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Sexually Transmitted Infections (STIs)

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This article provides an overview of sexually transmitted infections (STIs), including the organisms involved, symptoms, signs, investigations and management options. For more information on history taking, see our OSCE guide to taking a sexual history.

In the UK, chlamydia is the most common STI, with approximately 160,000 cases diagnosed by sexual health services in England in 2021.

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Risk factors

General risk factors for sexually transmitted infections include:

  • Unprotected sexual intercourse
  • Multiple sexual partners
  • 15-24 year-olds
  • Illicit drug use and alcohol use
  • Men who have sex with men (MSM)
  • Sex workers
  • Urban areas
Co-infection

The co-infection rate of sexually transmitted infections is approximately 40%. This means that if one STI is present, there is around a 40% chance of a second STI co-infection. It is important to conduct a full sexual health screen in all individuals diagnosed with an STI.


Chlamydia

Causative organism

Chlamydia trachomatis

Transmission

These obligate intracellular bacteria are transmitted via sexual contact. However, perinatal transmission from an infected mother to her baby during vaginal birth can lead to neonatal conjunctivitis (ophthalmia neonatorum) and pneumonia.

Clinical presentation

Chlamydia is asymptomatic in approximately 70% of females and 50% of males.

Typical clinical features of chlamydia in males include:

  • Urethral discharge (usually clear)
  • Dysuria
  • Proctitis

Epididmyo-orchitis is a complication of chlamydial infection, and patients may present with scrotal pain.

Typical clinical features of chlamydia in females include:

  • Vaginal discharge
  • Proctitis
  • Post-coital bleeding (may indicate cervicitis)
  • Intermenstrual bleeding
  • Cervicitis on vaginal examination

Chlamydial infection can lead to pelvic inflammatory disease (PID), and patients may present with pelvic pain with signs of systemic infection,

Ophthalmic chlamydial infection can cause conjunctivitis, and oropharyngeal infection can cause pharyngitis. 

Investigations

Chlamydia is diagnosed using a Nucleic Acid Amplification Test (NAAT). This detects the presence of the organism’s DNA. A NAAT test involves a first-pass urine sample (if sampling from the penis) or a vaginal/throat/rectal swab. A vulvovaginal swab is more sensitive than a urine sample in women. 

A self-taken sample in individuals who are asymptomatic is now a common method of chlamydia screening, and widely available. 

There is a 14-day window period following infection with chlamydia or gonorrhoea. If there has been exposure during the window period, testing should be repeated 14 days after the last exposure. 

As the NAAT detects DNA, it cannot indicate if the organism is living or dead. Therefore the swab may remain positive for up to 5 weeks following treatment for chlamydia. 

Management

The first-line management for chlamydia is:

  • Doxycycline 100mg orally twice daily for 7 days (contraindicated in pregnancy) OR
  • Azithromycin 1g oral, followed by 500mg daily for 2 days

Individuals diagnosed with chlamydia should undergo a full sexual health screen. Contact tracing and partner notification need to be undertaken.

All forms of sexual intercourse need to be avoided until all parties are tested and have completed treatment. 

A test of cure at around 5 weeks should be offered in cases of rectal infection or pregnancy. 

Complications

Complications of chlamydia include:

  • Pelvic inflammatory disease (PID): increases the risk of ectopic pregnancy and infertility
  • Epididmyo-orchitis (leading to scrotal pain and swelling)
  • Prostatitis
  • Reactive arthritis
Lymphogranuloma venereum (LGV)

Lymphogranuloma venereum (LGV) is caused by a serotype of Chlamydia trachomatis. It is most commonly tested for in men who have sex with men (MSM) presenting with anal discharge and pain, or anyone presenting with rectal chlamydia. 

LGV requires a longer course of antibiotics (21 days), and there are higher rates of HIV and hepatitis C co-infection. 


Gonorrhoea

Causative organism 

Neisseria gonorrhoeae

Transmission

This gram-negative diplococcus bacteria is predominantly transmitted via sexual contact. Mucosal epithelium lining the genital tract, oropharynx and rectum are most commonly infected.

Like chlamydia, vertical transmission of gonorrhoea during childbirth can cause ophthalmia neonatorum. However, the onset of gonococcal conjunctivitis is earlier than chlamydial conjunctivitis.

Clinical features

Typical clinical features of gonorrhoea in males include:

  • Mucopurulent urethral discharge
  • Dysuria

Typical clinical features of gonorrhoea in females include:

  • Mucupurulent discharge
  • Dysuria

Rectal gonococcal infection can cause proctitis leading to rectal pain, bleeding and discharge. 

Oropharyngeal gonococcal infection can cause pharyngitis.

Ophthalmic gonococcal infection can cause conjunctivitis.

Investigations

Like chlamydia, gonorrhoea is diagnosed using a Nucleic Acid Amplification Test (NAAT). This detects the presence of the organism’s DNA. A NAAT test involves a first-pass urine sample (if sampling from the penis) or a vaginal/throat/rectal swab. A vulvovaginal swab is more sensitive than a urine sample in women. 

In addition to a NAAT, all patients with symptoms should have a culture swab taken from the symptomatic area (urethral, cervical, anal, conjunctival or oropharyngeal) before treatment. This is to test for sensitivities and monitor patterns of antimicrobial resistance.

Where microscopy is available, Gonorrhea can be diagnosed (and immediately treated) at the point of examination by identifying Gram-negative intracellular diplococci. 

Management

The first-line management for gonorrhoea is:

  • Ceftriaxone 1g intramuscular injection as a single dose (often mixed with lidocaine to reduce injection site pain) OR
  • Ciprofloxacin 500mg orally as a single dose (only used when antimicrobial sensitivities are known before treatment) OR

Individuals diagnosed with gonorrhoea should undergo a full sexual health screen. Contact tracing and partner notification need to be undertaken.

All forms of sexual intercourse need to be avoided until all parties are tested and have completed treatment. 

A test of cure should be routinely performed at least 14 days after completing treatment (if diagnosed via NAAT). 

Complications

Complications of gonorrhoea include:

  • Pelvic inflammatory disease (PID): increases the risk of ectopic pregnancy and infertility
  • Epididmyo-orchitis (leading to scrotal pain and swelling) and balanitis
  • Prostatitis
  • Disseminated gonococcal infection (DGI): a rare complication of gonococcal infection leading to systemic features (including arthritis, skin lesions and arthralgia)

Syphilis

Causative organism

Treponema pallidum

Transmission

Syphilis is either acquired or congenital:

  • Acquired: usually via direct sexual contact (exposure to a lesion) but can occur via the blood-borne route
  • Congenital: occurs in infants occurs due to trans-placental transmission, which increases the risk of miscarriage, stillbirth and congenital abnormalities.

Clinical features

Syphilis is a multi-stage and multi-system disease. 

Congenital syphilis (i.e., present from birth) can be broken down into two stages:

  • Early congenital syphilis: presents in those <2 years old
  • Late congenital syphilis: presents in those >2 years old

Two-thirds of infants with congenital syphilis will be asymptomatic at birth. Most will develop symptoms by five weeks. 

Acquired syphilis can be broken down into:

  • Primary: development of an indurated painless ulcer (chancre) on the genitals
  • Secondary: widespread non-pruritic maculopapular rash (if involving the soles and palms, is almost pathognomonic for syphilis)
  • Latent: asymptomatic infection (confirmed by positive diagnostic serology) acquired within the last two years
  • Late latent: asymptomatic infection (confirmed by positive diagnostic serology) acquired more than two years prior
  • Tertiary (occurs >2 years following infection): gummatous (formation of granulomatous inflammatory lesions), cardiovascular (aortitis) or late neurological involvement. 

Investigations

In patients with lesions, syphilis can be diagnosed using a syphilis PCR swab of the lesion. Where available, dark ground microscopy can show the motile spirochete.

The mainstay of diagnosis of syphilis is by using serology. Point-of-care testing is available in some clinics.

Interpretation of syphilis serology is an art and not an exact science. The serology must be interpreted alongside a comprehensive sexual history to reach an accurate diagnosis.

Immunological medical conditions (e.g. SLE and HIV) and pregnancy can affect the interpretation of syphilis serology. 

Tests performed include:

  • Treponemal enzyme immunoassay (EIA)
  • T.pallidum haemagglutination test (TPHA)
  • T. pallidum particle agglutination test (TPPA)
  • Rapid Plasma Reagin (RPR)
  • Venereal Disease Research Laboratory (VDRL)

RPR and VDRL are often used to monitor disease activity following treatment. 

The window period for syphilis is 12 weeks. Therefore, testing should be repeated at least 12 weeks following the last exposure. 

Management

Treatment of syphilis is with benzathine benzylpenicillin (IM). The exact treatment regime will vary depending on the stage of syphilis. 

Individuals diagnosed with syphilis should undergo a full sexual health screen. Contact tracing and partner notification need to be undertaken.

All forms of sexual intercourse need to be avoided until all parties are tested and have completed treatment.

Following successful treatment, parts of the syphilis serology will remain positive (can be lifelong). 

Complications

Complications of syphilis include:

  • Neurosyphilis: occurs when Treponema pallidum crosses the blood-brain barrier causing neurological symptoms
Jarisch-Herxheimer reaction

A Jarisch-Herxheimer (JH) reaction can occur following the initial treatment of syphilis in some patients. This phenomenon causes a sepsis-like picture due to the release of toxins from treponemal bacterium breakdown.


Herpes simplex virus (HSV)

Causative agent

HSV-1 and HSV-2

Transmission

HSV are double-stranded DNA viruses transmitted through mucosal surfaces or broken skin.

Only approximately one-third of patients will develop symptoms at the time of initial acquisition of the virus. After initial infection, the virus lies dormant within local sensory ganglia. HSV is, therefore, a lifelong infection with periods of reactivation and symptoms.

HSV-1 is the traditional cause of labial (oral) herpes infections and is now the most common cause of anogenital herpes. Anogenital infections caused by HSV-2 are around four times more likely than HSV-1 to cause recurrent symptoms.

Infection with HSV-1 does not confer immunity to HSV-2 and vice versa. 

Clinical features

As discussed above, infection with HSV can be asymptomatic.

Symptoms of infection include blisters which progress to painful ulcers around the anogenital area. This can be accompanied by dysuria, discharge and inguinal lymphadenopathy. In the initial infection, systemic symptoms (e.g. pyrexia and myalgia) may occur. 

Symptoms from initial infection tend to be more severe than those of recurrent infection/reactivation episodes. 

Investigations

HSV is diagnosed using PCR from swabs of lesions. When taking the swab, the lesion should be burst and a swab taken from the base of the ulcer.

If no lesions are present, testing for HSV can not be performed. 

HSV testing can be undertaken by serology in some places. However, this is not routinely performed and may be unreliable.

Management

HSV episodes are managed with aciclovir (an antiviral). Symptomatic episodes can be treated with aciclovir 400mg orally TDS for five days. Treatment should be commenced within five days of symptom onset. 

Saltwater baths, topical petroleum jelly, oral analgesia and topical lidocaine gel can be used for pain control.

Patients with lesions should undergo a full sexual health screen. There is no requirement for contact tracing. However, patients should be advised to refrain from intercourse when they have lesions and disclosure in relationships is advised. Condoms can reduce the rate of spread and should be encouraged.

Patients who experience recurrent episodes become aware of prodromal symptoms (e.g. tingling sensation). In these patients, treatment can be started before lesions develop to prevent or minimise an outbreak (‘rescue medication’). 

Prophylactic treatment (aciclovir 400mg BD daily) can be considered in patients experiencing multiple episodes or wishing to reduce onward transmission. 

Complications

Complications of HSV include:

  • Urinary retention: may require catheterisation
  • HSV keratitis: dendritic lesion on the cornea
  • Aseptic meningitis
  • Herpes proctitis
  • Neonatal HSV: an increased risk if the mother becomes infected in the third trimester
  • Herpetic whitlow

Genital warts

Causative agent

Human papillomavirus (HPV) 6 and 11 (in most cases)

Transmission

This double-stranded DNA virus is mainly transmitted via direct skin-to-skin contact, and more rarely can be transmitted perinatally. Infection with HPV is very common in sexually active individuals, and most will not develop visible warts. The incubation period from exposure to infection can be up to 8 months.

Clinical features

Anogenital warts can vary in size, number, colour and texture but mostly appear as textured, soft growths. The anus, cervix and urethral meatus can all be affected.

Anogenital warts can be keratinised (hard surface) or non-keratinised (soft surface). 

Genital warts are usually asymptomatic. However, itching, bleeding and pain can occur.

Investigations

The diagnosis of anogenital wards in clinical, based on characteristic examination findings. However, if lesions appear atypical or suspicious, a biopsy should be performed to exclude an oncogenic HPV virus type. 

Management

A shared decision taking into account patient preferences, wart characteristics and tolerability of treatments, should be made regarding managing anogenital warts. 

Treatment of the visible wart may not clear the virus from the body. Therefore, warts may reoccur following treatment. 

Smoking is known to increase the risk of recurrence.

Topical treatment options include:

  • Topical podophyllotoxin (Warticon® and Condyline®)
  • Topical imiquimod (patients should be made aware that this damages condoms)

Physical ablation therapy includes:

  • Cryotherapy
  • Surgical excision

Patients with anogenital warts should undergo a full sexual health screen. Contact tracing is not required. The use of condoms should be encouraged.

The HPV vaccine has not been shown to be effective in treating existing anogenital warts.

Complications

Complications of anogenital warts include:


Trichomoniasis

Causative organism

Trichomonas vaginalis

Transmission

This flagellated protozoan is transmitted via sexual intercourse.

Clinical features

Trichomoniasis is asymptomatic in up to 50% of cases.

Typical clinical features in females include:

  • Vaginal discharge (thin, frothy yellow coloured)
  • Strawberry cervix on speculum examination
  • Vulval pruritus
  • Vulvovaginitis
  • Dysuria
  • Dyspareunia

Typical clinical features in males include:

  • Urethral discharge
  • Urethral irritation/itching
  • Dysuria
  • Balanitis

Investigations

Testing for trichomoniasis does not form a part of a routine asymptomatic sexual health screen in the UK.

In some centres, diagnosis can be made at the time of assessment by detecting motile trichomonads on wet mount microscopy. 

Point-of-care testing (using a posterior vaginal fornix swab) is becoming increasingly popular, due to improved sensitivity over microscopy. 

Alternatively, a NAAT can be performed on vulvovaginal swabs or first-pass urine samples in men.

Trichomonas vaginalis can also be detected from a culture of vaginal or urethral discharge, or of first-pass urine in men.

Management

Treatment of trichomoniasis is with metronidazole 400-500mg twice daily for 5-7 days (first line) or 2g oral as a single dose (alternative). Alcohol should be avoided during treatment and for 72 hours afterwards.

Individuals diagnosed with trichomoniasis should undergo a full sexual health screen. Contact tracing and partner notification need to be undertaken.

All forms of sexual intercourse need to be avoided until all parties are tested and treated. A test of cure is not routinely required.

Complications

Complications of trichomoniasis include:

  • Pelvic inflammatory disease: increases the risk of ectopic pregnancy and infertility
  • Altered vaginal flora
  • Prostatitis
  • In pregnancy, there is an increased risk of premature rupture of membranes and preterm birth.

Mycoplasma genitalium (M. gen)

Causative organism

Mycoplasma genitalium is a tiny self-replicating bacterium.

Transmission

Mycoplasma genitalium is a sexually transmitted, the bacteria invade epithelial cells and can remain there for many years if left untreated.

Clinical features

In the vast majority of cases, infection with Mycoplasma genitalium is asymptomatic.

If symptoms occur, males present with urethral discharge(often clear), dysuria or epididymo-orchitis. Females present with dysuria, post-coital bleeding, or signs of pelvic inflammatory disease.

Proctitis can also occur. 

Investigations

Testing for Mycoplasma genitalium does not form a part of a routine asymptomatic sexual health screen in the UK. Testing should occur in patients with symptoms.

NAATs are the gold standard for diagnosis. First void samples urine in men and vaginal swabs in women should be used. 

Management

Management of Mycoplasma genitalium depends on the clinical presentation and any known resistance patterns.

For patients with uncomplicated urogenital infection: doxycycline 100mg twice daily for 7 days followed by azithromycin 1g as a single dose then 500mg daily for 2 days (total 10 days of antibiotic treatment)

For patients with complicated infection (PID/epididymo-orchitis): moxifloxacin 400mg daily for 14 days

Different treatment regimens are used in pregnancy.

Individuals diagnosed with Mycoplasma genitalium should undergo a full sexual health screen. Contact tracing and partner notification need to be undertaken.

All forms of sexual intercourse need to be avoided until all parties are tested and treated. A test of cure is performed at 5 weeks following initiation of treatment. 

Complications

Complications of Mycoplasma genitalium include:

  • Sexually acquired reactive arthritis (SARA)
  • Pelvic inflammatory disease: increases the risk of ectopic pregnancy and infertility
  • Pre-term delivery

Reviewers

Dr Grace Farrington

GP Trainee

Dr Ashley Jefferies

Community Sexual and Reproductive Health Registrar


References

  1. BMJ Publishing Group (2018) Overview of sexually transmitted diseases, Available at: [LINK]
  2. British Association for Sexual Health and HIV (2015; Updated 2018) 2015 UK national guideline for the management of infection with Chlamydia trachomatis, Available at: [LINK]
  3. British Association for Sexual Health and HIV (2018) 2018 UK national guideline for the management of infection with Neisseria gonorrhoeae, Available at:[LINK]
  4. British Association for Sexual Health and HIV (2015; Updated 2019) UK national guidelines on the management of syphilis 2015, Available at:[LINK]
  5. British Association for Sexual Health and HIV (2014) 2014 UK national guideline for the management of anogenital herpes, Available at: [LINK]
  6. British Association for Sexual Health and HIV (2015) UK National Guidelines on the Management of Anogenital Warts 2015. Available at:[LINK]
  7. National Health Service UK (2019) HPV vaccine overview. Available at: [LINK]
  8. British Association for Sexual Health and HIV (2014). United Kingdom National Guideline on the Management of Trichomonas vaginalis 2014. Available at: [LINK]
  9. British Association for Sexual Health and HIV (2018). British Association for Sexual Health and HIV national guideline for the management of infection with Mycoplasma genitalium (2018). Available at: [LINK]
  10. UK Health Security Agency. Sexually transmitted infections and screening for chlamydia in England: 2021 report. 2022. Available from: [LINK]

 

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