The number of West Nile virus cases is climbing in parts of Ontario, experts say.

Toronto Public Health said the city has seen 38 human cases this summer as of Monday.

"It's worrisome," Ontario's Associate Chief Medical officer of Health Dr. Doug Sider said of the nearly 80 cases in the province. "What we're looking at is a fairly significant swing in the number of cases."

This summer follows a decade of ebbs and flows in the number of human cases in Canada, which stood at 62 as of Aug. 18, according to the Public Health Agency of Canada. To date, human cases have been reported in Quebec, Ontario, Manitoba, Alberta and Saskatchewan, the agency said.

In 2002, 414 cases were reported to the Public Health Agency of Canada. Last year, there were 101.

Ontario's record year was 2002, when the province had 394 cases and 19 deaths.

"In 2002, our epidemic year, we had the same number of positive mosquitoes in any given calendar week as we do now," said Prof. Fiona Hunter, a medical entomologist at Brock University in St. Catharines, who tests flying insects from across the province.

"So that would tell me we're in for another epidemic year."

Sider and Hunter attributed the upswing centred in Toronto and Peel to the west to heat early in the summer, which created breeding opportunities for mosquitoes.

There is a risk of getting West Nile virus into the first few weeks of September, Phil Curry, an entomologist with the Saskatchewan's health ministry, said last week.

West Nile is transferred through mosquitoes that carry the virus, but officials say most individuals who are exposed show no symptoms.

Recommended preventive measures include:

  • Using mosquito repellent with DEET.
  • Wearing long sleeved light coloured shirts and pants.
  • Make sure that door and window screens fit tightly and have no holes that may allow mosquitoes indoors

Roughly 20 per cent of people who are infected may show some flu-like symptoms such as headaches, muscle aches and pain.

A very small number of people who are infected could develop neurological symptoms that can cause paralysis, coma or — in rare cases — death.

With files from CBC's Dave Seglins and Kelly Crowe