The association between human perceived heat and early-stage syphilis and its variance: Results from a case-report system

Background: The relationship between temperature and syphilis is poorly understood despite clues and suggestions from previous studies. We examined the association between human perceived temperature (humidex) and the relative risk of early-stage syphilis in the most affected area of China, as well as its variance across social-economic status and age groups.

Methods: Information regarding early-stage (primary and secondary) syphilis cases reported to the China Case Report System between 2005-2013 from Guangdong province was analyzed in this study. Weather data were obtained from the National Meteorological Information Center. Distributed lag nonlinear models were applied to assess the relationship between humidex and the relative risk of early-stage syphilis. Results were further stratified by social-economic status and age groups.

Results: The relative risks of early-stage syphilis mainly increased with increased humidex, and were elevated when the humidex was around 30 or greater than 38. The humidex-RR association for the 15-40 age group was similar to the scenario pooled across the age groups, particularly in the Pearl-River Delta Region. The relative risk of syphilis in the elderly for the non-Pearl-River Delta Region did not show a clear change with humidex, whereas that in the Pearl-River Delta Region substantially increased, particularly when humidex was above 25.

Conclusions: Human perceived temperature was positively related to the relative risk of early-stage syphilis. People in the Pearl-River Delta Region tended to be more sensitive, with relative risk elevated at potential comfortable times or under excessively hot conditions. The vulnerability of the elderly in the Pearl-River Delta Region is concerning.