Two mosquito species that were tested as part of a surveillance project have shown positive results for a virus that can cause illness in people.
The surveillance project was launched in summer 2025 in response to a cluster of pediatric encephalitis cases – swelling of the brain – in the Whistler area where invasive species and evidence of California serogroup virus among mosquitoes in the Sea-to-Sky region was detected. Three pediatric cases were identified in 2024.
The pilot project was a partnership between the B.C. Centre for Disease Control, Vancouver Coastal Health, UBC and the Provincial Health Services Authority, as well as the Squamish and Líl̓wat nations.
Dr. Anya F. Smith said the research team collected and identified mosquitoes along the Sea-to-Sky corridor, from Squamish to Pemberton, looking for invasive and endemic mosquito species. Smith was the principal investigator and the BCCDC’s public health response senior scientist.
“Two of the mosquito species that we tested showed positive results for CSGv (California serogroup virus) — a group of mosquito-borne viruses that can cause illness in people.”
From June 16 to Aug. 25, 2025, mosquitoes were collected in traps placed at 11 sites in the Squamish, Whistler and Pemberton areas. A total of 2,575 mosquitoes from 27 different species were collected and identified at the UBC Department of Zoology.
More than 170 mosquito pools, or groups of the same species found around the same time and location, were tested for California serogroup virus and West Nile virus at the BCCDC laboratory.
California serogroup virus was detected in two mosquito pools: Culex pipiens/restuans and Aedes cinereus.
However, the levels were low and the team was unable to confirm the specific strain California serogroup virus that was found.
West Nile virus was not detected in any of the pools tested.
California serogroup virus is a mosquito-borne pathogen that can cause infections in people. It includes 18 viruses, with Snowshoe Hare and Jamestown Canyon being the most commonly found in Canada.
People infected will either be asymptomatic or will have mild flu-like illness. In rare cases, they can cause encephalitis or meningitis in humans.
Between 2009 and 2024, there were 15 known cases of California serogroup virus in B.C. There were no known cases in 2025.
Stefan Iwasawa, a BCCDC vector specialist, said the pilot project gave the team a “snapshot” of what’s happening with mosquitoes in one part of B.C. Isawa added that with climate change causing temperatures to rise, it may lead to an increase in exposure to mosquitoes.
“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 related to CSGv infections.”