Brain-swelling virus type found in Sea-to-Sky mosquitoes after three children fell ill

A mountain destination better known for bike trails and ski-friendly powder has drawn attention from health officials for a very different reason.

An outbreak of brain swelling in three children in August 2024 led scientists to an unexpected first in British Columbia's Sea-to-Sky corridor: a specific set of mosquito-borne viruses present in local insects.

What happened?

An encephalitis cluster in the Whistler area about two years ago may have been linked to mosquito-borne viruses, according to Earth.com, setting off a broader search for what may be circulating in the region.

A team led by Anya Smith from the British Columbia Center for Disease Control reportedly spent the summer of 2025 trapping mosquitoes at 11 sites between Squamish and Pemberton.

That summer, Smith's team collected 2,575 mosquitoes from 27 species. Those samples were screened for West Nile virus and California serogroup virus, a family of mosquito-borne viruses.

Lab testing found no West Nile virus, but two pools were positive for California serogroup virus.

Even though the viral signal was too weak to identify the exact strain, the result still marked the first documented detection of these viruses in mosquitoes from the Sea-to-Sky corridor. 

The survey also identified several invasive species, including northern house mosquitoes.

How is the risk level impacted?

The discovery reshapes the picture of risk in a place where many people may not think much about mosquito-borne illness.

Most infections caused by California serogroup viruses are mild or go unnoticed, but in rare cases, they can lead to serious neurological illness, including brain swelling.

Improved testing in recent years has also suggested such infections may be more common than previously believed, especially in summertime cases of unexplained brain inflammation.

"With climate change causing temperatures to rise, we may see an increase in our exposure to mosquitoes," Stefan Iwasawa, a mosquito specialist at the BCCDC, said in a statement.

Overall, the risk appears to remain low in the area. British Columbia had 15 confirmed human cases from 2009 to 2024, according to a BCCDC statement cited by The Canadian Press, with no Sea-to-Sky cases reported in 2025.

Detecting the virus in mosquitoes gives doctors, public health teams, and communities a clearer sense of what to watch for. It also shows how surveillance can help officials respond earlier, instead of trying to piece things together only after people become sick.

What's being done?

According to Earth.com, BCCDC scientists teamed up with university zoologists, the regional health authority, and leaders from the Squamish and Líl̓wat Nations to decide where to put traps and map which mosquito species live in the corridor.

That kind of monitoring gives officials a baseline for the future. It can help guide testing, track whether invasive species are becoming established, and support doctors evaluating unusual fever or neurological symptoms during mosquito season.

The practical advice remains familiar: Dump standing water where mosquitoes can breed, wear approved repellent, and cover exposed skin around dusk and other peak mosquito hours. Those steps will not eliminate risk entirely, but they can reduce the chances of being bitten.

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