Briefing Document: Spatial and Temporal Analysis of West Nile Virus Infection in Ontario, Canada (2003-2022)
Citation: Talbot, B., Ludwig, A., O’Brien, S.F. et al. Spatial and temporal analysis of West Nile virus infection in mosquito and human populations based on arboviral detection testing data. Sci Rep 14, 31343 (2024). https://doi.org/10.1038/s41598-024-82739-3
Dates: Received - 09 July 2024 | Accepted - 09 December 2024 | Published - 28 December 2024
Prepared for: Saleh Lab
Executive Summary:
This study analyzed over two decades of data from Ontario, Canada, to understand factors influencing West Nile virus (WNV) infection in humans. Researchers integrated mosquito surveillance, human blood donation testing, and environmental and socioeconomic data. Their findings indicate that higher habitat suitability for the primary mosquito vector in southern regions and lower socioeconomic status are significant risk factors for human WNV infection. The research also identified climatic variables like temperature and precipitation as important for mosquito populations and WNV transmission. Ultimately, this spatio-temporal analysis aims to inform public health interventions to reduce the risk of mosquito-borne diseases in Canada.
Main Themes and Important Ideas/Facts:
1. Endemic Nature and Surveillance of WNV in Ontario:
- WNV was introduced to North America in 1999 and Canada in 2001, rapidly spreading and becoming endemic in much of the continent.
- Ontario has experienced an increase in WNV cases over time with interannual fluctuations and several epidemics.
- Since 2003, regular WNV detection testing has been conducted on most blood donations in Canada to prevent transmission to recipients.
- While not perfectly representative of the general population, blood donor testing data offers a valuable "representative snapshot of spatio-temporal variations in WNV infection risk in the human population."
- Traditional human case reporting has limitations, including underreporting of asymptomatic infections.
2. Key Mosquito Vectors and Their Dynamics:
- Culex pipiens/restuans and Aedes vexans are identified as important mosquito vectors for WNV in northeastern North America, acting as bridges between avian hosts and humans.
- Cx. pipiens/restuans had a higher minimum infection rate (MIR) for WNV compared to Ae. vexans. The overall MIR was higher in the latter half of the study period (2013-2022).
- The abundance and occurrence of Ae. vexans generally decreased across the study years, while these remained relatively stable for Cx. pipiens/restuans.
- Peak abundance and occurrence of both vector species, as well as WNV infection prevalence in mosquitoes and human blood donors, occurred in August and September.
- "No WNV-positive mosquito pools were observed from November to May of any year."
- Interestingly, Culex tarsalis, a WNV vector more common in western North America, was also observed in Ontario.
3. Environmental Factors Influencing WNV Risk:
- Ecological niche modeling revealed that temperature was the most important explanatory variable for the habitat suitability of both mosquito species and WNV.
- Precipitation had a strong effect on Ae. vexans habitat suitability, with average values being optimal.
- Land cover had a moderate importance, with wetlands and residential areas with low to medium green cover showing high suitability for both mosquito species and WNV. Grasslands were suitable for mosquitoes but not WNV.
- The highest projected habitat suitability for both mosquito species and WNV was concentrated in southern Ontario, particularly the corridor from Windsor to Toronto. WNV habitat suitability was more confined to major urban centers.
4. Socio-economic and Demographic Factors Associated with Human WNV Infection:
- Statistical modeling at the census subdivision level identified lower income (proportion of households earning less than $20,000) as a strong positive predictor of human WNV infection (IRR = 2.8).
- Conversely, a high proportion of residents spending 30% or more of their income on shelter showed a strong negative association with WNV infection (IRR = 0.7).
- Habitat suitability index for Cx. pipiens/restuans was also a significant positive predictor of human WNV infection (IRR = 2.0).
- "Our results show that high habitat suitability of the main WNV vector in southerly locations of this region, Cx. pipiens/restuans (IRR = 2.0), and variables related to lower income (IRR = 2.8), and shelter infrastructure spending (IRR = 0.7), were key risk factors associated with WNV infection among blood donors from 2003 to 2022 across Ontario (R2 = 0.67)."
- Population density and residential land cover did not show a significant effect on human WNV infection in the multivariable models in this study, contrasting with some previous research.
5. Individual-Level Factors:
- Analysis at the individual blood donation level found that donor age, year of detection test, and month of detection test had statistically significant positive associations with WNV infection. However, the model had poor statistical power due to the low number of positive cases.
- "Donor age, year of detection test and month of detection test had a slope coefficient between 1.002 and 1.003 (Table 4), suggesting a positive association of these variables with WNV infection."
- The cumulative WNV infection rate was slightly higher for male donors and much higher for donors older than 65.
6. Implications for Public Health Interventions:
- The study highlights modifiable risk factors, such as housing infrastructure and socio-economic status, that can be targeted for interventions.
- "For example, our study supports the need for government education campaigns and incentives facilitating renovations aimed at reducing mosquito habitat and/or exposure to mosquitoes, especially in areas with highly suitable habitat for Cx. pipiens/restuans in Southern Ontario..."
- The findings underscore the need for rigorous surveillance in southern Ontario due to the strong influence of temperature on mosquito vector habitat and subsequent human infection risk, especially in the context of climate change.
- The study provides a model for using blood donor testing data for widespread arboviral surveillance.
7. Limitations:
- Nucleic acid tests used have a potential for cross-reactivity with other viruses in the Japanese encephalitis serocomplex.
- Mosquito sampling and human donor data are highly concentrated around urban and metropolitan areas.
- Blood donor population may not perfectly represent the general population.
Quotes:
- "While the blood donor population isn’t absolutely synonymous with the general population, data from WNV detection tests on blood donations offer a representative snapshot of spatio-temporal variations in WNV infection risk in the human population."
- "Mosquito species of the genus Culex, such as Culex pipiens and Culex restu...