With St. Patrick’s Day around the corner, Toronto revellers may have more to worry about than just finishing their green-dyed beer.

Another five cases of mumps have been confirmed in Toronto, bringing the total to 22 this year. Toronto public health officials said the virus has hit mostly people between the ages of 18 and 35 who frequented west-end bars in the city —specifically west of Yonge St., east of Lansdowne Ave. and south of Bloor St. W.

Overall, 35 cases have been confirmed in Ontario.

Toronto normally averages about four cases per year, according to Dr. Vinita Dubey, associate medical officer of health with Toronto Public Health. And yet more cases could be on the way.

“I think we can expect more,” she said. “How many more, it’s hard to tell.”

Combined, those affected reported visiting a list of bars numbered in the “high teens,” Dubey said. Toronto Public Health has sent letters to 1,300bars and alcohol-serving restaurants in the area asking that they follow food safety guidelines, check their staff’s immunization records, and watch for signs and symptoms of the virus.

A letter was also sent to local school boards to be distributed to parents, asking that they immunize themselves and their children, and watch for symptoms, Dubey said.

The mumps virus spreads through coughing, sneezing, and coming into contact with a person’s saliva by sharing drinks or utensils, food or water bottles, or by kissing.

Common symptoms of mumps include swelling and pain in at least one salivary gland, fever, headache, muscle aches and pains, fatigue and loss of appetite.

Dr. Sarah Wilson, a medical epidemiologist with Public Health Ontario, said those predominantly affected by the mumps outbreak in Toronto, aged between 18 and 35, are “more susceptible to mumps” because of their behaviours.

“That’s an age group that tends to have a more active social life in terms of going out with friends to bars and restaurants, sharing food and drinks, talking close at a crowded noisy party or in a restaurant environment.”

Some in this age group might only have received one dose of the vaccine, as the second dose was introduced in Ontario in the mid-1990s, when it became apparent it was necessary, Wilson said.

Public health officials say the outbreak serves as a reminder of the importance of proper immunization, particularly two doses of the measles-mumps-rubella (MMR) or measles-mumps-rubella-varicella (MMRV) vaccines.

Of those who contracted the virus in Toronto, 40 per cent had received two doses of the vaccine and 60 per cent had received either no doses of the vaccine, one dose, or the number of doses was unknown, Dubey said.

Two doses of the vaccination are 85 to 88 per cent effective, according to Dubey, but individuals who still contract the virus experience a “milder illness.”

“The vaccines provide very good protection against mumps infection but it’s not 100 per cent,” Wilson said. “If you have a lot of exposure to the virus, it is still possible.”

Recent mumps outbreaks have not been limited to Ontario.

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Between Sept. 1, 2016 and March 2, 2017 there have been 184 confirmed cases of mumps in Manitoba, according to Manitoba Health, Seniors, and Active Living. Alberta Health Services reported 21 confirmed cases of mumps in 2017 as of March 2.