ICU · Infectious Diseases

Acute severe community-acquired pneumonia: Legionella

Also known as Legionella pneumophila · Legionnaires disease · Atypical pneumonia · Pontiac fever · Legionellosis

Legionella pneumophila is a gram-negative facultative intracellular pathogen that causes two syndromes: Pontiac fever (a mild self-limiting flu-like illness, no pneumonia) and Legionnaires' disease (severe atypical CAP). It is acquired by inhalation of aerosolised water from cooling towers, spa pools, hot-water systems, showers, fountains and nebulisers; there is NO person-to-person spread. The organism is fastidious — it requires buffered charcoal yeast extract (BCYE) agar supplemented with L-cysteine and iron, and does NOT grow on routine blood/Chocolate agar. Clinical clues suggesting Legionella over pneumococcus: hyponatraemia (44.4 percent of Legionnaires' disease vs 8.2 percent of other pneumonias), diarrhoea, confusion and relative bradycardia. Diagnosis rests on the urinary antigen (detects serogroup 1, the agent of most cases; pooled sensitivity 0.79, specificity 1.00) plus BCYE culture (gold standard) and PCR; in Australia and New Zealand consider Legionella longbeachae from potting mix, which the urinary antigen misses. Treatment is a macrolide (e.g. azithromycin) or fluoroquinolone (e.g. levofloxacin) — meta-analysis favours fluoroquinolones for mortality and length of stay, azithromycin performed similarly in direct comparison, and beta-lactams have no role. Hyponatraemia correction is limited to 10 mmol/L per day (4-6 in high-risk patients). Legionellosis is notifiable.

Checked against its sources on 5 Sept 2026