Defining core gonorrhea transmission utilizing spatial data

Spatial distribution of repeat and singly occurring gonorrhea in Baltimore, Maryland, was examined to identify clusters of core transmitters. Gonorrhea reported between 2001 and 2002 was geocoded and mapped. Those with more than one gonorrhea infection separated by more than 14 days during the 2-year period were considered repeaters. Other cases were treated as isolated events. Six definitions of core transmission were examined by census tracts with the highest quintiles of 1) cases for 2001, 2) cases for 2002, 3) rates for 2001, 4) rates for 2002, 5) counts of repeaters over the 2-year period, and 6) proportion of total cases that were repeaters. Of the 6,108 gonorrhea cases analyzed, 9% were repeaters. Repeaters were more likely to be female and younger. Core areas identified by definitions based on overall disease burden agreed well with each other but had poor agreement with definitions based upon repeat infections. Repeaters clustered to a greater extent at smaller distances than did singly occurring gonorrhea cases. Repeat gonorrhea infections are prevalent in Baltimore and likely represent behavior consistent with core transmission. Census tracts of core transmission defined by geographic distribution of repeat infections may indicate foci of highest risk sexual behaviors and high transmission.