Overview
More than 30 different bacteria, viruses and parasites are known to be transmitted through sexual contact. Eight of these pathogens are linked to the greatest incidence of sexually transmitted disease. Of these, 4 are currently curable: syphilis, gonorrhoea,
chlamydia and trichomoniasis. The other 4 are viral infections which are incurable: hepatitis B, herpes simplex virus (HSV or herpes), HIV and human papillomavirus (HPV).
STIs are spread predominantly by sexual contact, including vaginal, anal and oral sex. Some STIs can also be transmitted from mother-to-child during pregnancy, childbirth and breastfeeding.
A person can have an STI without showing symptoms of disease. Common symptoms of STIs include vaginal discharge, urethral discharge or burning in men, genital ulcers and abdominal pain.
Scope of the problem
STIs have a profound impact on sexual and reproductive health worldwide.
More than 1 million STIs are acquired every day. In 2020, WHO estimated 374 million new infections with one of four STIs: chlamydia (129 million), gonorrhoea (82 million), syphilis (7.1 million) and trichomoniasis (156 million).
More than 490 million people were estimated to be living with genital HSV (herpes) infection in 2016, and an estimated 300 million women have an HPV infection, the primary cause of cervical cancer. An estimated 296 million people are
living with chronic hepatitis B globally. Both HPV and hepatitis B infections are preventable with vaccination.
STIs can have serious consequences beyond the immediate impact of the infection itself.
- STIs like herpes, gonorrhoea and syphilis can increase the risk of HIV acquisition.
- Mother-to-child transmission of STIs can result in stillbirth, neonatal death, low-birth weight and prematurity, sepsis, pneumonia, neonatal conjunctivitis and congenital deformities. Approximately 1 million pregnant women were estimated to have active syphilis in 2016, resulting in over 350 000 adverse birth outcomes, of which 200 000 occurred as stillbirth or neonatal death.
- HPV infection causes cervical cancer. Cervical cancer is the fourth most common cancer among women globally, with an estimated 570 000 new cases in 2018 and over 311 000 cervical cancer deaths each year (2).
- Hepatitis B resulted in an estimated 820 000 deaths in 2019, mostly from cirrhosis and hepatocellular carcinoma (primary liver cancer).
- STIs such as gonorrhoea and chlamydia are major causes of pelvic inflammatory disease (PID) and infertility in women.
Prevention of STIs
When used correctly and consistently, condoms offer one of the most effective methods of protection against STIs, including HIV. Condoms also protect against unintended pregnancy in mutually consented sexual relationships. Although highly effective, condoms do not offer protection for STIs that cause extra-genital ulcers (i.e., syphilis or genital herpes). When possible, condoms should be used in all vaginal and anal sex.
Safe and highly effective vaccines are available for 2 viral STIs: hepatitis B and HPV. These vaccines have represented major advances in STI prevention. By the end of 2020, the HPV vaccine was introduced as part of routine immunization programmes in 111 countries, most of them high- and middle-income. HPV vaccination could prevent the deaths of millions of women over the next decade in low- and middle-income countries, where most cases of cervical cancer occur, if high (>80%) vaccination coverage of young women (ages 11–15) can be achieved.
Research to develop vaccines against herpes and HIV is advanced, with several vaccine candidates in early clinical development. There is mounting evidence suggesting that the vaccine to prevent meningitis (MemB) has cross-protection against gonorrhoea. More research into vaccines for chlamydia, gonorrhoea, syphilis and trichomoniasis are needed.
Other biomedical interventions to prevent some STIs include adult male circumcision and microbicides.
Diagnosis of STIs
Accurate diagnostic tests for STIs are widely used in high-income countries. These are especially useful for the diagnosis of asymptomatic infections. However, diagnostic tests are largely unavailable in low- and middle-income countries. Where testing
is available, it is often expensive and geographically inaccessible, and patients often need to wait a long time (or need to return) to receive results. As a result, follow-up can be impeded and care or treatment can be incomplete.
The only inexpensive, rapid tests currently available for STIs are for syphilis, hepatitis B and HIV. The rapid syphilis test is already in use in some resource-limited settings. A rapid dual HIV/syphilis test is now available whereby a person can be
tested for HIV and syphilis from a single finger-stick and using a single testing cartridge. These tests are accurate, can provide results in 15 to 20 minutes, and are easy to use with minimal training. Rapid syphilis tests have been shown to increase
the number of pregnant women tested for syphilis. However, increased efforts are still needed in most low- and middle-income countries to ensure that all pregnant women receive a syphilis test at the first antenatal care visit.
Several rapid tests for other STIs are under development and have the potential to improve STI diagnosis and treatment, especially in resource-limited settings.
Treatment of STIs
Effective treatment is currently available for several STIs.
- Three bacterial STIs (chlamydia, gonorrhoea and syphilis) and one parasitic STI (trichomoniasis) are generally curable with existing single-dose regimens of antibiotics.
- For herpes and HIV, the most effective medications available are antivirals that can modulate the course of the disease, though they cannot cure the disease.
- For hepatitis B, antiviral medications can help to fight the virus and slow damage to the liver.
Antimicrobial resistance (AMR) of STIs – in particular gonorrhoea – to antibiotics has increased rapidly in recent years and has reduced treatment options. The Gonococcal AMR Surveillance Programme (GASP) has shown high rates of resistance to many antibiotics including quinolone resistance, increasing azithromycin resistance and emerging resistance of extended-spectrum cephalosporins, a last-line treatment, increasing the risk that gonorrhoea will be untreatable (4).
AMR for other STIs, though less common, also exists, making prevention and prompt treatment critical.
STI case management
Low- and middle-income countries rely on identifying consistent, easily recognizable signs and symptoms to guide treatment, without the use of laboratory tests. This is called syndromic management. This approach, which often relies on clinical algorithms,
allows health workers to diagnose a specific infection on the basis of observed syndromes (e.g., vaginal discharge, urethral discharge, genital ulcers, abdominal pain).
Syndromic management is simple, assures rapid, same-day treatment, and avoids expensive or unavailable diagnostic tests for patients that present with symptoms. This approach results in overtreatment and missed treatment as the majority of STIs are asymptomatic.
Thus, WHO recommends countries enhance syndromic management by gradually incorporating laboratory testing to support diagnosis. In settings where quality assured molecular assays are available, it is recommended to treat STIs based on laboratory tests.
Moreover, STI screening strategies are essential for those at higher risk, such sex workers, men who have sex with men, adolescents in some settings and pregnant women due to potential severe consequences to the babies.
To interrupt transmission of infection and prevent re-infection, treating sexual partners is an important component of STI case management.
Controlling the spread
Behaviour change is complex
Despite considerable efforts to identify simple interventions that can reduce risky sexual behaviour, behaviour change remains a complex challenge. Research has demonstrated the need to focus on carefully defined populations, consult extensively with
the identified target populations, and involve them in design, implementation and evaluation.
Education and counselling can improve people’s ability to recognize the symptoms of STIs and increase the likelihood that they will seek care and encourage a sexual partner to do so. Unfortunately, lack of public awareness, lack of training among health workers, and long-standing, widespread stigma around STIs remain barriers to greater and more effective use of these interventions.
Health services for screening and treatment of STIs remain weak
People seeking screening and treatment for STIs face numerous problems. These include limited resources, stigmatization, poor quality of services and often out-of-pocket expenses.
Marginalized populations with the highest rates of STIs – such as sex workers, men who have sex with men, people who inject drugs, prison inmates, mobile populations and adolescents – often do not have access to adequate and friendly health services.
In many settings, STI services in low- and middle-income countries are often neglected and underfunded. These problems lead to difficulties in providing screening for asymptomatic infections, insufficient number of trained personnel, limited laboratory
capacity and inadequate supplies of appropriate medicines.
WHO response
Our work is currently guided by the Global health sector strategy on sexually transmitted infections, 2016–2021.
Within this framework, WHO:
- develops global norms and standards for STI treatment and prevention
- support the estimation the disease and economic burden of STIs
- globally monitors antimicrobial resistance to gonorrhoea
- leads the setting of the global research agenda on STIs, including the development of:
- affordable and easy to use diagnostic tests
- STI vaccines
- additional drugs for gonorrhoea and syphilis.
As part of its mission, WHO supports countries to:
- implement health interventions to decrease their burden of STIs
- enhance and scale-up health intervention for impact, such as:
- hepatitis B and HPV vaccination
- syphilis screening of pregnant women and populations at increased risk of STIs
- strengthen their capacity to monitoring trends in STIs to improve their programs
- monitor and respond to antimicrobial resistance in gonorrhoea.
- James C, Harfouche M, Welton NJ, et al. Herpes simplex virus: global infection prevalence and incidence estimates, 2016. Bull World Health Organ. 2020;98(5):315-329. doi:10.2471/BLT.19.237149
- Bray F, Ferlay J, Soerjomataram I, Siegel RL, Torre LA, Jemal A. Global cancer statistics 2018: GLOBOCAN estimates of incidence and mortality worldwide for 36 cancers in 185 countries. CA Cancer J Clin. 2018 Nov;68(6):394-424. doi: 10.3322/caac.21492. Epub 2018 Sep 12. Erratum in: CA Cancer J Clin. 2020 Jul;70(4):313. PMID: 30207593. https://doi.org/10.1371/journal.pone.0211720
- Unemo M, Lahra MM, Escher M, Eremin S, Cole MJ, Galarza P, Ndowa F, Martin I, Dillon JR, Galas M, Ramon-Pardo P, Weinstock H, Wi T. WHO global antimicrobial resistance surveillance (GASP/GLASS) for Neisseria gonorrhoeae 2017-2018: a retrospective observational study. Lancet Microbe 2021; 2: e627–36 https://doi.org/10.1016/S2666-5247(21)00171-3