Objectives: To examine changes over time in the distribution of primary and secondary syphilis cases across states, using Lorenz curves and Gini coefficients.
Methods: For each year from 1985 to 2007, the Gini coefficient for the state-level distribution of male and female syphilis cases was calculated. The Gini coefficient can range from 0 (indicating equality in syphilis rates across states) to 1 (indicating complete inequality such that all syphilis cases occur in one state).
Results: The Gini coefficients for men are notably lower (indicating more equality in the distribution of syphilis across states) in recent years (in which men who have sex with men (MSM) account for most syphilis cases) than in the heterosexual epidemic of the late 1980s and early 1990s. Although syphilis rates in men were similar in 1995 and 2007 (6.7 and 6.6 per 100,000, respectively), the Gini coefficient for male syphilis decreased from 0.523 in 1995 to 0.333 in 2007. For syphilis in women, Gini coefficients have not shown the same degree of decline.
Conclusions: The spread of MSM syphilis cases to urban areas across the nation is likely to be a main cause of the decreased concentration of male syphilis cases across states from 1995 to 2007.