Mosquitoes in Sea-to-Sky region carry virus

Two mosquito species in the Sea-to-Sky corridor tested positive for California serogroup virus, a mosquito-borne pathogen that can cause illness in people, according to a B.C. Centre for Disease Control surveillance project.

The pilot project ran from June to August 2025 in response to a 2024 cluster of pediatric encephalitis cases in the Whistler area. Researchers collected and identified mosquitoes at 11 sites across Squamish, Whistler, and Pemberton, examining 2,575 specimens from 27 different species.

Of more than 170 mosquito pools tested at the BCCDC laboratory, California serogroup virus was detected in two: Culex pipiens/restuans and Aedes cinereus. The levels were low, and researchers were unable to confirm the specific strain.

West Nile virus was not detected in any pools.

California serogroup virus includes 18 distinct viruses. Snowshoe Hare and Jamestown Canyon are the most common in Canada. Most infections in people are asymptomatic or cause mild flu-like illness, but in rare cases the virus can lead to encephalitis or meningitis.

Between 2009 and 2024, B.C. recorded 15 known cases of California serogroup virus. There were no known cases in 2025.

Stefan Iwasawa, a BCCDC vector specialist, said the project provided a "snapshot" of what's happening in one part of the province. With climate change raising temperatures, exposure to mosquitoes may increase. "Having a better understanding of the mosquito species that people living in BC are likely to encounter — and the viruses some of them can carry — will help inform preparedness, prevention and response efforts," Iwasawa said.

The project was a partnership between the B.C. Centre for Disease Control, Vancouver Coastal Health, UBC, the Provincial Health Services Authority, and the Squamish and Líl'wat nations.