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There have been 13 reported cases of West Nile virus in humans in Ottawa so far in 2017 — the most since the virus made its first appearance in Canada in 2001.

Before this summer, the highest number of West Nile cases was eight in 2012, said Ottawa Public Health on Wednesday. But with temperatures at summertime levels over the past week and hot weather predicted into next week, public health officials warn that virus-carrying mosquitoes will continue to bite until there have been several hard frosts.

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The mosquitoes most likely to transmit West Nile are Culex pipiens, the northern house mosquito. These are found in urban areas and around homes. Testing has shown mosquitoes carrying the virus in all urban areas of Ottawa.

There are no vaccines to prevent or medications to treat West Nile. Most infected people do not have any symptoms, but about 20 per cent may experience flu-like symptoms including fever, headache and muscle aches. In less than one per cent of cases, West Nile invades the central nervous system. Older adults, the elderly and people with weakened immune systems are most at risk.