Introduction Anorectal Chlamydia trachomatis (Ct) testing in women is not standard. In some countries, women are being tested based on reported anal intercourse. However, universal anorectal Ct testing in STI clinics revealed prevalences over 10%, irrespective of anal intercourse, and showing high co-occurrence with urogenital infections. To inform control strategies in women, this study explores different transmission mechanisms that can explain the high observed Ct prevalences using mathematical modelling.
Methods We developed a pair compartmental model of heterosexuals aged 15–29 years. To inform the model, data on anorectal and urogenital infections are used from heterosexual men and women attending STI clinics. In the model, women can have urogenital and anorectal infections, men only urogenital infections. At all sites, individuals can either be susceptible (S), infected (I) or recovered (R). All partnerships engage in vaginal intercourse, and a fraction of partnerships will also have anal intercourse. We developed models including different transmission mechanisms, e.g. transmission by anal sex and autoinoculation between anatomic sites, and explored which mechanisms or combinations thereof fit the observed data best.
Results Most models did fit to the observed prevalence of male and female urogenital Ct: 13.6% (95% CI: 10.7–17.2) and 13.0% (95% CI: 12.4–13.7), female anorectal Ct: 10.6% (95% CI: 8.0–13.9) and both sites: 9.9% (95% CI: 7.4–13.1). Models that assumed autoinoculation between anatomic sites fitted the data best, compared to models that focused on anal sex only. The model will be used to further determine the impact of testing strategies (i.e. universal irrespective of anal intercourse) and treatment strategies (i.e. azithromycin or doxycycline) on population prevalence.
Conclusions The results are suggestive of a Ct autoinoculation process between anatomic sites in women. This has potential consequences for future chlamydia control strategies including testing and treatment.
Disclosure of interest statement The National Institute of Public Health and the Environment is funded by the Ministry of Health, Welfare and Sport. The authors declare no conflict of interest.
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