This undated photo provided by the Northwest Mosquito Abatement District shows a Culex pipiens, left, the primary mosquito that can transmit West Nile virus to humans, birds and other animals. It is produced from stagnant water. The bite of this mosquito is very gentle and usually unnoticed by people. At right is an Aedes vexans, primarily a nuisance mosquito produced from freshwater. It is a very aggressive biting mosquito but not an important transmitter of disease.Courtesy of the Northwestern Mosquito Abatement District/The Associated Press
People are not the only ones enjoying the summer heat. After a distinct lack of buzzing pests last summer, mosquitoes are back in bigger numbers. The hot weather has been so intense in Ontario and parts of the Prairies that scientists are anticipating a bounce-back of the West Nile virus.
So far, 89 mosquito pools – standing water in which the insects lay their eggs – in Ontario have tested positive for West Nile – the highest number since 2002 at this point in the year. A 68-year-old Windsor, Ont., man was hospitalized with inflammation in his brain after contracting the disease, and the virus was found in a handful of people in Regina, Quebec and Ontario.
The U.S. Centers for Disease Control and Prevention has reported 241 cases of West Nile disease this summer, including four deaths – the highest number in one year since 2004. Almost 80 per cent of the cases were in Texas, Mississippi and Oklahoma.
"It is not clear why we are seeing more activity than in recent years," said Marc Fischer, a medical epidemiologist with CDC's arboviral diseases branch.
Transmitted from infected birds to mosquitoes and then to humans, the West Nile virus was isolated in Uganda in 1937. The first recorded outbreak was in New York in 1999.
Since its first appearance in Toronto in 2001, the virus has spread west and been on and off the public's radar screen depending on how many people have been affected. Four in five infected adults will show no symptoms, but the virus can lead to fever, headache, nausea, vomiting and rashes. In some extreme cases, the virus can cause permanent brain damage or death.
Canada began surveillance in 2002, and the worst year has so far been 2007, with more than 2,200 cases, including 12 deaths, according to the Public Health Agency of Canada, with the Prairies the worst hit.
Mosquitoes, specifically the Culex tarsalis species, which carries West Nile, flourish with a wet start to the period for laying eggs and hatching larvae followed by a hot and dry summer in which adults can thrive.
It's too early to say exactly how this year will shape up, as the prime weeks for transmission are in August and early September.
But, said Phil Curry, an entomologist and the West Nile co-ordinator for Saskatchewan, the weather conditions have been optimal for mosquitoes in parts of Ontario, Saskatchewan and Manitoba.
"This will be a build-up year," Mr. Curry said, adding that the earlier prevalence of West Nile virus in mosquito pools is a warning sign. "I don't think it will reach the worrying levels as we had a couple of years ago, but the virus tends to be cyclical … and this is it coming back."
The West Coast, however, has seen almost no evidence of the virus in its mosquitoes.
The number of mosquitoes that can carry West Nile found in traps in British Columbia has risen to unprecedented levels – but none has tested positive yet.
"We would normally see 100 to 200 Culex mosquitoes per night in our traps in a normal summer," said Bonnie Henry, the medical director of communicable disease prevention at the B.C. Centre for Disease Control. "Now, we're finding somewhere between 300 and 500 per night."
Manitoba's medical officer of health, Richard Rusk, speculates the numbers have gone up because the spring did not bring the kind of floods that last year prevented stagnant pools of water from forming.
Most often, heat is to blame.
A map showing the hottest areas will almost match up with a map showing the outbreaks of West Nile virus, Mr. Curry said.
He theorized that the nesting season for birds had a long overlap with the time that mosquitoes emerged as adults this year.
"When you have young nestlings that cannot defend themselves, they're more likely to be bitten by these mosquitoes that will then get infected by the virus," he said. "And then the cycle continues from there."
Dr. Rusk is worried Canadians might have become too complacent about West Nile virus and aren't taking precautions such as using bug spray outdoors and getting rid of standing water on their property.
"You can't just look at a mosquito and say that's a nuisance one and that one carries the West Nile virus," he said. "Our big worry is that the virus will ramp up quickly because people aren't being careful."
Municipalities such as Toronto, Winnipeg and Portage La Prairie, Man., are increasing efforts to curb mosquito populations. Manitoba Health ordered the spraying of the insecticide malathion in Portage La Prairie and a three-kilometre area surrounding the city.
And Toronto officials are treating mosquito pools in roadside sewers and catch basins with a chemical that prevents larvae from developing into adults.
No vaccines are available for West Nile. And while some speculate humans may develop immunity, very little is also known about the long-term effects of infection, precisely because West Nile is relatively new to North America.
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WHAT TO DO TO PROTECT YOURSELF
Avoid being out from dusk to dawn as this when mosquitoes that carry WNV are most active.
Wear protective clothing, including long-sleeve shirts or jackets and long pants. Tuck your pants into socks for extra protection.
Avoid dark-coloured clothing as it can attract mosquitoes.
Use mosquito netting for babies and toddlers in cribs and strollers.
Use federally registered mosquito repellant that contains DEET (N,N-Dethyl-m-toluamide) or PMD (lemon-eucalyptus oil)repellent.
There are many repellents that have been shown not to protect against mosquito bites, including bug zappers, devices that give off sound waves and Citrosa plants.
Source: B.C Centre for Disease Control