Sexually Transmitted Infections: Old Foe, New Opportunities for Control

Abstract More than 30 pathogens, including bacteria, viruses, protozoa, and ectoparasites, cause sexually transmitted infections (STIs). Approximately 1 million curable STIs, comprising Neisseria gonorrhoeae, Chlamydia trachomatis, Treponema pallidum, and Trichomonas vaginalis, are acquired everyday worldwide. The most common of the treatable STIs is a protozoon, T. vaginalis, causing approximately 156 million new infections in 2020. Sexually transmitted viral infections are also prevalent worldwide, of which the most important are the human immunodeficiency virus (HIV), herpes simplex virus types 1 and 2, and the human papillomavirus (HPV). STIs impact people’s lives through their impact on reproductive health and child health, as well as through the facilitation of sexual transmission of HIV infection and, with some, such as HPV, as precursors of anogenital cancers. People face enormous challenges with access to health services for STI care. Furthermore, some STIs commonly exist as asymptomatic infections, particularly among adolescents. In addition, even with symptoms, some individuals have difficulty accessing affordable healthcare services because of anticipated stigma. The epidemiology of STIs is influenced by an interplay of the determinants of spread of infections and human behavior. At the individual level, determinant factors include ignorance of STIs, sexual behavior, sexual preferences, sexual networks, sex work, healthcare-seeking behaviors, and whether or not use is made of old and newer biomedical HIV and STI prevention interventions, such as condoms, medical male circumcision, and pre-exposure prophylaxis (e.g., PrEP for HIV); availability of, and access to, post-exposure prophylaxis; and prophylactic STI vaccines. At the population level, the determinants include demographic factors, socioeconomic factors, geographical settings, cultural ramifications, political commitment and health system responses. STIs can be prevented through modification of sexual behavior toward “safer sex.” The interventions implemented by countries, to varying degrees of coverage, include behavioral interventions, promotion of use of barrier methods, vaccinations, screening for STIs, and case-finding in people attending healthcare services for conditions other than for STI care. In persons with established infections, the focus is on averting short-term and long-term sequelae of untreated STIs, such as pelvic inflammatory disease, tubal-factor infertility, adverse pregnancy outcomes, and cervical cancer. This includes screening for asymptomatic STIs, cervical cancer, and the early diagnosis and treatment of HIV infection. Regular screening has been shown to reduce the population prevalence of STIs, prevention of some adverse outcomes such as congenital syphilis, and improved prognosis in such cases as early treatment of HPV-associated cervical cancer. The evidence and cost-effectiveness of screening for C. trachomatis and N. gonorrhoeae to prevent infertility and improve pregnancy outcomes is limited. Also, screening programs result in increased use of antibiotics in certain population groups, with concerns of increase in the development of antimicrobial resistance, especially in N. gonorrhoeae. The detection and management of STIs has been revolutionized since the 1990s by the development of molecular detection tests, the advent of STI vaccines, and the advent of new treatment molecules, such as antiretroviral treatments, which have facilitated timely diagnosis of both symptomatic and asymptomatic infections and converted some fatal infections into manageable chronic infections.

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