"This year the risk seems to be a bit higher than what we usually experience," said Dr. Howard Shapiro, the agency's associate medical officer of health.
Health officials who trap mosquitoes at 43 locations across the city say the virus has shown up earlier this year and in record numbers. They fight the virus by dropping larvicide 'bug bombs' in 120 roadside catch basins.
West Nile was first discovered in Toronto in 2001, and in 2002 there were 163 cases and 11 deaths. But there have been no deaths in Toronto from West Nile since 2005, and only one human case since 2010.
No human cases of the virus have been reported so this year, but health officials don't expect that to last. Given the numbers of infected larvae, the Toronto health authority says some people will likely get sick.
Hotter weather is associated with higher viral loads in mosquitoes, which pick up the virus from birds they bite and then spread it to people.
1 in 150 infected develop severe symptoms
Only about one in five infected people get sick. One in 150 infected people will develop severe symptoms including neck stiffness, disorientation, coma and paralysis.
The best way to prevent West Nile disease is to avoid mosquito bites. Insect repellants, screens on doors and windows and wearing long sleeves and pants are some of the recommended strategies. Experts also suggest emptying standing water from buckets, kiddie pools and other places to discourage mosquito breeding.
In recent days, two human cases have been confirmed in Winnipeg and one in Windsor, Ont. All of the cases were mild and were discovered by Canadian Blood Services after blood donations.
In the U.S., more serious illnesses from West Nile virus have been reported so far this year than any since 2004.
Through the end of July, 241 human cases have been reported in 22 states, including four deaths. Texas, especially around the Dallas area, has seen the bulk of them.