Sexually transmitted infection (STI) refers to infection with a pathogen that is transmitted through blood, semen, vaginal fluids, or other body fluids during oral, anal, or genital sex with an infected partner. Sexually transmitted disease (STD) refers to a recognizable disease state that has developed from an STI (1). STIs can be caused by a number of microorganisms that vary widely in size, life cycle, susceptibility to available treatments and the diseases and symptoms they cause.
Bacterial STIs include
Viral STIs include
Parasitic infections that can be sexually transmitted include
Trichomoniasis (caused by protozoa)
Scabies (caused by mites)
Pediculosis pubis (caused by lice)
Many other infections not considered primarily to be STIs—including salmonellosis, shigellosis, campylobacteriosis, amebiasis, giardiasis, hepatitis (A, B, and C), Zika virus infection, monkeypox (mpox), and cytomegalovirus infection—can be transmitted sexually.
Because sexual activity includes close contact with skin and mucous membranes of the genitals, mouth, and rectum, many organisms are efficiently spread between people. Some STIs cause inflammation (eg, in gonorrhea or chlamydial infection) or ulceration (eg, in herpes simplex, syphilis, or chancroid), which predispose to transmission of other infections (eg, HIV).
STI prevalence rates remain high in most of the world, despite diagnostic and therapeutic advances. In the US, over 25 million new cases of STIs occur each year; about half occur in people aged 15 to 24 years (see also Centers for Disease Control and Prevention [CDC]: Sexually Transmitted Disease Surveillance 2020).
Factors impeding control of STIs include
Unprotected sexual activity with multiple partners
Difficulty talking about sexual issues for both physicians and patients
Inadequate funding for implementing existing diagnostic tests and treatments and for developing new tests and treatments
Susceptibility to reinfection if both partners are not treated simultaneously
Incomplete treatment, which can lead to development of drug-resistant organisms
International travel, which facilitates rapid global dissemination of STIs
Reference
1. Workowski KA, Bachmann LH, Chan PA, et al: Sexually Transmitted Infections Treatment Guidelines, 2021. MMWR Recomm Rep 70(4):1-187, 2021. doi:10.15585/mmwr.rr7004a1
Symptoms and Signs of STIs
Symptoms and signs of STIs vary depending on the infection. Many STIs cause genital lesions (see table Differentiating Common Sexually Transmitted Genital Lesions).
Diagnosis of STIs
History and physical examination
Gram staining and culture or nucleic acid amplification tests (NAATs)
Blood tests
A medical history, including sexual history, is taken to assess for risk factors for STIs. Physical examination is performed if symptoms are associated with potential STIs.
Diagnostic testing may include blood tests or collection of specimens from lesions or potential sites of infections for Gram staining and culture or NAATs.
STIs are diagnosed and treated in a variety of settings; sometimes, diagnostic tests are limited or unavailable or patient follow-up is uncertain. Thus, identification of the causative organism may not occur. In such situations, diagnosis is based on clinical findings.
Treatment of STIs
Usually antimicrobials
Treatment of complications, if present
If possible, simultaneous treatment of sex partners
Most STIs can be effectively treated with antimicrobial medications. However, drug resistance is an increasing problem.
Clinical diagnosis alone may be used to determine treatment in the following situations:
Diagnostic testing is limited or not available.
The infection is severe and treatment cannot be delayed while waiting for a diagnostic test result.
Clinical suspicion of a particular infection is high based on history or physical examination, particularly if patient follow-up is uncertain.
Patients who are being treated for a bacterial STI should abstain from sexual intercourse until the infection has been eliminated from them and their sex partners. Sex partners should be evaluated and treated simultaneously.
Viral STIs, especially herpes and HIV infection, usually persist for life. Antiviral medications can control but not yet cure most of these infections, and patients should be counseled about safer sex practices to prevent transmission.
Prevention of STIs
STI control depends on
Education of health care practitioners and the public
Avoidance of high-risk behaviors by patients
Adequate access to facilities and trained personnel for diagnosis and treatment
Public health programs for locating and treating recent sex partners of patients
Follow-up for treated patients to ensure that they have been cured
Condoms and vaginal condoms or dental dams, if used correctly, greatly decrease risk of some STIs. For some infections and patients, pre-exposure prophylaxis with immunizations or medications is appropriate.
Vaccines are not available for most STIs, except for hepatitis A, hepatitis B, and human papillomavirus infection.
More Information
The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
CDC: Sexually Transmitted Disease Surveillance 2020: Reference document with statistics and trends for STIs in the US through 2020 for policy makers, researchers, and others who are concerned with the public health implications of these diseases
