Environmental Drivers of Rocky Mountain Spotted Fever in Tennessee Open Access
Jones, Bryant (2016)
Published
Abstract
Introduction: Rickettsia rickettsii is one of the most pathogenic ricketsial strains infecting humans and can be found throughout North and South America as well as some areas in the Eastern hemisphere. The primary vector is the American dog tick (Dermacentor Variabilis) which is distributed throughout the Eastern U.S. and contributes to the particularly high incidence of RMSF in Tennessee. This study aims to utilize GIS technologies and data available from various sources to estimate the association between presence of RMSF with environmental variables.
Methods: Passive surveillance data of Rocky Mountain spotted fever cases throughout the state of Tennessee were used to document both RMSF cases and non-cases at the census tract level. Case locations were determined using the address of the infected individual as a proxy for actual tick bite location. Clusters of high prevalence census tracts were used to create a dichotomous outcome variable for use in logistic regression analysis. Analysis was performed using the environmental factors as independent exposure variables and clustered versus non-clustered census tracts as the outcome.
Results: Cluster analysis of positive cases showed areas of High-High clustering west of the capital. Significant environmental variables for the logistic regression model included land cover, elevation, and average annual precipitation. Geological classes and soil order were not deemed significant but were retained for the final model based on a priori knowledge. The predictive risk map shows the majority of the state with the 25 to 50% probability range while areas of higher risk are scattered throughout the state.
Discussion: Research has shown that the distribution of tick vectors is associated with environmental variables. This project serves as a starting point to better understand the role of environmental variables in the distribution of Rocky Mountain spotted fever. More in-depth studies on the subject could lead to new health policy initiatives regarding tick borne disease in Tennessee.
Table of Contents
Chapter 1: Introduction………………………………………………………………1
Rocky Mountain spotted fever in Tennessee…………………………………….1
The Vectors……………………………………………………………………………..2
Using GIS and mapping to estimate disease risk……………………………....3
Chapter 2: Methods……………………………………………….………………....4
Study Area………………………………………………………………………………4
Geospatial Analysis…………………………………………………………………..4
Statistical Method…………………………………………………………………….8
Chapter 3: Results……………………………….…………………………………...9
Figure 1: Prevalence of RMSF per 100,000 by Census Tract………………..10
Descriptive Statistics………………………………………………………………..11
Figure 2: Cluster Analysis of Census Tract using Anselin Local Moran's I…11
Table 1: Bivariate and Multivariate Analysis of Environmental Variables…14
Modelling Results…………………………………………………………………….14
Figure 3: Risk map of probability of RMSF presence in Tennessee………..15
Chapter 4: Discussion………………………………………………………...….....16
Chapter 5: References………….………………………………………………….…19
Appendix I: Maps…………….……………………………………………………….23
Figure 4: Soil Orders in Tennessee………………………………………….…….23
Figure 5: Land Cover in Tennessee……………………………………………..…24
Figure 6: Geologic Classes in Tennessee………………………………..……....25
Figure 7: Elevation in Tennessee…………………………………….……………26
Figure 8: Average Annual Precipitation in Tennessee………………………..27
Figure 9: Average Annual Temperature in Tennessee……………………..…28
Figure 10: Positive RMSF Cases in Relation to Population and Prevalence by Census Tract..29
Figure 11: Reclassified Soil Orders……………………………………………….30
Figure 12: Reclassified Land Cover Type………………………………………..31
Figure 13: Reclassified Geological Classes……………………………………..32
Figure 14: Mean Elevation by Census Tract…………………………………....33
Figure 15: Mean Average Annual Precipitation by Census Tract..…………34
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