This map shows the distribution of positive detections for West Nile Virus (WNV) and St. Louis Encephalitis Virus (SLEV) for mosquito pool and bird specimens in California between 2003 and 2018 and was created using Mapbox GL JS. Hexbin layers were used to represent both viruses to show density of positive detections in mosquito pool and bird specimens. Individual point layers for each virus are also available for display along with information about each point of detection. West Nile Virus (WNV) and St. Louis encephalitis Virus (SLEV) are "arthropod-borne flaviviruses" that have a presence in Californa (Oyer, Beckham, Tyler 433 'Handbook of Clinical Neurology: Chapter 20 - West Nile and St. Louis encephalitis viruses, 2014). WNV is a well known commodity at this point and typically catches local news headlines when the virus has been found in a deceased bird or a mosquito pool has tested positive, but there has been a reemergence of SLEV in California. This reemergence began in 2015 after a long period without detection.* Both viruses can be found throughout California and typically cycle between mosquitoes and birds. Infection in humans is a byproduct of living in close proximity to both hosts (birds) and vectors (mosquitoes) for WNV and SLEV. Most infections of WNV and SLEV in humans are asymptomatic and result in mostly flu-like symptoms, but in a small number of cases those infected "develop neuroinvasive infection (meningitis, encephalitis, and myelitis) that can cause significant illness and death" (Oyer, Beckham, Tyler).
* Diana A. Singh, Maria L. Salas, Robert E. Snyder, Oliver Oyler, Sarah Skallet, Tina Feisli, Chris Preas, and Jill Hacker. The Reemergence of St. Louis Encephalitis Virus In California. Poster presented at California Department of Public Health, Richmond, California
The California county boundary data comes from the Census Bureau's Tiger Line/Shape Files website. West Nile Virus and St. Louis Encephalitis data were scraped from the California Vectorborne Disease Surveillance Map Gateway. Cleaned data and Jupyter notebooks used to clean the data can be found in the map's GitHub repo.
An analysis was performed on this WNV and SLEV data to determine whether there may be higher numbers of positive detections in California's highly vulnerable census tracts among the mosquito pool and bird samples collected. The CDC has released data files for 2010, 2014, and 2016 where it has determined which census tracts may be the most vulnerable to events such as natural disasters or disease outbreaks. A simple point-in-polygon analysis was performed using PostgreSQL and PostGIS within a Docker container to determine how many of the positive detections for given years fall within the CDC determined vulnerable census tracts. In order to look at the most vulnerable census tracts and what points may fall within these tracts, a CDC Social Vulnerability Index value of .75 or higher was used. All commands for the installation of a PostgreSQL/PostGIS docker container, along with commands used for the point-in-polygon analysis, can be found here.
In 2010, of the 8048 census tracts in California, 2001 had a CDC SVI value greater than or equal to .75. This means that about a quarter of all California census tracts were deemed highly vulnerable according to the CDC. There were also 1041 positive detections of WNV across California in 2010 for mosquito pools and bird specimens. Looking at how many of these detections occurred in highly vulnerable census tracts, we see that there were 287 positive detections within these census tracts. Given these values this means that ~28% of positive detections for mosquito pool and bird specimens for WNV occurred within highly vulnerable census tracts.
2014 saw the CDC determine that 2003 of California's census tracts were highly vulnerable, a slight increase in vulnerable census tracts compared to 2010 which had 2001 highly vulnerable census tracts. Even with the slight increase, about a quarter of California's census tracts were deemed highly vulnerable. California also saw 3550 positive detections for WNV in 2014 for mosquito pools and bird specimens. Of these positive detections, 528 occurred in California's highly vulnerable census tracts. With these values we see that ~15% of California's positive detections among mosquito pools and bird specimens occurred within highly vulnerable census tracts.
In 2016 the CDC classified 1995 of California's census tracts as highly vulnerable, which is still around a quarter of all census tracts.Positive detections for WNV in 2016 for mosquito pool and bird specimens were 2527. Of these 2527 positive detections, 426 occurred in California's highly vulnerable census tracts. With these values we see that ~17% of California's positive detections among mosquito pools and bird specimens occurred within highly vulnerable census tracts.
SLEV was relatively dormant for quite some time in California until its reemergence in 2015. Since there is no data for the previous years, 2016 is the only year we can look at for SLEV to determine whether detections fell disproportionately in highly vulnerable census tracts. Remember that in 2016 the CDC classified 1995 of the census tracts in California as highly vulnerable, about a quarter of all census tracts. There were 186 positive detections for SLEV in mosquito pools and bird specimens in California in 2016. The number of positive detections that occurred in highly vulnerable census tracts was 122. Of all positive detections, ~66% occurred within census tracts that were deemed highly vulnerable by the CDC. Without data for previous or ensuing years it's difficult to determine any sort of pattern with regards to SLEV detections for mosquito pools and bird specimens falling within highly vulnerable census tracts.
If we look across 2010, 2014, and 2016 for WNV in California, 2010 started with a high of ~28% of detections occurring within highly vulnerable census tracts with a considerable decline going into 2014 (~15% of detections) and a slight increase in 2016 (~17% detections). What could account for this decline? Education efforts may have been ramped up during the years following 2010, along with greater mosquito control efforts. Could there be an environmental explanation for the decline? In "Drought-Induced Amplification and Epidemic Transmission of West Nile Virus in Southern Florida", Shamana, Day, and Stieglitz argue that periods of prolonged spring drought lead to greater exposure between birds (hosts) and mosquitoes (vectors) which in turn has the potential for greater amplification of WNV. In California, 2014 - 2016 were exceptional drought years and this environmental cause for the decline would not seem to hold.
|Year||SVI 0 - .24 as % of Total Detections||SVI .25 - .49 as % of Total Detections||SVI .50 - .74 as % of Total Detections||SVI >=.75 as % of Total Detections|
|* Points that fell within census tracts with uncalculatable SVI values were dropped (2 points dropped for 2010, 9 points dropped for 2014, and 12 points dropped for 2016)|
How do detections look when taking into account other census tracts within the index? One thing that we do see across these years is that highly vulnerable census tracts do not make up the majority of areas where positive detections occurred. We see that those tracts that that are part of the mid to mid-high range of the CDC's index make up a greater percentage of the positive detections. Unfortunately, these same comparisons could not be made SLEV given its reemergence in 2015 and the most recent SVI data files from the CDC are for 2016. For 2014 and 2016, it's the low to mid range census tracts that make up the index that have the greatest percentages of WNV detections. While we may not see the majority of positive detections for WNV in highly vulnerable census tracts among mosquito pool and bird specimens, there is still potential that these highly vulnerable census tracts may be burdened with a higher percentage of the human cases of WNV that occur and is an area for further study. Residents of highly vulnerable census tracts may need to travel to census tracts that are less vulnerable, which may also have a higher percentage of positive detections as seen in 2014 and 2016, and their line of work may expose them to mosquitoes more so than in their home census tract. With regards to SLEV, one area of possible study would be its slow creep north from 2015 - 2018 which can be seen in the map when using the year slider to visualize the point data. It starts off in 2015 solely in Riverside county and then begins its expansion north in the ensuing years.