BMS1026 Stewart Lecture 4 Sexually Transmitted Infections

Learning Outcomes

  • List the major sexually transmitted infections (STIs).

  • Understand their importance to public health.

  • Identify and describe the causative agents of STIs.

  • Outline clinical presentations of bacterial STIs.

  • Describe laboratory diagnosis of the different STIs.

Sexually Transmitted Infections (STIs)

  • Definition: A disease caused by a pathogen transmitted during sexual acts;

    • Sexually Transmitted Disease (STD): A term used interchangeably with STI.

  • Microorganisms involved: STIs comprise a diverse range of pathogens, categorized primarily into bacteria, viruses, fungi, and protozoa.

Major Pathogens Causing STIs

  1. Bacteria

    • Gonorrhoea: Neisseria gonorrhoeae

    • Syphilis: Treponema pallidum

    • Chlamydia: Chlamydia trachomatis

    • Chancroid: Haemophilus ducreyi

  2. Viruses

    • Genital Herpes: Herpes simplex virus type 2

    • Genital Warts: Papilloma virus

    • AIDS: Human Immunodeficiency virus (HIV)

  3. Fungi

    • Candidiasis (Thrush): Candida albicans

  4. Protozoa

    • Trichomoniasis: Trichomonas vaginalis

Public Health Importance of STIs

  • STIs are significant contributors to morbidity and mortality worldwide.

    • Syphilis leads to approximately 300,000 neonatal deaths annually.

    • HPV is linked to 250,000 cervical cancer deaths.

    • HIV/AIDS results in about 700,000 deaths each year.

  • Other consequences of STIs include:

    • Infertility

    • Heart disease

    • Blindness

General Characteristics of STIs

  • Infectious agents are not shed in large quantities but can be efficiently transmitted through person-to-person contact, particularly in moist genitourinary (GU) tracts.

  • Agents tend to be sensitive to environmental factors such as drying, light, and heat.

  • Symptoms are often absent or unclear, leading to many undetected cases of STIs.

  • Co-infections are common; one infection may be masked by another, leading to delayed treatment.

Classification of STIs

  1. STIs causing discharge:

    • Gonorrhoea

    • Chlamydial infection

    • Trichomoniasis

    • Candidal infection

    • Bacterial vaginosis

  2. STIs causing ulceration:

    • Syphilis

    • Chancroid

Epidemiology of STIs

  • STIs often have higher incidence rates in specific demographics, particularly young people.

  • Statistical representation of STI diagnoses by age and gender shows significant trends in incidence rates.

Gonorrhoea: Neisseria gonorrhoeae

  • Overview: Widespread infection; diagnosed with around 70,000 cases in the UK alone in 2019.

  • Transmission: Occurs via person-to-person contact; primarily through mucous membranes of the genitourinary tract.

Clinical Presentation

  • Uncomplicated Infection:

    • Men: Painful urethra infection; primary site of infection is the urethra, with the rectum and pharynx as potential sites.

    • Women: Often mild symptoms leading to unnoticed infections; cervix is the primary site; may include vaginitis.

  • Complicated Infection:

    • Men: Can progress to prostatitis and epididymitis.

    • Women: Can progress to salpingitis and pelvic inflammatory disease (PID).

Pathogenesis

  • N. gonorrhoeae adheres to columnar epithelial cells using pili, invades epithelial cells, and may trigger inflammation leading to mucosal damage.

Diagnosis and Treatment

  • Diagnosis: Microscopy of urethral smears for intracellular diplococci; culture on selective media.

  • Treatment: A single dose of penicillin can cure but resistance is increasing; alternatives include ciprofloxacin, cefixime, and ceftriaxone; doxycycline for concurrent chlamydia treatment is prudent.

  • Antibiotic Resistance: Growing concerns regarding reduced susceptibility requiring combination treatments (e.g., injectable ceftriaxone and oral azithromycin).

Chlamydia: Chlamydia trachomatis

  • Characteristics: Obligately intracellular parasite, cannot be cultured on agar; has two forms:

    • Elementary bodies (extracellular)

    • Reticulate particles (intracellular)

Clinical Presentation

  • Women: Mucopurulent cervicitis, pelvic inflammatory disease (major infertility cause).

  • Men: Often asymptomatic; may cause non-gonococcal urethritis with discharge.

Diagnosis and Treatment

  1. Diagnosis Options:

    • Growth in tissue culture (difficult, variable sensitivity)

    • Immunofluorescence (highly sensitive)

    • Immunoassays (applicable for larger specimens)

    • Molecular techniques (PCR for high sensitivity, costly)

  2. Treatment: Tetracycline or doxycycline.

  • Epidemiology: Common in young populations; overall incidence remains high but stabilized since 2006.

Syphilis: Treponema pallidum

  • Infection Phases:

    1. Primary: painless ulcer

    2. Secondary: systemic illness with fever

    3. Latent: no symptoms

    4. Tertiary: cardiovascular or neurological issues

  • Transmission occurs via broken skin.

Diagnosis and Treatment

  • Diagnosis through dark-ground microscopy; detection of antibodies is preferred.

  • Treatment: Penicillin.

Epidemiology

  • Rising cases observed since the 1990s, particularly among males and men who have sex with men.

Chancroid: Haemophilus ducreyi

  • Overview: Common in the Tropics; shallow painful ulcers, potential lymphadenopathy.

  • Diagnosis: Based on clinical presentation, microscopy, and exclusion of syphilis.

  • Treatment: Erythromycin and ciprofloxacin are effective, though resistance can occur.