Legionella pneumonia (Legionnaires' disease)

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How the diagnosis is reached

A pneumonia four days into amoxicillin and still deteriorating has largely excluded the organisms amoxicillin covers, and the sputum says the same thing more directly: abundant neutrophils, no visible organism. A plainly purulent pneumonia whose pathogen refuses to appear on the film is the signature of Legionella, which stains too faintly to be seen on a routine Gram preparation. Blood cultures sterile at 48 hours argue further against pneumococcus, which is frequently bacteraemic when disease runs this severe. Then comes the systemic reach that pneumococcal pneumonia lacks: confusion, headache and watery diarrhea out of proportion to the chest, a pulse of 82 at 39.6 °C, sodium of 124 mmol/L, raised transaminases, hypophosphatemia and a creatine kinase of 1240 U/L from rhabdomyolysis. The hyponatremia discriminates best, running both commoner and deeper here than in other community-acquired pneumonia. The hotel spa pool supplies the epidemiology, an aerosol-generating warm water system being how the organism reaches a lung at all. Treatment then follows from where the organism lives rather than what it is built from. Legionella is a Gram-negative bacillus and does have a cell wall; amoxicillin failed because the organism replicates inside alveolar macrophages, where the drug does not follow it. That calls for an agent which concentrates intracellularly: a respiratory fluoroquinolone such as levofloxacin, or a macrolide such as azithromycin, for 7 to 10 days and longer if he proves immunocompromised. A negative urinary antigen would have excluded nothing, since the test detects only serogroup 1, and the case is notifiable so the hotel's water system can be traced.

Key clinical clues

• Pneumonia failing to improve on amoxicillin, which cannot reach an intracellular organism
• Prominent confusion, headache and diarrhea out of proportion to the chest findings
• Relative bradycardia despite a temperature of 39.6 °C
• Hyponatremia with deranged transaminases and a raised creatine kinase
• Exposure to an aerosol-generating warm water system (hotel spa pool)

The case in full

1

Presenting complaint

Patient history

A 62-year-old smoker has five days of fever, headache and dry cough, plus three days of watery diarrhea. His wife says he grew confused yesterday and could not recall their address. They returned a week ago from a hotel where they used the spa pool daily.

2

Vital signs

Vital signs

He is febrile and tachypneic.

Measurements

Parameter	Value	Reference
Temperature	39.6 °CRef 36.1–37.5	36.1–37.5
Heart rate	82 bpmRef 60–100	60–100
Respiratory rate	28 /minRef 12–20	12–20
Blood pressure	112/68 mmHg	–
SpO₂ (room air)	88 %Ref 95–100	95–100

3

Chemistry panel

Laboratory results

Serum chemistry is sent as part of the sepsis workup.

Measurements

Parameter	Value	Reference
Sodium	124 mmol/LRef 135–145	135–145
Phosphate	1.6 mg/dLRef 2.5–4.5	2.5–4.5
ALT	148 U/LRef 7–56	7–56
AST	132 U/LRef 10–40	10–40
Creatine kinase	1240 U/LRef 30–200	30–200
Creatinine	1.6 mg/dLRef 0.7–1.3	0.7–1.3

4

Inflammatory markers and blood count

Laboratory results

A full blood count and inflammatory markers are sent.

Measurements

Parameter	Value	Reference
White blood cells	16.8 ×10⁹/LRef 4.5–11.0	4.5–11.0
Lymphocytes	0.6 ×10⁹/LRef 1.0–4.0	1.0–4.0
C-reactive protein	312 mg/LRef <5	<5
Procalcitonin	4.8 ng/mLRef <0.5	<0.5
Platelets	168 ×10⁹/LRef 150–400	150–400

5

Chest radiograph and treatment response

Imaging

A chest radiograph is taken. His family doctor started amoxicillin four days ago; despite this his oxygen requirement has continued to climb.

6

Microbiology

Laboratory results

Blood cultures are negative at 48 hours and sputum Gram stain shows abundant neutrophils but no organisms. A urinary antigen test is sent, and it returns positive.