From Jan. 27 to Jan. 31, two children and two adults were diagnosed with measles. There are no apparent links between the four and none had recently travelled outside the country.
The lack of connections and travel history suggests there is probably at least one other case that hasn't been detected and potentially more, Dr. Lisa Berger, of Toronto Public Health, said in an interview Monday.
"That's why we're launching our investigation because we're looking for the source for these people. Source or sources," said Berger, an associate medical officer of health.
Berger said none of the four received the recommended two doses of a measles vaccine.
Ontario Health Minister Dr. Eric Hoskins said the development underscores the importance of maintaining a high level of vaccine coverage against measles, one of the most contagious of the so-called childhood diseases.
"Unless everybody, or as close to everybody as possible, is vaccinated ... there is risk," Hoskins said.
Measles is best known for triggering a widespread red rash. Common symptoms include high fever, a cough, a runny nose and red, watery eyes.
But the virus can make people who contract it — especially young children — very sick. While most people recover fully, the U.S. Centers for Disease Control says one in 20 children with measles will develop pneumonia. Permanent brain damage and deafness are also complications of measles.
The disease can be fatal too. The CDC estimates that for every 1,000 children who contract measles, between one and three children will die.
Berger said anyone who suspects they or their children have measles should call ahead before going to a doctor's office, a clinic or a hospital. A child with measles could infect others in a crowded waiting room and could pose a danger to babies less than 12 months old who are too young to have received their first measles shot.
Measles was widespread in Canada before the vaccine was introduced in 1970. So people born before that year are believed to be immune because the vast majority would have been infected in childhood.
In the late 1990s, Canada eliminated measles, meaning the virus does not spread here on a continuous basis. The same is true of the United States, which is currently experiencing a large measles outbreak linked to the Disneyland theme parks in California. The U.S. recorded 102 measles cases in January, with infections popping up in 14 states.
The Public Health Agency of Canada does not have statistics available for measles cases in Canada so far this year.
When cases occur in Canada, it is due to the importation of measles virus, either by a Canadian who became infected abroad or someone from another country travelling here while harbouring the virus.
Measles importations can trigger one or a few infections, or can ignite large outbreaks, such as the 2011 outbreak in Quebec in which more than 750 people were infected, or last year's outbreak in B.C.'s Lower Mainland, which involved more than 400 infections.
The director of York University's centre for disease modelling said someone who is asymptomatic — infected but not sick — may have brought the virus to Toronto.
"It's possible that there's an index case that came in, say, maybe from Disney World or somewhere else measles is being transmitted right now, and then they may have had contact with these individuals or other people," said Jane Heffernan, who researches the spread of infectious diseases.
"So there probably could be more cases."
Samples of the viruses from the four people will be analyzed at the National Microbiology Laboratory in Winnipeg, which will be able to compare the viral sequences to others in a global database maintained by the World Health Organization. That should provide clues about the origin of virus. Results of that work aren't likely to be available until late next week, Berger said.
Note to readers: This is a corrected story. A previous version stated that five children in Canada died from measles between 2000 and 2011
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