As a dead crow tested positive for West Nile virus in Ontario this week, Canadians may be wondering: just how prevalent are such mosquito-borne viruses in Canada, and how risky is the summer camping trip?
Mosquitoes are the most dangerous animals on Earth, carrying many diseases. Malaria alone takes hundreds of millions of lives each year. Cases of dengue, yellow fever and chikungunya are projected to rise globally due to climate and land-use change.
While Canada’s cold weather currently protects us from the most serious mosquito-borne diseases, local species do transmit lesser-known seasonal viruses like California serogroup viruses, Eastern Equine encephalitis and West Nile virus.
As temperatures rise, tropical species such as the Asian tiger mosquito are also becoming more established in Canada, along with the deadly viruses they transmit.
Surveillance is our best defence. Our lab group has collaborated with Agriculture and Agri-Food Canada and the Public Health Agency of Canada to survey mosquitoes in the South Nation River watershed near Ottawa since 2017. We’ve developed cost-effective strategies to improve the detection of mosquito-borne viruses.
West Nile virus
While West Nile virus is becoming more prevalent in Canada, the good news is that only around 20 per cent of infections result in flu-like symptoms. Less than one per cent of cases develop life-threatening neuroinvasive symptoms like meningitis or encephalitis.
West Nile virus can be deadly for the most vulnerable, however. Research shows organ transplant recipients have up to a 40 per cent chance of developing neuroinvasive symptoms.
The risk of encephalitis is also significantly higher for adults aged over 60, or those with co-morbidities like heart disease, hypertension and diabetes.
What’s troubling is that West Nile virus and other mosquito-borne illnesses may be under-detected. In 2022, there were 47 officially diagnosed cases in Canada; of these, 64 per cent included neuroinvasive symptoms, suggesting that milder infections remain undiagnosed.
(Wikimedia Commons/Alexis Lours), CC BY
Mosquito surveillance
Ontario, Manitoba, Saskatchewan and the Northwest Territories are tracking West Nile virus using mosquito surveillance, but these programs have some limitations.
Current surveillance aims to assess human risk by monitoring areas where humans and mosquitoes overlap. In Ontario, because sites are monitored only after documented cases, high-density areas like cities and suburbs are disproportionately represented, even though West Nile virus is primarily spread in rural areas.
While humans can be infected with West Nile virus, we don’t carry high enough levels of the virus to spread it to an uninfected mosquito that bites us. The virus can only proliferate through an infection cycle between certain mosquito and bird species — both present in high numbers in rural and agricultural regions. West Nile virus has been detected in over 300 species of bird and is most common for birds in the passerine family, like the crow and the American robin.
How can we confidently predict an outbreak if we aren’t properly monitoring these areas?
Our lab group has been sampling mosquitoes in the Greater Ottawa area, along with our collaborators. By creating a surveillance program that monitors different habitat types in Ontario, we can get ahead of West Nile virus.
(Unsplash/Bruno Mauricio)
Genomics as a tool
How do we pick the right areas to monitor? In our work, we’re interested in using genomics to understand how and where the virus is spreading.
The first possible approach is phylogeography — the study of how evolutionary processes affect the genetic structure of lineages across space and time. Exciting research has used phylogeography to understand how temperature has impacted the speed and location of West Nile virus spread across North America. The researchers even assessed whether birds or mosquitoes were the most likely culprits in spreading the virus.
This information helps pinpoint broad areas at risk of the virus and the possible transmission routes. But what if your area already has West Nile virus?
In that case, you need to look at these patterns at a local level. A landscape genetics approach can help us match patterns in genetic relatedness across space to specific landscape and climate features. This tells us how and where the virus is circulating in your local community, helping inform where we need surveillance most.
While this method has yet to be used in West Nile virus surveillance, it has been successfully applied by some researchers to study how human movement and the landscape have impacted the spread of malaria in western Kenya. Their work can now be used to inform targeted control strategies in the region.
The future of surveillance
Monitoring and prevention are inseparable. Only after we identify what and where a pathogen is can we make informed decisions about what steps to take. We cannot control mosquito-borne diseases without knowing where they are and how they spread.
We need to establish a two-pronged approach: habitat and genomics-informed surveillance. These tools are essential for controlling the mosquito-borne viruses currently here, and those that could threaten us in the future.
But please, keep camping. Just remember your long-sleeved shirt, bug spray and — for the super keen — a net over your cap.
Roqeeb Akinbile, a master’s student in bioinformatics at Carleton University, co-authored this article.
