Abstract
West Nile Virus (WNV) first arrived in Ontario, Canada in 2001 and has since spread throughout most of the province, causing disease in humans. The provincial government established a province-wide surveillance program to monitor WNV transmission throughout the 36 regional health units. Here we have acquired records of WNV human and mosquito surveillance from 2002 to 2013 to describe seasonal and geographic trends in WNV activity in southern Ontario. Additionally, we obtained climate data from seven municipalities to investigate how temperature and precipitation affect WNV transmission dynamics. We identified a strong quadratic relationship between the number of confirmed human cases and positive Culex mosquito pools recorded at the end of each year (R2 = 0.9783, p < 0.001). Using Spearman rank correlation tests, we identified that the minimum infection rate of Culex pipiens/restuans pools are the strongest predictor of human cases at a 1 week lag period. We also identified positive correlations between minimum infection rates, temperature, vector abundance, and cumulative precipitation. Global Moran’s I index indicates strong positive autocorrelation and clustering of positive Culex pool counts in southern Ontario. Local indicators of spatial association tests revealed a total of 44 high-high and 1 high-low trap locations (n = 680). In the current work we have identified when and where hot spots of WNV activity have occurred in southern Ontario. The municipalities surrounding the western shore of the Lake Ontario and Windsor-Essex County have the largest records of positive mosquitoes and human cases. We identified that positive mosquitoes are a strong indicator of human cases to follow in the coming weeks. An epidemic action threshold of cumulative positive Culex pools was established, allowing Ontario public health officials to predict an epidemic at epidemiological week 34 (rho = 0.90, p < 0.001). These data have the potential to contribute to more efficient larvicide programs and awareness campaigns for the public.
Highlights
Despite more than a decade of pesticide use and awareness campaigns, West Nile virus (WNV; Family Flaviviridae, genus Flavivirus), an arthropod-borne virus that is transmitted through the bite of an infected mosquito, continues to be the leading cause of mosquito-borne disease in Canada [1,2,3]
West Nile virus was first detected in Ontario, Canada in Peel regional (PEE) on 31 August 2001 from a Cx. pipiens/restuans pool
We identified that the Spearman Rank correlation coefficient given a ten-week lag in data was 0.90 and only slightly increased to 0.91 by lag twelve, indicating that the cumulative number of Culex pools recorded by epi-week 34 is a sufficient action threshold for WNV epidemics is Ontario
Summary
Despite more than a decade of pesticide use and awareness campaigns, West Nile virus (WNV; Family Flaviviridae, genus Flavivirus), an arthropod-borne virus that is transmitted through the bite of an infected mosquito, continues to be the leading cause of mosquito-borne disease in Canada [1,2,3]. WNV is a member of the Japanese encephalitis virus serogroup along with other viruses that cause encephalitic disease in humans such as Japanese encephalitis virus and St. Louis encephalitis virus [4,5]. If infection does occur in humans, the severity can vary greatly; *80% of infections are asymptomatic, *20% develop into West Nile fever, and < 1% develop into deadly neuroinvasive disease [6,7]. The Public Health Agency of Canada (PHAC) estimates that an additional 18,000–27,000 human WNV cases may have occurred and gone unreported since most WNV cases are asymptomatic [3]
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