Sexually transmitted infections (STIs) are infections that are spread primarily through person-to-person sexual contact. There are more than 30 different sexually transmissible bacteria, viruses and parasites. Several, in particular HIV and syphilis, can also be transmitted from mother to child during pregnancy and childbirth, and through blood products and tissue transfer.
Some of the commonest sexually transmitted pathogens can be divided into those caused by bacteria, viruses and parasites.
According to 1999 WHO estimates, 340 million new cases of curable STIs (syphilis, gonorrhoea, chlamydia and trichomoniasis) occur annually throughout the world in adults aged 15-49 years. (These are the most recent available data. New estimates up to 2005 are under development for publication towards the end of 2007.)
In developing countries, STIs and their complications rank in the top five disease categories for which adults seek health care. Infection with STIs can lead to acute symptoms, chronic infection and serious delayed consequences such as infertility, ectopic pregnancy, cervical cancer and the untimely death of infants and adults.
STIs are the main preventable cause of infertility, particularly in women. Between 10% and 40% of women with untreated chlamydial infection develop symptomatic pelvic inflammatory disease. Post-infection tubal damage is responsible for 30% to 40% of cases of female infertility. Furthermore, women who have had pelvic inflammatory disease are 6 to 10 times more likely to develop an ectopic (tubal) pregnancy than those who have not, and 40% to 50% of ectopic pregnancies can be attributed to previous pelvic inflammatory disease. Infection with certain types of the human papillomavirus can lead to the development of genital cancers, particularly cervical cancer in women.
Untreated sexually transmitted infections are associated with congenital and perinatal infections in neonates, particularly in the areas where rates of infection remain high.
In pregnant women with untreated early syphilis, 25% of pregnancies result in stillbirth and 14% in neonatal death – an overall perinatal mortality of about 40%. Syphilis prevalence in pregnant women in Africa, for example, ranges from 4% to 15%. Up to 35% of pregnancies among women with untreated gonococcal infection result in spontaneous abortions and premature deliveries, and up to 10% in perinatal deaths. In the absence of prophylaxis, 30% to 50% of infants born to mothers with untreated gonorrhoea and up to 30% of infants born to mothers with untreated chlamydial infection will develop a serious eye infection (ophthalmia neonatorum), which can lead to blindness if not treated early. It is estimated that, worldwide, between 1000 and 4000 newborn babies become blind every year because of this condition.
The presence of an untreated ulcerative or non-ulcerative (those STIs which cause ulcers or those which do not) infection increases the risk of both acquisition and transmission of HIV by a factor of up to 10. Thus, prompt treatment for STIs is important to reduce the risk of HIV infection. Controlling STIs is important for preventing HIV in people at high risk, as well as in the general population.
The most effective means to avoid becoming infected with or transmitting a sexually transmitted infection is to abstain from sexual intercourse (i.e., oral, vaginal, or anal sex) or to have sexual intercourse only within a long-term, mutually monogamous relationship with an uninfected partner. Male latex condoms, when used consistently and correctly, are highly effective in reducing the transmission of HIV and other sexually transmitted infections, including gonorrhoea, chlamydial infection and trichomoniasis.
Some sexually transmitted infections often exist without symptoms. For example, up to 70% of women and a significant proportion of men with gonococcal and/or chlamydial infections may experience no symptoms at all. Both symptomatic and asymptomatic infections can lead to the development of serious complications, as outline above.
Although many different pathogens cause STIs, some of them give rise to similar or overlapping clinical appearances, known as signs (what the individual or the health-care provider sees on examination) and symptoms (what the patient feels, such as pain or irritation). Some of these signs and symptoms are easily recognizable and consistent, giving what is known as a syndrome that signals the presence of one or a number of pathogens. For example, a discharge from the urethra in men can be caused by gonorrhoea alone, chlamydia alone or both together.
The traditional method of diagnosing STIs is by laboratory tests. However, these are often unavailable or too expensive. Since 1990 WHO has recommended a syndromic approach to diagnosis and management of STIs in patients presenting with consistently recognized signs and symptoms of particular STIs.
The syndromic approach is a scientifically derived approach and offers accessible and immediate treatment that is effective. The syndromic approach using flowcharts to guide diagnosis and treatment is more accurate than diagnosis based on clinical judgment alone, even in experienced hands, and more cost-effective for some syndromes than use of laboratory tests. However, the specificity of this approach for vaginal discharge is low for gonorrhoea and chlamydia and treating all women with vaginal discharge for cervical infection (as if they have a sexually transmitted cause for the discharge) has led to unacceptable levels of over-treatment in some settings. Women with abnormal vaginal discharge should be treated primarily for vaginal infections, and treatment for gonorrhoea and chlamydia only included according to the prevalence of or risk for these pathogens. The organisms causing any particular syndrome need to be determined locally and flow charts adapted accordingly.
Furthermore, regular monitoring of the organisms causing each syndrome should be conducted on a regular basis to validate the treatment recommendations.
The control of STIs remains a priority for WHO. The World Health Assembly endorsed the global strategy for the prevention and control of STIs in May 2006. The strategy urges all countries to control the transmission of STIs by implementing a number of interventions, including the following: