Modeling a Syphilis Outbreak Through Space and Time Using the Bayesian Maximum Entropy Approach

Purpose

The aim of the study is to describe changes in the spatial distribution of syphilis before, during, and after an outbreak in Baltimore, MD, by using Bayesian maximum entropy (BME), a modern geostatistical technique for space–time analysis and mapping.

Methods

BME was used to conduct simple and composite space–time analyses of the density of syphilis infection based on primary, secondary and early latent syphilis cases reported to the Baltimore City Health Department between January 1, 1994, and December 31, 2002.

Results

Spatiotemporal covariance plots indicated that the distribution of the density of syphilis cases showed both spatial and temporal dependence. Temporally dependent disease maps suggested that syphilis increased within two geographic core areas of infection and spread outward. A new core area of infection was established to the northwest. As the outbreak waned, density diminished and receded in all core areas. Morbidity remained elevated in the two original central and new northwestern core areas after the outbreak.

Conclusions

Density of syphilis infection was a simple informative measure easily compared across years. The BME approach was useful for quantitatively and qualitatively describing the spatial development and spread of syphilis. Our results are specific to Baltimore; however, the BME approach is generalizable to other settings and diseases.

Introduction

During the 1990s, heterosexually transmitted syphilis outbreaks in the United States were associated with crack cocaine, exchange of sex for drugs, and prostitution 1, 2, 3, 4. In Baltimore, MD, rates peaked in 1996 when the primary and secondary syphilis rate exceeded 100 cases/100,000. Before 1995 to 1996, syphilis morbidity in Baltimore was geographically defined as “core” in two distinct areas, one in the central eastern end and the other in the west of the city 5, 6.

Spatial analysis and mapping of sexually transmitted infections (STIs) has been useful for defining and describing core areas of infection 7, 8, 9, 10, 11, 12, providing insight into patterns of STI transmission 6, 12, 13, 14 and identifying sexual networks and interrupting syphilis transmission during outbreaks (15). However, many of these previous analyses were limited to descriptive approaches 7, 8, 9, 10, 14, 15. Applying analytic methods to space–time variation of an epidemic could be useful for understanding how disease spreads spatially and, in turn, preventing the geographic progression of future outbreaks.

Our objective is to describe the movement of early syphilis for Baltimore from 1994 to 2002 both qualitatively and quantitatively by using Bayesian maximum entropy (BME), a modern geostatistical technique for space–time analysis and mapping. We hypothesized that the syphilis epidemic in Baltimore originated from two core areas, expanded, and established new core areas of infection, especially in the north, that persisted as the epidemic receded. We explored our hypothesis by mapping the spatial distribution of reported early syphilis (defined as reported cases of primary, secondary, and early latent syphilis) before, during, and after the outbreak.