28th July 2023

Sexually transmitted infections (STIs)

Most STIs are caused by Chlamydia trachomatis or Neisseria gonorrhoeae. Both are Gram negative bacteria, and N. gonorrhoea is particularly notable for its diplococci morphology, which makes it distinctive in microscopy.

Both chlamydia and gonorrhoea cause a similar broad spectrum of disease, ranging from asymptomatic infection to pelvic inflammatory disease.

Diagnosis is either made on microscopy of swab smears or by a nucleic acid amplification test (NAAT).

Treatment of gonorrhoea has changed over the past few decades as resistance has emerged. Currently, ceftriaxone (1g, intramuscularly) is the treatment of choice. For chlamydia, a single dose treatment of azithromycin is the preferred treatment. Neonates may develop conjunctival disease after exposure to chlamydia in the birth canal, which is an important potential complication of chlamydia to be aware of.

Syphilis is caused by Treponema pallidum. It is one of medicine’s great mimickers and can be confused for many other diseases. It usually starts with a primary, painless ulcer (chancre) at the initial infection site, often the genitals. A secondary syphilis rash may occur a few weeks after inoculation before the bacteria enters the latent phase, which may last for decades.

If untreated, after this latency period, tertiary syphilis may develop, which can have life-threatening complications. Tertiary syphilis manifestations include aortic root disease, neurological disease, and gummatous disease of the skeleton.

Penicillin is the treatment of choice for syphilis. The dose and route depend on stage and organ involvement, summarised in the British Association for Sexual Health and HIV (BASHH) guidelines, which can be found in the References section.