Two neighbouring Ontario health units confirmed their first human cases of West Nile virus of the season within roughly 24 hours of each other this week, and a mosquito species found in the region in August complicates the standard advice both are giving residents.
Hamilton Public Health confirmed the city’s first local human case on Sept. 8 and its medical officer of health raised the local West Nile risk level from moderate to high, according to CHCH News. The following afternoon, Niagara Region Public Health reported the year’s first laboratory-confirmed human case in its area, in St. Catharines.
Both units issued similar guidance. Use insect repellent containing DEET or Icaridin, wear long sleeves and long pants, and empty standing water from buckets, animal dishes, bird baths, flowerpots, tarps and old tires. Hamilton advised anyone entering wooded areas, golf courses or gardens at dawn and dusk to cover up, because mosquitoes are most active at those hours. The city said it has completed a third round of larvicide treatment on street catch basins and has treated surface water on public land.
That dawn and dusk guidance describes the behaviour of Culex mosquitoes, which drive most West Nile transmission in Ontario. It does not describe the species Niagara Region Public Health turned up in August. Through its ongoing surveillance program, the health unit identified a single Aedes albopictus, the Asian tiger mosquito, in Lincoln, Ont. The insect tested negative for mosquito-borne viruses. Unlike Ontario’s established mosquitoes, it is an aggressive daytime biter, which is why Niagara has told residents to protect themselves throughout the day rather than only at dawn and dusk.
The tiger mosquito has been detected in Windsor for about a decade, so the Lincoln finding represents movement within the province rather than a first arrival in Canada. The species is capable of transmitting West Nile as well as dengue, chikungunya and Zika. One insect is not an established population, and no health unit has connected it to either of this week’s cases. But it does mean a resident in the Niagara area who follows only the dawn and dusk rule is following advice built around a different mosquito.
The timing of a first case is also easy to misread. Up to 80 per cent of people infected with West Nile develop no symptoms at all, according to the Public Health Agency of Canada. Niagara Region Public Health says about 20 per cent experience flu-like symptoms and fatigue, and roughly one per cent develop severe illness, which can include severe headaches, neck stiffness, confusion, weakness and sensitivity to light. Older adults and people with compromised immune systems face the greatest risk of severe outcomes.
Because most infections produce nothing a person would take to a doctor, and because confirmation requires laboratory testing, a first confirmed case in early September is a lagging signal rather than a starting gun. Transmission has typically been underway for weeks by the time a case is reported. Hamilton’s move to a high risk level was not triggered by the single patient either. Those designations track mosquito surveillance results collected across the season.
“At this point in the summer season, conditions are ideal for mosquito activity,” Dr. Hamidah Meghani, Niagara Region’s associate medical officer of health, said in the health unit’s statement. “Now is the time for everyone to be proactive in protecting themselves and their families by taking precautions to reduce mosquito bites and remove mosquito breeding grounds around their homes.”
Mosquito season in southern Ontario generally runs until the first hard frost. Until then, the practical takeaway from reading the two announcements together is narrower than either one alone. Repellent and standing-water removal still matter most. In the Niagara area, it is the clock-based part of the advice that should be held loosely.
via CHCH News, reporting by Julia Vellucci and Mason Goodfellow. Additional detail from CP24, Global News and the Public Health Agency of Canada.
