Common Symptoms of Legionnaires' Disease
Legionnaires' disease presents with high fever, respiratory symptoms (cough, dyspnea), and hyponatremia, but cannot be reliably distinguished from other pneumonias based on clinical features alone, requiring specific laboratory testing for definitive diagnosis. 1, 2
Core Respiratory and Systemic Symptoms
Fever and Respiratory Manifestations:
- High-grade, persistent fever is the hallmark feature, typically accompanied by chills 2, 3
- Cough with or without sputum production 2
- Dyspnea (shortness of breath) and respiratory distress 2, 3
- Rales and altered breath sounds on physical examination, though these are neither sensitive nor specific 2
- Respiratory failure is the main cause of admission in 90% of severe cases 3
Characteristic Laboratory and Extrapulmonary Features
Highly Suggestive Laboratory Findings:
- Hyponatremia (low serum sodium) is a characteristic and highly suggestive finding 1, 4, 2
- Elevated lactate dehydrogenase (LDH) levels, particularly >700 U/mL 1, 4, 2
- Leukocytosis with left shift in complete blood count 4
Central Nervous System Manifestations:
- Confusion, altered mental status, or encephalopathy can occur 1, 4, 2
- These CNS symptoms are part of the constellation of features that suggest Legionnaires' disease 1
Severe Disease Manifestations in High-Risk Populations
Multi-Organ Involvement:
- Multi-organ failure occurs in 60% of severe cases requiring intensive care 3
- Acute kidney injury develops in 50% of severe cases, sometimes with rhabdomyolysis 5, 3
- Gastrointestinal symptoms may be present 6
- Atypical presentations can include elevated liver enzymes with a 2:1 AST:ALT pattern 6
Important Clinical Context:
- The elderly, smokers, and immunosuppressed patients (transplant recipients, hematologic malignancies, end-stage renal disease, diabetes, chronic lung disease) are at markedly increased risk 1, 2
- Mortality rates range from 5-25% in immunocompetent hosts but reach 40% in nosocomial cases, ICU patients, and immunocompromised individuals 1, 4, 2, 5
Critical Diagnostic Pitfall
The constellation of high fever, hyponatremia, CNS manifestations, and elevated LDH may suggest Legionnaires' disease, but several studies demonstrate that individual cases cannot be reliably distinguished from other causes of pneumonia based on initial clinical findings alone 1. This is why specific diagnostic testing (urinary antigen assay or culture on selective media) is essential for any patient with severe pneumonia, ICU admission, failure to respond to β-lactam antibiotics, or relevant epidemiologic risk factors (recent travel, spa/hot tub exposure, plumbing changes) 1, 4, 2.