Ontario is reporting that as of earlier this week, 49 confirmed and probable human cases of the disease have been spotted.
Public Health Ontario's weekly West Nile report shows that the lion's share of the cases have occurred in Toronto, where 30 infections have been reported.
The news comes a day after Alberta announced it has seen its first human case since 2010 in a southern Alberta woman who is expected to make a full recovery.
The Ontario report says three other provinces have also reported cases so far this year — Manitoba (five), Saskatchewan (one) and Quebec (2).
U.S. authorities say they've never seen as many cases of West Nile infection as early in the year as they are seeing this year.
"We're in the midst of one of the largest West Nile outbreaks ever seen in the United States," said Dr. Lyle Petersen, who oversees mosquito-borne illness programs at the Centers for Disease Control and Prevention in Atlanta.
During a teleconference Wednesday, Peterson said 1,118 illnesses have been reported so far this year in the U.S., about half of them in Texas. In an average year, fewer than 300 cases are reported by mid-August.
There have also been 41 deaths this year, the CDC said.
The mild winter, early spring and hot summer in parts of the country have officials concerned this could be a bad year for West Nile virus in Canada as well.
"I suspect this year will be another outbreak year maybe even as big as 2007 or 2003 given the early activity in Ontario," said Dr. Bonnie Henry, medical director of communicable disease prevention and control services at the B.C. Centre for Disease Control.
Henry said B.C. hasn't yet seen cases, but is entering the highest-risk period of the summer so that could soon change.
— With files from The Associated PressSuggest a correction