Description
1. Canada’s Yearly West Nile Virus Human Cases
Description of the bar chart: Y-Axis (Vertical Axis): Represents the number of human cases. The scale ranges from 0 to 2,500 cases. X-Axis (Horizontal Axis): Represents time, measured in years. Each bar’s height corresponds to the total number of human WNV cases confirmed in that specific year. The chart includes data from 2002 to 2022, providing a comprehensive view of the virus’s activity over a 21-year period.
Trends and Patterns:
Initial Introduction and Rapid Escalation (2002-2003): The virus was first identified in North America in 1999 and arrived in Canada around 2001-2002. The chart shows a significant number of cases in its first major year (2002), followed by a massive peak in 2003. This is a classic pattern for a newly introduced pathogen, finding a susceptible population and favorable conditions.
Major Peak and Subsequent Decline (2003-2008): The years following the 2003 peak show a volatile but generally declining trend. There is a noticeable secondary peak around 2007, though it is smaller than the 2003 peak. This period represents the virus becoming endemic (established within the ecosystem) but with fluctuating case numbers based on environmental factors.
Period of Relative Quiescence (2009-2017): For nearly a decade, case numbers remained persistently low, with many years likely showing only a few hundred cases. This period indicates a potential equilibrium between the virus, mosquito vectors, bird hosts, and human populations, possibly influenced by control measures and acquired immunity in bird populations.
Significant Resurgence (2018 onwards): The chart shows a dramatic resurgence of cases starting around 2018, with a new, sharp peak that appears to rival or even exceed the initial 2003 outbreak. This is the most striking feature of the recent data.
Explanation of Results
The trends observed in the chart can be explained by a combination of ecological, climatic, and epidemiological factors:
- The Initial Peak (2003): This explosion in cases is characteristic of a naïve population—both human and avian—with no prior immunity to the virus. The mosquito vectors (Culex species) found abundant bird hosts to amplify the virus and efficiently transmitted it to humans.
- Volatility and Decline (2004-2008): After the initial outbreak, several factors contributed to the fluctuating decline:
- Public Awareness and Control Measures: Mosquito control programs (larviciding, spraying) and public health messaging (removing standing water, using repellent) became widespread, reducing human exposure.
- Acquired Immunity in Birds: High mortality in certain bird species (e.g., crows, jays) from the virus may have temporarily reduced the reservoir host population, slowing transmission. Surviving birds developed immunity.
- Climatic Variability: WNV transmission is highly dependent on weather. Warm springs and hot, humid summers accelerate mosquito breeding and the replication of the virus within mosquitoes. Cooler, drier years naturally suppress case numbers.
- The Recent Resurgence (2018-Present): The dramatic return of high case numbers is strongly linked to climate change:
- Warmer Temperatures: Consistently warmer temperatures expand the geographic range of mosquito vectors, lengthen the seasonal transmission period, and accelerate the virus’s extrinsic incubation period (the time it takes for a mosquito to become infectious after biting an infected bird).
- Extreme Weather Events: Increased frequency of heatwaves and heavy rainfall events that create ideal breeding grounds for mosquitoes.
- New Naïve Populations: A new generation of birds and humans without immunity, combined with optimal climatic conditions, can fuel large outbreaks even in areas where the virus has been present for years.
Public Health Implication
West Nile Virus in Canada has evolved from an emerging epidemic into an endemic public health concern, a threat now exacerbated by climate change. The recent resurgence is a powerful reminder that endemic diseases are not static threats.
The key takeaway is that West Nile Virus is a climate-sensitive illness. Its incidence is no longer predictable based on past patterns alone. Public health strategies must now adapt to this new reality, emphasizing:
- Enhanced and earlier mosquito surveillance.
- Climate-informed predictive modeling for outbreak risk.
- Sustained public education on personal protective measures, especially during increasingly warm and long summers.
The bar chart underscores the ongoing and potentially growing threat of vector-borne diseases in a warming world.
https://www.canada.ca/en/public-health/services/diseases/west-nile-virus/surveillance-west-nile-virus.html
2. Tracking WNV in Canadian Birds: Positive Test Results Since 2009
The data reveals a critical shift starting in 2018. After a period of relatively low activity, positive tests in birds surged dramatically, marking a new and persistent phase of high viral circulation. This elevated state has not subsided. The years 2022 and 2023 show consistently high values, confirming the virus is firmly established at a heightened level within the avian population. Most alarmingly, the data indicates that 2024 has reached the highest level on record, surpassing even the 2018 peak.
This trend is highly significant. Birds are the primary reservoir for West Nile Virus, and soaring infection rates among them directly predict increased human risk. The unprecedented high in 2024 suggests environmental conditions—such as warmer temperatures and altered precipitation patterns linked to climate change—are creating ideal and sustained conditions for mosquito breeding and viral transmission.
In summary, the chart indicates that West Nile Virus has not just resurged but has entered a new era of consistently high activity in Canada’s bird population, breaking records and posing a continued and growing public health threat. This necessitates enhanced surveillance and proactive public health measures.
3. Tracking WNV in Canadian Birds by Province Since 2009
https://www.canada.ca/en/public-health/services/diseases/west-nile-virus/surveillance-west-nile-virus.html
4. Human WNV neuroinvasive cases and deaths in Saskatchewan 2010–2024
5. West Nile Virus in British Columbia (2004-2024).
6. Search_Canada_Mosquito_borne_disease_surveillance data (Human, Bird, Horse and Mosquito) [admin]
7. Number of reported human West Nile virus cases, Ontario, by Year, 2003-2023 (Chart)
8. Number of reported human West Nile virus cases, Quebec, by Year, 2003-2023 (Chart)
Note:! Sorry, this project is focused on developing a data portal for Canada mosquito and mosquito-borne disease research at LAMPS. Therefore, most information is only available to authorized users (LAMPS members).