Summer still has a few weeks to go in Canada, which means that West Nile season is staying with us, as a mosquito trapped in an area outside of Owen Sound, Ontario, has tested positive for the disease.
Yesterday, the Grey Bruce Health Unit confirmed the West Nile virus is present in local pools of mosquitos, although no human cases have yet been confirmed in the area.
“It’s the first (positive test) of the year so we’re just using it as a reminder for our standard message to use proper control measures, to avoid being bitten and prevent mosquito eggs being laid in standing water around your house,” said public health inspector Stephanie Nickels to the Owen Sound Sun Times.
So far across Ontario, 260 mosquito pools have tested positive for West Nile in 2018. Last year, 154 human cases were reported in Ontario and 200 cases for the whole of Canada. In late July, the first human case of the disease in Toronto was reported, while in August, two dead ravens found in southeastern BC tested positive for the disease, marking the first evidence of West Nile this year in British Columbia.
Although West Nile has the distinction of being the most widely distributed vector-borne disease in North America, it wasn’t until 2002 that the first human cases were documented in Canada. That year, 414 cases were recorded, all in Quebec and Ontario, but by the following year, West Nile had spread to five other provinces: Nova Scotia, New Brunswick, Manitoba, Saskatchewan and Alberta.
80 per cent of people who contract West Nile will not show any symptoms at all, while close to 20 per cent may develop flu-like symptoms such as fever, headache, muscle aches and swollen glands. A smaller percentage, about one out of every 150 people infected, will develop serious symptoms including, in some cases, neurological effects such as encephalitis or meningitis and long-term health problems such as muscle weakness, paralysis, confusion, depression and problems with cognitive functioning.
While scientists are unclear on why the disease impacts some people while others experience no symptoms, the spread of West Nile and other insect-carrier diseases and their relationship to our warming weather due to climate change is now being explored.
Originating from Uganda in Africa, the West Nile virus thrives where its host species of mosquito —the Culex tarsalis— does, namely, in hot and dry climates. The same goes for that other vector-borne illness on the rise across Canada, Lyme disease, which has grown more prevalent as climate change has allowed for the expansion of its host’s range, the black-legged tick, further northward.
As the Canadian Public Health Agency (CPHA) reports, another species of mosquito, the Asian tiger mosquito, which acts as a carrier for a number of diseases such as dengue, zika, yellow fever and the chikungunya virus, has been identified in the northeastern United States but could make its way into Canada as our winters become milder.
“Insects, the vectors for many illnesses, are dependent on a consistent climate for survival, reproduction and development,” says Catherine Hierlihy of the CPHA. “Changes in temperature, precipitation and humidity can alter their distribution, potentially increasing the risk of disease transmission.
Across the Atlantic, West Nile infections in Europe have seen a marked rise this summer, with the increase attributed to spikes in temperatures. Health officials are warning that warmer weather is likely to make both Southern and Northern Europe more susceptible to the Asian tiger mosquito and its related diseases.
“Mosquitos and ticks are cold-blooded and are affected by higher temperatures,” says Jan Semenza of the European Centre for Disease Prevention and Control (ECDC), to the Guardian. “At higher temperatures, mosquitos replicate faster. Pathogens in the mosquito also replicate faster. Everything is speeded up and you get higher turnover, bigger populations of mosquitoes and a growing epidemic potential for viruses.”
Health Canada reports that as of August 18, 12 clinical cases of West Nile virus have been reported across the country.