1. West Nile Virus Assessment: Louisiana
Andrea Krutulis
Samantha Reichle
Sean Weyrich
Louisiana:
The Pelican State
What did we find?
Abstract
The occurrence of West Nile Virus has been a concern in the United States ever since its introduction in New York in Since then, the disease has continued to spread throughout the country. However, the factors influencing the spread still are not understood to a large extent. In analyzing the growth of the disease in the state of Louisiana from 2002 to 2004, we felt that population density, major highways, wetlands areas, as well as precipitation rates and temperature will all play significant roles in determining the areas of highest West Nile Virus activity. A base map showing population density, road networks, and wetlands, as well as a graph showing average precipitation and temperature, were produced. A second map showing disease incidents was then generated, and the two were overlaid to better observe any correlations. A map showing the avian flight path through the state was also produced to analyze the potential bird species that introduced the virus to the state. Finally, a graphical representation of disease incidents versus climate data was used to analyze correlation between incidence rates and temperature and precipitation. Results showed that population density, precipitation rates, and temperature all played significant roles in establishing areas of highest disease incidence, which occurred in Central, Southwest, and South Central Louisiana. While insufficient data concerning wetlands areas limits conclusive results, population and climatic conditions seem to have the greatest effects on West Nile Virus occurrence.
Methods
Why worry about
West Nile?
Increases in West Nile Virus Activity in Louisiana:
- Added a map of United States with county boundaries from UNC library
- Performed an attribute query for Louisiana counties
- Exported this data as a new layer to a new map
- Added five new fields in attribute table to display 2002 West Nile Virus data by county from USGS for each of the
following: avian, human, mosquito, sentinel and veterinary
- Entered data from USGS
- Added five more fields in attribute table to display 2004 West Nile Virus data by county from USGS for each of the
- Added five fields in attribute tale to display West Nile virus increase by county for each of the five categories
- Used the field calculator in attribute table to subtract 2002 West Nile Virus data from 2004 West Nile Virus data
for each category
- In first new data layer named “bird” did a definition query to find where avian increase was greater than 0
- Imported a picture of a bird
- Displayed data from definition query as a proportional symbol map using the imported picture of a bird
- Repeated last three steps for each category (human, mosquito, sentinel and veterinary)
- Added each layer to final map
Cases of West Nile Virus in Louisiana
- Important features within Louisiana..
- Added transportation terminals, drainage, major highways, states, places people live, water bodies, climate
divisions to the map.
- I clipped such that they focused on the designated area for our project Louisiana.
- After I overlaid the layers, I selected by attribute for highways only in the state from the major highways layer.
- I exported the data as a layer and then overlaid the new layer while removing the previous highways layer.
- I performed the same step as above for the places people live layer.
- I selected by attribute for cities that contain more than 15,000 people.
- I then exported that into a new layer which I then overlaid onto the map.
- Climate Divisions: Buffered regions within the state of Louisiana.
- The map included transportation terminals, water bodies, and climate divisions.
- Divisions were based on the several climate divisions within the state which detail average rainfall, temperature.
- I performed a buffer around towns/cities that contain more than 15,000The buffer had a distance of 2 miles from
the center. This captured towns and transportation terminals.
- My final step was to select by attribute for the water bodies layer.
- I selected for wetlands, lakes, and reservoirs.
Flight paths through Louisiana:
- Added US state boundaries and US county boundaries layers
- Exported Louisiana county data to ArcCatalog and then re-added as new Flightpath layer
- Created polygon in Flightpath later to show the boundary of the Mississippi Flyway
- Added Mississippi Flyway bird species and Positive US Avian West Nile Virus species metadata to an excel
document. Resorted data alphabetically
- Selected by attribute for species common to both groups and created a new column of Common Species
- To add common species metadata to map:
- Option 1 (method used): create a new field in the Mississippi Flyway attribute table and, using spatial analyst
editor, manually add species names
- Option 2 (alternative method): create a unique identifier based on FID and join data to table by attribute
West Nile Virus is a potentially serious disease. It was first introduced into
North America in the year 1999 into New York City. Since that time, it has spread rapidly
across the continental United States. Disease activity increase during mosquito season,
particularly hot summer months. It is a disease vectored in mosquitoes. A mosquito
Infected with the disease agent then has the ability to pass it to any human or animal it
may bite. The primary host of the virus is birds, which pass it to the mosquitoes and vice
versa. Humans are considered “dead-end” hosts as they cannot pass it to each other
without an intermittent vector. According to the US Geological Survey, less than one
percent of infected humans develop more severe illnesses from the virus, including
meningitis and encephalitis. Because of this health risk and the diseases wide-spread
activity in countries such as Africa, West Nile Virus has been carefully tracked since its
introduction.
There are many possible factors that affect the spread of West Nile Virus.
The largest of these is mosquito travel and activity. Mosquitoes thrive and feast more on
human blood during periods of increased temperatures. They also thrive in areas with
available standing water. Bird activity is thought to also correlate with virus activity, as
birds are the primary hosts.
Our case study focuses on West Nile Virus occurrence in the state of
Louisiana, observing change in the number of cases reported between the years 2002
and 2004, in order to demonstrate the virus's progression throughout the country. By
analyzing the geography of various physical and biological features throughout the state,
we hope to determine possible factors that brought the disease to Louisiana. We predict
that the number of positive West Nile Virus cases will increase in areas with wet
conditions, high population densities, and areas with higher average temperature
and precipitation.
- Level of avian West Nile Virus activity was greatest in Central, Southwest, and South Central Louisiana
Areas with the greatest increase in West Nile Virus activity typically correlated with the highest observed average precipitation and temperatures
- Significant numbers of disease incidents were not reported for wetlands areas
- Over 150 bird species were found to be potential carriers of the disease agent through Mississippi avian flyways
Final Thoughts &
Next Steps
Results offer partial support for the original hypothesis. Population density proved to be an influential factor in the increase in the West Nile Virus incidence rate, as all three areas with the highest levels of West Nile Virus activity were home to large urban areas. There was not as strong a correlation between disease occurrence and proximity to large highways, despite the suggestion that human transportation networks also serve as a means of improved travel for mosquito vectors. While wetlands areas showed little disease activity, particularly for mosquitoes, these results may be inconclusive due to lack of testing data for 2002 in many counties, particularly those in wetland areas.
Temperature and precipitation rates appeared to be the most influential factor in the occurrence of West Nile Virus in Louisiana. Those areas with the highest disease rates also had some of the highest average temperature and precipitation measures. Due to the fact that mosquitoes thrive in moist, hot conditions, and all three high virus rate areas were located near wetlands areas, this once again suggests that wetlands may prove a much greater factor than available data would imply.
Lack of West Nile Virus incidence data for earlier years a major problem in conducting our analyses, as it served as a limiting factor leading to potentially misleading results. Insufficient data concerning specific bird species identified as positive West Nile Virus carriers in Louisiana also restricted the analysis of results. In subsequent analyses greater measures should be taken to fill in gaps in data. Additional factors should be taken into account, including elevation and land-cover data. Expanding the temporal and spatial scope of the data would be beneficial to reach more definite conclusions.
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<
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