RespiratoryTier 2Disease (DEADMAN)

Legionella infection

Description

Atypical bacterial pneumonia from Legionella pneumophila (Gram-negative, intracellular) - acquired from contaminated water aerosols (cooling towers, showers, spa pools), not spread person-to-person

Epidemiology

Outbreaks linked to cooling towers/water systems; risk factors: smoking, older age, immunosuppression, chronic lung disease, travel/hotel exposure ("Legionnaires' disease" was first described at a hotel convention)

Aetiopathogenesis

Inhaled aerosolised contaminated water -> intracellular replication within alveolar macrophages -> severe pneumonia +/- multisystem involvement

Diagnosis

Urinary Legionella antigen - rapid, detects serogroup 1 (the majority of cases) but misses non-1 serogroups
Sputum/respiratory culture on selective media (BCYE agar) for definitive diagnosis and outbreak strain typing
Clinical clues alongside pneumonia: relative bradycardia for the degree of fever, hyponatraemia, deranged LFTs, GI symptoms, confusion - suggestive but not sensitive/specific enough alone

Management

Macrolides (azithromycin) or fluoroquinolones (levofloxacin) - beta-lactams are ineffective (intracellular organism)
Notifiable disease - triggers public health investigation of the environmental source

Associations

Cooling towers, hot water systems, travel-associated pneumonia clusters, hyponatraemia

Natural history & complications

Can progress to severe pneumonia/ARDS and multiorgan involvement if untreated; mortality is significant in the immunosuppressed/elderly, low with prompt appropriate antibiotics

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