High STI incidence in racial geographic isolates

Background: Sexually transmitted infections (STIs) are historically concentrated among racial and ethnic minority populations. In 2010 it was estimated that there were 19 million new cases of STI infection in the United States. Among these individuals, African Americans had substantially higher rates of both gonorrhea and chlamydia than Caucasians and made up almost half of all cases of syphilis that year.

Methods: We used Geographical Information Systems to map incidence of chlamydia, gonorrhea, and syphilis in Flint, create maps of disease density, and aggregate incidence data at the Census block group level. We used linear regression models to predict the rate of STI incidence based on Block Group socio-demographics.

Results: The percent of the population that is African American and the percent of the population that is Caucasian provide the strongest predictors of STI incidence. Variation in the percent of a block group's population that is Caucasian alone accounts for 53.3% of the variation in STI incidence. Geographic isolation from other in-group members was related to higher STI incidence rates for both African Americans and Caucasians.

Conclusions: Because the highest incidences of STIs in African Americans occurred in areas with the lowest proportion of African Americans, and the same pattern was seen for Caucasians, interventions may be more effective by incorporating outreach to racial geographic isolates. Future studies should determine whether this trend replicates on a larger geographic scale and in other areas.