Legionella pneumophila

Diagnosis

Indications for Testing

• Community-acquired pneumonia with gastrointestinal symptoms
• Consider testing if nosocomial pneumonia occurs or if other known Legionella pneumonia is present in hospital

Laboratory Testing

• Legionellosis resources (CDC)
• Perform routine microbiology testing only for pathogens that would alter empirical decisions (recommendation from Infectious Diseases Society of America and American Thoracic Society)
• Nonspecific testing
  • Serum sodium – hyponatremia
  • Serum phosphate – hypophosphatemia
  • Urine analysis – hematuria
  • CBC – thrombocytopenia common
  • Liver function studies – elevated transaminases
• Consider testing for other common agents of community-acquired pneumonia – Streptococcus pneumoniae, Chlamydophila pneumoniae, Mycoplasma pneumoniae
• Rapid testing may aid in early initiation of treatment
  • Point of care testing – available for urine specimens (sensitivity 70-80%; specificity 97-100%)
  • Urine antigen testing (EIA)  
    • Urine antigen testing more sensitive than DFA and highly specific for serogroup 1  
    • Detects only serogroup 1 (most common serogroup causing disease)
    • Antigens or antibodies may be detected as early as 1 day after symptom onset; positivity of test may persist >60 days
  • DFA of respiratory specimen
    • Highly insensitive compared to PCR and culture
    • Can detect various serogroups
    • Technically demanding; requires large respiratory specimen
    • Urine antigen detects serogroup 1; DFA detects various serogroups
  • PCR testing of respiratory secretions – good sensitivity; rate of false positives is 5-10%
• Culture – considered gold standard in diagnosis
  • Best yield from sputum, which may be difficult to obtain with legionellosis
  • May be used to confirm PCR results
  • Takes at least 48 hours 
  • Genotyping isolates may help identify source of outbreak
• Serological testing may be used for retrospective diagnosis
  • Seroconversion may take several weeks

Imaging Studies

• Chest x-ray
  • Rapidly progressive asymmetrical infiltrates are common
  • Infiltrates may initially enlarge concurrent with clinical improvement while on appropriate antimicrobial therapy

Differential Diagnosis

• Pneumonia
  • Other atypical pneumonia (eg, mycoplasma)
  • Bacterial pneumonia (eg, S. pneumoniae)
  • Viral pneumonia (eg, influenza, parainfluenza, hantavirus)
  • Zoonotic pneumonias (Coxiella burnetii, C. pneumoniae and psittaci, Francisella tularensis)
• Gastrointestinal
  • Influenza
  • Diarrhea (viral, bacterial, parasitic)

Clinical Background

Legionellosis refers to two clinical syndromes caused by bacteria of the genus Legionella – Legionnaires’ disease and Pontiac fever. Legionnaires’ disease is considered an atypical pneumonia.

Epidemiology

• Incidence – >10,000 annually in U.S.
  • May be responsible for 3-8% of all community-acquired pneumonia
• Age – usually 45-64 years; rare cause of pneumonia in children
• Sex – M>F; 2-3:1
• Transmission
  • Mostly sporadic cases
  • More common in summer months – outbreaks of pneumonia have occurred after inhalation of contaminated water droplets from water sources such as evaporative coolers, hot tubs, heating systems
  • Person-to-person transmission has not been shown
  • Nosocomial cases are not uncommon
    • Sources of infection are often hospital showers, drinking fountains, and respiratory-therapy equipment

Organism

• Legionella is a fastidious gram-negative coccobacillus
  • Widespread in nature but live mainly in freshwater bodies
  • Parasitizes freshwater amoeba in the environment and pulmonary macrophages in humans
• >50 species compose at least 70 different serogroups of Legionellae
• L. pneumophila is major pathogen and most commonly encountered member of group
  • Serogroup 1 is most common infecting type

Risk Factors

• Adults
  • Older age (average ~55 years)
  • Cigarette smoking
  • Underlying chronic disease (eg, chronic obstructive pulmonary disease, diabetes, renal failure)
  • Compromised immune system (immunosuppressive therapy or T-cell dysfunction, including cancer)
  • Recent surgery
  • Alcoholism
  • Long-term use of breathing machines (ventilators)
• Children – prematurity, immunocompromised, bronchopulmonary dysplasia

Clinical Presentation

• Legionnaires' disease
  • Incubation period of 2-15 days
    • Prodromal phase of headache and myalgia progressing to fever with chills
    • Productive cough, possibly with hemoptysis 
    • Chest pain 
    • Shortness of breath
  • Symptoms may suggest pneumococcal pneumonia
  • Frequently includes gastrointestinal symptoms – primarily loose stools, diarrhea and/or abdominal pain
  • Frequently associated systemic involvement – kidney, liver and central nervous system dysfunction
    • Central nervous system – headache, lethargy, confusion, ataxia, encephalopathy 
  • Case fatality rate – 5-30%
    • Greater risk of death exists for elderly and immunocompromised  
• Pontiac fever
  • Self-limiting, febrile illness with non-pneumonic, influenza-like illness characterized by myalgia, malaise and fever
    • Usually affects healthy individuals

Treatment

• Begin treatment as soon as disease is suspected
• Supportive – oxygen, fluid, and electrolyte replacement
• Antibiotic therapy – usually quinolones and macrolides

Indications for Laboratory Testing

• Tests generally appear in the order most useful for common clinical situations
• Click on number for test-specific information in the ARUP Laboratory Test Directory

Click the plus sign to expand the table of additional tests.

Test Name and Number	Comments
Legionella pneumophila Antibody (Type 1), IgM by IFA 0050273 Method: Semi-Quantitative Indirect Fluorescent Antibody	Epidemiologic purposes only 40-60% sensitivity, 96-99% specificity
Legionella pneumophila Antibody (Types 1-6), IgM by IFA 0050274 Method: Semi-Quantitative Indirect Fluorescent Antibody	Epidemiologic purposes only 40-60% sensitivity, 96-99% specificity
Legionella pneumophila Antibody (Types 1-6), IgG by IFA 0050365 Method: Semi-Quantitative Indirect Fluorescent Antibody	Epidemiologic purposes only 40-60% sensitivity, 96-99% specificity
Legionella pneumophila Antibody (Type 1), IgG by IFA 0050376 Method: Semi-Quantitative Indirect Fluorescent Antibody	Epidemiologic purposes only 40-60% sensitivity, 96-99% specificity
Chlamydia pneumoniae by PCR 0060715 Method: Qualitative Polymerase Chain Reaction
Mycoplasma pneumoniae by PCR 0060256 Method: Qualitative Polymerase Chain Reaction
Streptococcus pneumoniae Antigen, Urine 0060228 Method: Qualitative Immunochromatography