What's driving an epidemic? The spread of syphilis along an interstate highway in rural North Carolina

Objectives. The purpose of this study was to determine whether county syphilis rates were increased along Interstate Highway 95 (I-95) in North Carolina during a recent epidemic. Methods. Ecological data on syphilis cases, demographic data, highway data, and drug activity data were used to conduct a cross-sectional and longitudinal study of North Carolina counties from 1985 to 1994. Crude and adjusted incidence rate ratios (IRRs) were determined by means of standard and longitudinal Poisson regression models adjusted for sociodemographic factors and drug use. Results. Ten-year syphilis rates in I-95 counties greatly exceeded rates in non-I-95 counties (38 vs 16 cases per 100000 persons) and remained higher after adjustment for race, age, sex, poverty large cities, and drug activity (adjusted IRR=2.05, 95% confidence interval [CI] = 1.84, 2.28). Syphilis rates were stable until 1989, when rates increased sharply in 1-95 counties but remained stable in non-I-95 counties. Increased drug activity in I-95 counties preceded the rise in syphilis cases. Conclusions. A better understanding of the relationship between highways and the spread of sexually transmitted diseases may guide future prevention interventions.