Invasive mosquitoes carrying brain virus found near Sea-to-Sky

Health officials in British Columbia have detected invasive mosquitoes and a virus that can cause brain swelling in humans in the Sea-to-Sky region north of Vancouver, following an investigation triggered by childhood encephalitis cases.

Researchers found two mosquito species testing positive for California serogroup virus — a group of mosquito-borne pathogens that can cause encephalitis and meningitis — after investigating three pediatric encephalitis infections in the region in August 2024. Among the detected mosquitoes was the northern house mosquito, a species native to Africa, Asia, and Europe.

Anya F. Smith, a senior scientist with the BC Centre for Disease Control, said in a statement that researchers detected low virus levels, preventing confirmation of the specific California serogroup strain.

Stefan Iwasawa with the BCCDC said the study helps inform future preparedness. "With climate change causing temperatures to rise, we may see an increase in our exposure to mosquitoes," he said. "Having a better understanding of the mosquito species that people living in B.C. are likely to encounter — and the viruses some of them can carry — will help inform preparedness, prevention and response efforts related to CSGv infections."

The BCCDC documented 15 known cases of California serogroup virus in B.C. between 2009 and 2024, with no known cases in the Sea-to-Sky corridor in 2025. The risk of serious illness in B.C. from the virus remains low.

The findings underscore how warming climates are shifting the range of disease-carrying insects northward — a shift public health agencies are racing to monitor and understand.