A mosquito surveillance project launched in response to a cluster of pediatric encephalitis cases in the Whistler area detected invasive species and evidence of California serogroup virus (CSGv) among mosquitoes in the Sea-to-Sky region. A collaboration with Sḵwx̱wú7mesh Úxwumixw (Squamish Nation), L̓il̓wat7úl (Líl̓wat Nation), the University of British Columbia (UBC) Department of Zoology, and Vancouver Coastal Health (VCH), the BC Centre for Disease Control (BCCDC) study emphasizes the importance of monitoring mosquito populations throughout BC to better understand the risks they can pose to human health.
“As part of a pilot project funded by the UBC Centre for Disease Control, we collected and identified mosquitoes along the Sea-to-Sky corridor, from Squamish to Pemberton, looking for invasive and endemic mosquito species," said principal investigator and BCCDC Public Health Response senior scientist Dr. Anya F. Smith.
“Two of the mosquito species that we tested showed positive results for CSGv — a group of mosquito-borne viruses that can cause illness in people."
“The pilot project gave us a snapshot of what's happening with mosquitoes in one part of BC," said Stefan Iwasawa, a BCCDC vector specialist. “With climate change causing temperatures to rise, we may see an increase in our 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."
- Teaming up with VCH, UBC, Squamish Nation and Líl̓wat Nation, BCCDC launched a surveillance project following three pediatric cases of encephalitis (swelling of the brain) caused by California serogroup virus, reported in the Sea-to-Sky region in August 2024.
- Work on Sḵwx̱wú7mesh Úxwumixw and L̓il̓wat7úl territory engaged leaders in both communities, who shared valuable information about their lands and helped facilitate the selection of sampling sites.
- From June 16 to August 25, 2025, mosquitoes were collected in traps placed at 11 sites in the Squamish, Whistler and Pemberton areas. 2,575 mosquitoes were collected, including 27 different species, and identified at the UBC Department of Zoology.
- 171 mosquito pools (groups of the same species, found around the same time and location) were tested for CSGv and WNv at the BCCDC Public Health Laboratory. CSGv was detected in two mosquito pools: Culex pipiens/restuans and Aedes cinereus.
- Since the detected virus levels were low, the team was unable to confirm the specific strain of California serogroup virus that was found. West Nile virus was not detected in any of the mosquito pools that were tested.
- Project leads from BCCDC and VCH shared the study results in-person, meeting with Squamish Valley Elders and attending a Líl̓wat Nation open house in early May, where community members asked questions and offered feedback
- CSGvs are mosquito-borne pathogens that can cause infections in people. Typically these infections are either asymptomatic or cause mild flu-like illness. In rare cases, they can cause encephalitis or meningitis in humans.
- The California serogroup includes 18 viruses, with Snowshoe Hare and Jamestown Canyon being the most commonly found in Canada.
- In 2024, a small cluster of pediatric CSG viral encephalitis cases were reported in the Sea-to-Sky corridor, bringing renewed attention to these types of viruses.
- Between 2009 and 2024, there were 15 known cases of CSGv in BC. There were no known cases in Sea-to-Sky corridor in 2025.
- The risk of developing serious illness from CSGvs in BC is still considered to be low.
- CSGvs are a reportable condition in BC as of May 15, 2026.
Outdoor recreation is an important part of everyday life. When you're outdoors having fun and staying active, take steps to protect yourself and your family:
- Take extra precautions around dusk to dawn when mosquitoes tend to be most active.
- Wear protective clothing outside.
- Put on long-sleeve shirts or jackets and long pants that mosquitoes cannot bite through and tuck your pants into socks for extra protection.
- Avoid dark coloured clothing as it can attract mosquitoes.
- Use an approved mosquito repellent. Putting on insect repellents that are registered by Health Canada, like those containing DEET or Icaridin, is a safe and effective way to protect yourself from mosquito bites.
- Remove or limit standing water around your home. Mosquitoes lay eggs in standing water, even small amounts.
- Clean up or regularly drain anything that can hold stagnant water (flowerpots, rain gutters, tarps, tires etc.).
- Change the water in birdbaths twice a week and install a fountain in ornamental ponds or stock them with fish.
- Regularly maintain swimming pools with disinfection and a working pump, and remove water that collects on pool covers.
- Use windows and door screens if possible. Check them for holes and make sure they fit snugly into frames so mosquitoes cannot get in.
“While the risk of serious illness from mosquito-borne viruses remains low in the Sea-to-Sky area, this project highlights the importance of ongoing monitoring and taking practical steps to prevent mosquito bites, like reducing standing water near your home, using approved bug spray, and wearing light, protective clothing," said Dr. Ceinwen Pope, medical health officer for Vancouver Coastal Health – Public Health.
- BCCDC, VCH and UBC are grateful to Sḵwx̱wú7mesh Úxwumixw (Squamish Nation) and L̓il̓wat7úl (Líl̓wat Nation) for their collaboration and valued partnership in a research project that took place on their ancestral homelands.
- The mosquito surveillance project was funded by a UBC CDC Strategies Initiatives STARTS Grant Program.
- For more information on mosquito-borne diseases in BC and Canada, visit the BCCDC and Public Health Agency of Canada websites.
