Summary

Legionnaires' disease is a severe atypical pneumonia caused by the Gram-negative bacterium Legionella pneumophila. The organism is found in water systems and is transmitted by inhalation of contaminated aerosols (air conditioning, cooling towers, showers). It is NOT transmitted person-to-person. The classic presentation is a severe community-acquired pneumonia with high fever, dry cough, GI symptoms (diarrhoea), confusion, and hyponatraemia. Diagnosis is by urinary antigen test (detects serogroup 1 only) or PCR. Treatment is with fluoroquinolones or macrolides - beta-lactams are INEFFECTIVE as Legionella is an intracellular organism. It is a notifiable disease with public health implications.

Key Facts

• Organism: Legionella pneumophila (Gram-negative, intracellular)
• Transmission: Inhalation of contaminated water aerosols (NOT person-to-person)
• Sources: Air conditioning, cooling towers, hot tubs, showers, fountains
• Classic Features: Pneumonia + Diarrhoea + Hyponatraemia + Confusion
• Diagnosis: Urinary antigen (serogroup 1), PCR, culture
• Treatment: Fluoroquinolone (Levofloxacin) or Macrolide (Azithromycin)
• Key Point: Beta-lactams (e.g., Amoxicillin) are INEFFECTIVE

Clinical Pearls

"Legionella Loves Low Sodium": Hyponatraemia (often <130) is a classic clue - think Legionella if CAP + low Na.

"Atypical Means Unusual Features": GI symptoms (diarrhoea), confusion, and liver involvement are common - not typical of streptococcal pneumonia.

"Penicillins Don't Work": Legionella is an intracellular pathogen. Beta-lactams cannot penetrate cells. Use fluoroquinolones or macrolides.

"Always Notify": Legionnaires' disease is notifiable. Public health must trace the water source to prevent outbreaks.

Vital Signs

• High fever (often >39.5°C)
• Tachypnoea
• Tachycardia
• May be hypoxic

Respiratory Examination

• Signs of consolidation (reduced breath sounds, bronchial breathing, crackles)
• Pleural effusion (in some)

Other Signs

• Confusion, reduced GCS (encephalopathy)
• Relative bradycardia (fever-pulse dissociation)
• Hepatomegaly (occasionally)

Mortality

• Community-acquired: 5-10%
• Nosocomial: 15-20%
• ICU cases: Up to 30%

Factors Affecting Prognosis

Recovery

• Most recover fully with appropriate treatment
• Some have prolonged fatigue
• Neurological sequelae rare

Key Guidelines

1. BTS Guidelines: CAP in Adults
2. NICE Pneumonia Guidelines
3. Public Health England: Legionnaires' Disease Guidance

Key Evidence

Antibiotic Efficacy

• Fluoroquinolones and macrolides are first-line
• No RCTs (ethical issues); based on observational data and in-vitro efficacy

Urinary Antigen

• Sensitivity 70-80% (serogroup 1 only)
• Specificity >95%

What is Legionnaires' Disease?

Legionnaires' disease is a type of lung infection (pneumonia) caused by bacteria called Legionella. It's spread by breathing in tiny water droplets from contaminated water systems, like air conditioning units or showers. You CANNOT catch it from another person.

What Are the Symptoms?

• High fever
• Dry cough
• Difficulty breathing
• Muscle aches
• Diarrhoea (unusual for pneumonia)
• Confusion

How is it Diagnosed?

A simple urine test can detect Legionella in most cases. Blood tests and chest X-rays are also done.

How is it Treated?

Antibiotics are very effective. You'll usually receive antibiotics through a drip in hospital initially, then tablets to take at home. Treatment usually lasts 1-2 weeks.

Is it Serious?

Legionnaires' disease can be serious, especially in older people or those with weakened immune systems. Most people recover fully with treatment, but some need intensive care.

Where Does it Come From?

• Hotel or hospital water systems
• Air conditioning cooling towers
• Hot tubs and spas
• Decorative fountains

If a case is diagnosed, public health authorities investigate to find and disinfect the source.