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West Nile virus found in Toronto mosquitoes

A Culex quinquefasciatus mosquito is shown on a human finger in this undated handout photograph from the Centers for Disease Control and Prevention (CDC). The Culex quinquefasciatus mosquito has been proven to be a vector associated with transmission of the West Nile virus, according to the CDC.


The West Nile virus has been found in Toronto mosquitoes, but the cool temperatures this summer made it later than usual.

In previous years, the virus has been discovered much earlier in the summer, with positive tests cropping up by mid-July for the past three years.

When temperatures climb, it makes it easier for the virus to grow in mosquitoes, making them more likely to become infectious. The cooler temperatures this summer probably contributed to the late arrival of infected mosquitoes, according to Elaine Pacheco, healthy environments manager at Toronto Public Health.

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"Temperatures do play a role in the timing and number of positive mosquitoes during the West Nile virus season. Higher temperatures suggest that mosquitoes will go through their life cycle more quickly and that the virus will increase faster in the mosquito," Ms. Pacheco said.

"Seasons with hotter weather, especially earlier in the season, tend to produce a higher risk of West Nile virus-positive mosquitoes and human cases."

Toronto Public Health has 43 mosquito traps placed around the city where it collects samples of the mosquito population for weekly tests. Until this week, all of the samples have tested negative for West Nile virus, but TPH says a trap in the north end of the city captured a mosquito with a positive test this week. There have still been no reported cases of human infection this year.

The West Nile virus can be deadly in rare cases and even mild infections of the virus can cause fever, aches and rashes. It was first discovered in Toronto mosquitoes in 2001, but recently the hot, long summer of 2012 brought a near-record number of cases with 100 positive mosquito samples and 94 human cases in Toronto and four deaths across the province related to the virus. The highest number of confirmed human cases in Toronto was in 2002, with 163 people infected.

The best way to avoid the West Nile virus is to avoid getting bitten by mosquitoes, according to TPH. It recommends using bug repellent, wearing light-coloured clothing and removing any standing water – where mosquitoes breed – from your around your property.

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