Risk of cancer among individuals with a history of bacterial sexually transmitted infections: A population‐based study in Alberta, Canada

We investigated cancer risk among individuals with a prior bacterial sexually transmitted infection (STI) diagnosis using a retrospective cohort study of all Albertan residents diagnosed with chlamydia, gonorrhea, or syphilis during 2000-2014, that was then linked to the Alberta Cancer Registry. Follow-up started 5 years from their first bacterial STI diagnosis and continued until the first instance of a cancer diagnosis, death, or the study end date (December 31, 2019). Internal comparisons between the bacterial STI groups were undertaken using adjusted hazard ratios, while sex-specific standardized incidence ratios (SIRs) were calculated to compare cancer risk with the Alberta general population. The likelihood of developing cancer was largely comparable within the bacterial STI cohort, though head/neck cancer was more common after only gonorrhea exposure, and lung cancer was more common after only syphilis exposure. When compared with the general population, statistically significant SIRs were observed among females for cervical cancer (SIR = 1.9, 95%CI = 1.5, 2.3) and thyroid cancer (SIR = 0.8, 95%CI = 0.6, 0.9); females exposed to chlamydia with other STIs, or gonorrhea with other STIs, were also 3.2- and 2.9-times more likely to develop colon cancer, respectively. In males, statistically significantly associations were identified for cancer overall (SIR = 1.1, 95%CI = 1.0, 1.2) and Hodgkin lymphoma (SIR = 1.8, 95%CI = 1.0, 2.9); males exposed to chlamydia only were also 1.5- and 1.6-times more likely to develop prostate and testicular cancer, respectively, while males exposed to only syphilis were 2.4-times more likely to develop lung cancer. Our findings are consistent with common bacterial STIs being correlates of risk of certain cancers, although the possible etiologic mechanisms may be indirect.