West Nile virus cases down because of cooler summer temperatures

Ottawa health officials say cooler summer temperatures are making it hard for the West Nile virus to develop, leading to fewer confirmed cases this year compared to others.

Ottawa's first confirmed test of the virus in a mosquito was last week, according to Ottawa Public Health, who said by this time last year there were 13 mosquito "pools" with a positive test.

Last year 17 mosquito populations tested positive for the virus, along with four humans.

There have been no human cases so far this year in Ottawa.

Craig Calder, a program manager with Ottawa Public Health, said the West Nile virus needs temperatures to stay above a certain level (there's debate among scientists over whether that's 14 C or 18 C) or it can't survive.

"West Nile virus and mosquito populations in general are very weather-dependent, specifically rainfall and temperature play a key role," he said.

"Because of the cooler weather in the summer of 2014 we've seen a marked decrease in West Nile virus being present in the mosquito population."

Calder said they don't keep track of the number of mosquitoes in Ottawa, but based on calls from the public they don't believe it's much lower than normal.

Fewer cases across Ontario

West Nile virus is an infection transferred to humans by mosquitoes that have bitten infected birds, causing a high fever with flu-like symptoms.

• West Nile virus: What you need to know

Ontario's Ministry of Health said there were 56 confirmed human casesin 2013.

A ministry spokesperson said so far this year, there have been two: one in Windsor-Essex County and one in the Muskoka region.

"The 2014 decrease over 2013 is likely the result of cooler summer temperatures in this year.Most cases occur between July and September.No scientific models can accurately predict the severity of WNV from one year to the next," the ministry said.

Calder said the mosquito season will last for another four to five weeks, meaning people should check screens in their homes for holes, wear long-sleeve shirts and use bug spray with DEET to avoid bites.

"The population of Ottawa should still be cautious," he said.