Legionella pneumonia

Overview and aetiology

• Legionella pneumonia (Legionellosis) is most commonly caused by Legionella pneumophila serogroup 1, although other species—particularly Legionella longbeachae—are also important in Australia .

• Infection occurs via inhalation of aerosolised water (eg, from cooling towers, air-conditioners, showers) or dust/soil containing Legionella (especially L. longbeachae in potting mix). Person-to-person transmission is extremely rare.

Risk factors

• Older age (>50 years)

• Smoking (active or past)

• Chronic lung disease

• Immunosuppression (eg, corticosteroid therapy, transplant recipients)

• Chronic renal impairment

• Travel (especially hotels or cruise ships with large water systems)

Clinical features

• Fever, often high and sometimes with rigours

• Respiratory symptoms: cough (often initially non-productive), dyspnoea

• Extrapulmonary manifestations:

  • GI: nausea, vomiting, diarrhoea, abdominal pain

  • Neurological: confusion, headache

• Hyponatraemia is classically associated with Legionella pneumonia

• Elevated liver enzymes can also be seen

In clinical practice, a constellation of pneumonia plus prominent gastrointestinal or neurological symptoms and laboratory findings (eg, hyponatraemia) should increase suspicion of Legionella infection.

Diagnosis

Investigations

• Chest imaging: Typically shows patchy or interstitial infiltrates, which may progress to consolidation.

• Urinary antigen test: Rapid and highly specific for Legionella pneumophila serogroup 1.

• PCR and culture: Sputum samples can be used for PCR and culture of Legionella species.

• Blood tests:

  • Electrolytes (looking for hyponatraemia)

  • Full blood count and inflammatory markers

  • Liver function tests (may show mild-to-moderate transaminitis)

When to suspect Legionella

Suspect Legionella pneumonia in patients with ‘atypical’ features of pneumonia (especially GI or neurological manifestations), unexplained hyponatraemia, or exposure histories (eg, contaminated water sources, potting mix)

Severity assessment

Use a validated pneumonia severity tool (eg, CORB, SMART-COP, or CURB-65) to determine if outpatient management is appropriate or if hospital admission is warranted. This step is critical in deciding the route of antibiotic administration and the need for supportive therapy.

Treatment

Legionella pneumonia often requires antibiotics that provide atypical coverage (as Legionella is not covered by beta-lactam monotherapy alone).

Low- to moderate-severity

One of the following (oral or enteral) regimens is recommended:

1. Azithromycin 500 mg once daily for 3 to 7 days

   • If pneumonia is low severity and the patient improves clinically, 3 days may suffice.

   • For moderate severity, up to 7 days based on clinical response.

2. Ciprofloxacin 750 mg twice daily for 5 to 7 days

   • Use 5 days if there is a rapid clinical response.

3. Doxycycline 100 mg twice daily for 10 to 14 days

   • Note: Legionella longbeachae appears more resistant to doxycycline in vitro, and clinical data on doxycycline outcomes are limited. Consider azithromycin or ciprofloxacin for L. longbeachae.

High-severity

Requires intravenous therapy initially:

1. Azithromycin 500 mg IV once daily, or

2. Ciprofloxacin 400 mg IV every 8 hours

3. Intravenous to oral switch is done once the patient demonstrates clinical improvement, with a total duration of 7 to 10 days usually recommended.

4. Combination therapy (eg, azithromycin plus ciprofloxacin, or rifampicin plus either azithromycin or ciprofloxacin) is sometimes used in intensive care settings for critical cases, though evidence is limited and toxicity risk is higher. Seek expert advice if considering combination therapy.

Additional management considerations

• Supportive care: Ensure adequate hydration, oxygen therapy, analgesia, and monitoring in moderate to severe cases.

• Monitoring: Regular review of clinical status, vital signs, and laboratory markers (including electrolytes) during therapy.

• Notification: Legionella pneumonia is a notifiable disease in Australia. Notify your state or territory public health authority promptly