a snapshot of what's happening with mosquitoes in one part of B.C.
When three children in the Whistler area developed brain inflammation in the summer of 2024, public health officials recognized in their suffering a question that demanded an answer. A year later, a coalition of researchers, health authorities, and Indigenous nations had combed the Sea-to-Sky corridor, trapping thousands of mosquitoes and testing them for the viruses that might explain what had happened. The discovery of California serogroup virus in two local mosquito species offers no definitive verdict — but it opens a door of understanding at a moment when a warming climate may be quietly expanding the reach of the insects that carry such pathogens.
- Three children developing encephalitis in the same region within a single summer was unusual enough to set off alarm bells among B.C. public health officials.
- A multi-partner surveillance team spent the summer of 2025 trapping over 2,500 mosquitoes across eleven sites in Squamish, Whistler, and Pemberton — a labor-intensive effort to find what was hiding in the local insect population.
- Two mosquito species tested positive for California serogroup virus, the same family of viruses capable of causing the rare but serious brain inflammation seen in the 2024 pediatric cases.
- The signal is real but faint — viral levels were low, the specific strain remains unidentified, and no human cases were recorded in B.C. during 2025.
- Researchers warn that climate change may expand mosquito ranges and human exposure, making this kind of baseline surveillance less a one-time project and more a long-term public health necessity.
In the summer of 2024, three children in the Whistler area developed encephalitis — swelling of the brain — in a cluster that public health officials found deeply unusual. The cases demanded explanation, and by the following year they had launched a coordinated surveillance operation across one of British Columbia's most travelled mountain corridors.
The pilot project, running from mid-June through late August 2025, brought together the B.C. Centre for Disease Control, Vancouver Coastal Health, UBC, the Provincial Health Services Authority, and the Squamish and Líl̓wat nations. Led by Dr. Anya F. Smith, the team placed traps at eleven sites across Squamish, Whistler, and Pemberton, collecting 2,575 mosquitoes from 27 species. The specimens were identified at UBC and pooled for laboratory testing, with each pool screened for California serogroup virus and West Nile virus.
Two species — Culex pipiens/restuans and Aedes cinereus — came back positive for California serogroup virus, a family of eighteen related viruses that most often causes no symptoms or mild illness, but can in rare cases trigger encephalitis or meningitis. West Nile virus was not detected. The findings carried important caveats: viral levels were low, the specific strain could not be confirmed, and B.C. recorded no human cases of the virus in 2025 — only fifteen confirmed cases had been documented across the entire province between 2009 and 2024.
Vector specialist Stefan Iwasawa described the project as a snapshot of one region at one moment in time, but framed its value in longer terms. As climate change pushes temperatures higher and potentially expands mosquito populations, he argued, the real worth of this work lies in the foundation it builds — knowing which species live where, and what they carry, so that public health officials are better positioned to act if children fall ill again.
In the summer of 2024, three children in the Whistler area developed encephalitis—swelling of the brain—in what public health officials recognized as an unusual cluster. The cases prompted urgent questions: What was causing this? Could it happen again? By the following year, those questions had launched a full surveillance operation across one of British Columbia's most populated corridors.
The pilot project began in June 2025, a partnership between the B.C. Centre for Disease Control, Vancouver Coastal Health, UBC, the Provincial Health Services Authority, and the Squamish and Líl̓wat nations. The team's mission was straightforward but labor-intensive: trap mosquitoes across the Sea-to-Sky region, identify them, and test them for viruses that might explain what had happened to those three children. Dr. Anya F. Smith, the principal investigator and BCCDC's public health response senior scientist, led the effort.
From mid-June through late August 2025, researchers placed traps at eleven sites spanning Squamish, Whistler, and Pemberton. The haul was substantial: 2,575 mosquitoes representing 27 different species. Each specimen was catalogued and identified at UBC's Department of Zoology. The team then pooled mosquitoes of the same species collected from the same location and time period, creating more than 170 pools for laboratory analysis. At the BCCDC laboratory, each pool was tested for two viruses: California serogroup virus and West Nile virus.
The results came back with a cautious signal. Two mosquito species—Culex pipiens/restuans and Aedes cinereus—tested positive for California serogroup virus. The virus itself is not new to Canada. It belongs to a group of eighteen related viruses, with Snowshoe Hare and Jamestown Canyon being the most commonly detected in the country. Most people infected with California serogroup virus either show no symptoms or experience mild, flu-like illness. But in rare cases, the virus can trigger encephalitis or meningitis—the very conditions that had alarmed public health officials the year before.
Yet the findings came with important caveats. The viral levels detected in the mosquito pools were low, and the team could not determine which specific strain of California serogroup virus had been found. West Nile virus, another mosquito-borne pathogen of concern, was not detected in any of the samples. Looking at the broader picture, B.C. had recorded only fifteen confirmed cases of California serogroup virus between 2009 and 2024—and none in 2025, the year of this surveillance project.
Stefan Iwasawa, a vector specialist at the BCCDC, described the pilot project as a "snapshot" of what is circulating in one region of the province at one moment in time. But he also pointed toward a larger concern. As climate change drives temperatures upward, mosquito populations may expand and shift, potentially increasing human exposure to the insects and the pathogens they carry. The real value of this surveillance work, Iwasawa suggested, lies not in any single finding but in the foundation it builds. Understanding which mosquito species live in British Columbia, where they congregate, and what viruses some of them harbor will allow public health officials to prepare, prevent, and respond more effectively if cases emerge again.
Citas Notables
Two of the mosquito species that we tested showed positive results for California serogroup virus—a group of mosquito-borne viruses that can cause illness in people.— Dr. Anya F. Smith, principal investigator, BCCDC
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.— Stefan Iwasawa, BCCDC vector specialist
La Conversación del Hearth Otra perspectiva de la historia
Why did it take a cluster of three cases in children to trigger a full surveillance operation? Wasn't the virus already known to exist in Canada?
California serogroup virus was known, yes—but finding it in this region, in this pattern, in children specifically, was the alarm. Three pediatric encephalitis cases in one area in one year is not random. That's what made officials ask: Is this circulating here? How many mosquitoes are carrying it? What's the actual risk?
And what did they find when they looked?
Two mosquito species tested positive, but the viral levels were low and they couldn't even identify which specific strain it was. So the answer is: yes, it's here, but we don't yet know how much of a threat it poses.
That sounds like a non-answer wrapped in reassurance.
It's more honest than that. They found evidence of the virus in the environment. They didn't find a surge or a clear public health emergency in 2025. But they now have baseline data. If cases spike again, they'll know what to look for and where to look.
Climate change keeps getting mentioned. Are we supposed to be worried?
Warmer temperatures mean mosquitoes thrive in places they didn't before, and for longer seasons. The concern isn't apocalyptic—it's practical. More mosquitoes, more exposure, more opportunity for viruses to spread. That's why this surveillance work matters now, before the problem grows.
So what happens next?
They keep watching. They keep trapping and testing. They build a clearer picture of what's circulating and where. If cases emerge again, they'll have the knowledge to respond faster.