have tended to trace back to isolated cases of unvaccinated travellers who went abroad. What is different now is that the ongoing scare is part of a wider North American phenomenon, in which it is difficult for doctors to follow the virus as it travels.

"As far as we know, other than the cluster in Niagara, these are not linked," Dr. Williams said of the current Ontario cases.

Four of the five Niagara Falls cases came to light over the weekend. These infections appear to have started spreading the weekend before last.

People who have been to Saint Michael High School and Our Lady of Mount Carmel, and a Boys and Girls Club event and the Lighthouse Church of God, at specific times between Feb. 6 and Feb. 10 in Niagara Falls, are being urged to double-check their vaccination records, watch for symptoms or contact doctors.

Those who may be affected are being asked to be vigilant until March 3. The long period reflects how the highly contagious virus tends to be infectious for more than a week before infected people show any signs of being ill.

The teenaged Christian attendees of the Feb. 6 weekend's "Acquire the Fire" gathering at a church on Toronto's Queensway are also being urged to double-check medical records or get themselves checked out.

"A teenager who did not realize they were incubating the measles virus went to this event," Dr. Williams said. "We eventually diagnosed [the teen] as having wild measles disease on Friday."

The rare public warning was issued because the implications are far-reaching.

It was "such a large event and went on for most of two days.… it was felt that it was more extensive exposure than riding on a subway, going to a mall or being at a hockey game," Dr. Williams said.

"If you are unvaccinated and attending this event please contact your local health department," she said. "If you develop any signs or symptoms, contact your health-care provider before turning up in their waiting room with measles."

Public-health officials point out that the measles vaccine is extremely commonplace in Ontario and highly effective. Unvaccinated people who are most at risk include children, pregnant women, and people with immunity disorders.

Doctors in Toronto are responding to a small measles outbreak in which four people – three of whom had not received their measles vaccine – were diagnosed with the infectious disease in less than a week. Health officials announced on Monday that tests confirmed infections in two adults and two children under the age of 2. One case was confirmed on Jan. 29, two on Jan. 30 and one on Feb.1. Public-health officials have not been able to find any connections between the four patients, nor have they been able to determine where they contracted the virus.

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