Identifying and Monitoring Chlamydia Incidence Rate: Application of Exploratory Spatial Data Analysis (ESDA) Methods in Geographic Information System (GIS)

Background:
ESDA methods can be a useful set of tools for identifying and monitoring sexually transmitted disease (STD) rates by offering a way to test the significance of variations in levels and changes.

Objective:
To demonstrate the application of ESDA methods in GIS to identify and monitor county-level chlamydia incidence in the state of Texas.

Method:
Data from the National Electronic Telecommunications System for Surveillance (NETSS) on county level chlamydia incidence rates for 2004 and 2005 (for all ages, genders and races) in the state of Texas were used to characterize spatiotemporal patterns and test for global and local outliers. We also examined relative change in smoothed rates from 2004 to 2005.

Result:
Smoothed chlamydia incidence rates were spatially dependent. Furthermore, the relative changes that occurred between 2004 and 2005 were also spatially dependent. Six counties (Bell, Falls, Potter, Taylor, Kleberg, and Lubbock) were in the highest ten counties for the two years examined. Erath County had significantly (p<0.05) higher smoothed rates (> 300 cases per 100,000 residents) than its contiguous neighbors (195 or less) in both years. Gaines County experienced the highest relative increase in smoothed rate (173% - 139 to 379). The relative change in smoothed CT rates in Newton county was significantly (p<0.05) higher than its contiguous neighbors.

Conclusion:
The procedure is limited in the sense that it only helped to identify high morbidity counties. However, it can assist program officials in using surveillance data to identify outliers, as well as relevant changes in STDs. Thus more investigative research is needed to understand the reasons behind the results.

Implications:
For a better up-to-date assessment, this would have to be done annually (or as permitted by data availability). The results also underscore the need for more regionally integrated approaches to STD prevention and control programs.