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

Scientists began looking for the invasive mosquitoes between Squamish and Pemberton after three pediatric encephalitis infections in the region in August 2024.

Anya F. Smith, a senior scientist with the B.C. Centre for Disease Control (BCCDC), says in a statement that researchers found two mosquito species that tested positive for California serogroup virus, a group of mosquito-borne viruses that can cause encephalitis and meningitis in humans.

Among the mosquitoes detected with the virus were the northern house mosquito, which is native to Africa, Asia and Europe. 

A statement issued by the BCCDC, Vancouver Coastal Health, the University of B.C. and the Provincial Health Services Authority says that the detected virus levels were low, so the team was unable to confirm the specific strain of California serogroup virus that was found.

Stefan Iwasawa with the BCCDC says the study can help inform the response to future infections.

“With climate change causing temperatures to rise, we may see an increase in our exposure to mosquitoes,” said Iwasawa. 

“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 [California serogroup virus] infections.”

The statement says there were 15 known cases of the virus in B.C. between 2009 and 2024, and no known cases in the Sea-to-Sky corridor in 2025.

It says the risk of serious illness in B.C. from California serogroup virus is low.