Toronto Public Health said there are now 17 lab-confirmed cases of mumps in Toronto.
The infected people are all 18- to 35-year-olds who have frequented bars in the city's west end, TPH spokesperson Lenore Bromley confirmed on Friday.
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Those numbers are up from 14 confirmed cases on Wednesday.
Dr. Jeff Blackmer, vice-president of the Canadian Medical Association, said the number of cases is "on the high side" and above what the medical community typically expects for this time of year.
"It's not isolated cases, and it has been spreading," he said.
The mumps virus is spread through coughing, sneezing, and coming into contact with a person's saliva by sharing drinks, cutlery, water bottles, or through kissing.
Signs of mumps range from flu-like symptoms, including swelling or pain in the cheeks and jaw, fever, fatigue, and muscle aches, to painful swelling of the testicles or ovaries — and more severe complications including meningitis or brain inflammation.
Bromley said five of the Toronto cases are men with inflammation of one or both of their testicles.
The condition is "very painful" and, in rare cases, can lead to sterility or decreased reproductive capacity, said Ian Culbert, executive director of the Canadian Public Health Association.
People born before 1996 may be at higher risk
"It's always concerning to see outbreaks of vaccine-preventable diseases because we have the tools to prevent them," Culbert said.
He noted that when the vaccine for mumps was first introduced in 1970, the initial recommendation was for a single dose — but two doses has been the recommendation since 1996.
Are you vaccinated for mumps? Adults born after 1970 should get two doses of the mumps vaccine. Learn more: https://t.co/kYrduG1ZqW— @TOPublicHealth
Anyone born before then may be "underimmunized" and at a higher risk for infection, Culbert said.
Having a third dose of the vaccine — an idea under discussion in the American medical community — is also on the radar in Canada, Blackmer said.
"I would say that the jury is still out on that, and there's not great evidence in terms of whether or not that would be efficacious," he added. "There are still studies going on."
TPH said people should check their immunization record to make sure they're up-to-date with the mumps vaccination.
If someone thinks they have mumps, TPH recommends contacting a doctor, and letting them know you might have the viral infection before heading to a clinic.