OTTAWA | February 10, 2012

Mosquitoes could buzz into new frontiers: study

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More Canadians living along the United States border and in the Maritimes could be swatting away West Nile virus-carrying mosquitoes as the country’s warming climate opens up new terrain for the pests, a new study suggests.

New research suggests mosquitoes could invade larger areas of Canada if climate change trends continue.

There are 74 known species of the insect buzzing in various parts of the country during the usual May-to-September mosquito season, according to the Public Health Agency of Canada.

Only three of those species can carry West Nile virus, which is transmitted by mosquitoes who pick it up while feeding on infected birds.

New research published in the academic journal Applied Geography suggests habitats for one of those species — Culex pipiens, the common house mosquito — could grow dramatically this century.

West Nile can spread to humans bitten by mosquitos, causing symptoms ranging from flu-like conditions to neurological damage.

Roughly one in five people with the virus feel sick but recover on their own, while under one per cent of infections lead to hospitalization.

Warming good news for mosquitoes

Culex pipiens thrives in urban centres, breeding in stagnant water sources like ditches and bird baths.

The authors of the study used models from the UN’s climate-change panel to calculate warming temperatures and precipitation changes in Canada through to the 2090s.

That information was then matched with existing data on known habitats for Culex pipiens, the most-monitored mosquito species.

The researchers conclude the weather shuffle could play a role in boosting the number of places the insect can survive.

The worst-case climate scenario — warming by 3.4 degrees by century’s end — may make all populated areas along the U.S. border, as well as the Maritimes and roughly half of Alberta, suitable for the mosquito.

“There are clearly serious health risks to Canadians that wouldn't be there were it not for climate change.”
— Barry Smit, Canada research chair in global environmental change

Currently only Nova Scotia and southern Ontario and Quebec have a high concentration of habitats, according to the study, which did not include B.C.

Holding temperature growth to 1.75 degrees over that time would create new habitats only in southern Alberta and Newfoundland.

The projections mean more parts of Canada will face West Nile concerns as temperatures warm up earlier in the spring and stay hotter further into the fall. That change would result in melting snow or erratic rainstorms forming more water sources that mosquitoes can breed in.

Other factors not tied to weather also determine the range of the virus. Hotter days could aide its spread, said Valerie Hongoh, lead author of the study.

“The longer your summer, the higher your mosquito population, the higher the risk,” said Hongoh.

The study was based on her research as a graduate student at McGill University.

Itching to move

Winnipeg is no stranger to mosquitoes but hasn’t seen Culex pipiens buzzing within its borders for several decades, says Taz Stuart, the entomologist who leads the city's campaign against mosquitoes.

However, he explained that temperature increases of two or three degrees could open the door to the insect cropping up in Winnipeg.

Two other new-in-town mosquito species have been detected in recent years, though Stuart cautions there isn’t evidence directly linking the arrival of the foreign insects to hotter days.


A map of Canada and the USMore parts of Canada will become home to a common mosquito type as the temperature rises.

However it's a possibility he's not ruling out.

“As we get warmer, temperature is highly correlated to how effective (mosquitoes are) and how species can survive," Stuart said. "Simple as that."

Barry Smit, research chair in global environmental change at University of Guelph, said the study’s results hint at how warmer weather can impact Canadians’ health.

“This is just one simple little sliver of the implications,” he said.

“There are clearly serious health risks to Canadians that wouldn't be there were it not for climate change.”

A spokesman for Canada's public-health agency said in an email that West Nile doesn’t necessarily move in lock-step with hotter temperatures.

Robert Cyrenne said interconnected factors like the number of birds and the strength of their immune systems could also shape the range of the disease.

“Even with the Culex pipiens mosquito and the West Nile virus in a new locality, it does not necessarily mean we would see major shifts in West Nile virus distribution,” Cyrenne says.

Forty-two Canadians have died after contracting West Nile since the country saw its first case in 2002. About 100 infections were reported last year — far more than the averages of the two preceding years.

The study was co-authored by a scientist at the Public Health Agency of Canada and two McGill University professors.

Thumbnail image courtesy of John Tann

What is West Nile virus?

West Nile virus is found mainly in birds. However, the virus can be transferred to humans or other mammals if they are bitten by mosquitoes that have fed on infected birds. The virus can only be transmitted from person-to-person if someone receives blood or an organ from someone who's infected.

Many people with West Nile virus may never even know they are infected. Most afflicted individuals experience mild flu-like symptoms or no symptoms at all. That said, people with weak immune systems and people living with chronic diseases may become paralyzed or suffer brain inflammation if they contract West Nile virus. These conditions can ultimately cause death, since there are no medications to combat the virus.

Canadians are most at risk of becoming infected with the virus during mosquito season, which can last as long as mid-April until late October.

The best precaution against contracting West Nile virus is to avoid areas with dense mosquito populations and wear long-sleeved shirts and pants when outside.

Source: Government of Canada

A brief history of West Nile virus

1937 — West Nile virus is discovered for the first time in humans living in the West Nile region of Uganda.

1999 — In August, West Nile virus is found for the first time in North America in dead crows in New York City. By October, the virus is found in people.

2000 — West Nile virus spreads throughout the northeastern United States.

2001 — The discovery of dead infected birds in southern Ontario marks the spread of the virus to Canada. The virus continues to spread throughout the United States.

2002 — Canada sees its first confirmed human cases of West Nile virus in Ontario and Quebec. The virus spreads across Canada and infected birds are found from Saskatchewan to Nova Scotia.

2003 — More than 1,400 Canadians across the country contract West Nile virus.

2007 — Canada sees its worst year for West Nile virus with more than 2,200 confirmed cases. Twelve people die.

Public Health Agency of Canada and Government of Canada