Prefecture-Level Association Between Reported Syphilis Cases and Sexually Oriented Establishments in Japan: An Ecological Study

Background and aim The number of reported syphilis cases has increased in Japan in recent years, while the contribution of structural and regional factors remains unclear. Japan maintains a distinctive legal and operational framework for the sex industry, characterized by geographic concentration of establishments and expansion of non-storefront services. This study aimed to examine the prefecture-level association between reported syphilis cases and the number of sexually oriented establishments in Japan. Methods This ecological study analyzed publicly available prefectural data on reported syphilis cases from April 2023 to March 2024, obtained from the national infectious disease surveillance system. Data on the number of sexually oriented establishments were derived from statistics published by the Japan National Crime Prevention Association. For comparison, prefecture-level data on chlamydia and gonorrhea were also analyzed. Infection rates were calculated per 1,000,000 population. Simple linear regression analyses were conducted to evaluate associations between infection rates and the number of establishments. Results A significant positive association was observed between the number of sexually oriented establishments and the reported incidence of syphilis (β=0.005535, R²=0.5157, p<0.0001). No statistically significant associations were identified for chlamydia (p=0.8485) or gonorrhea (p=0.3457). Conclusions At the prefectural level in Japan, the number of sexually oriented establishments was significantly associated with reported syphilis incidence, whereas no such association was observed for chlamydia or gonorrhea. Although causality cannot be inferred from this ecological analysis, these findings suggest that regional structural factors may be associated with syphilis transmission dynamics. Public health strategies in high-burden regions may benefit from continued emphasis on education, testing, and timely treatment.