Legionella pneumophila (Legionellosis) - Pontiac Fever and Legionnaires’ Disease

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.

  • Diagnosis
  • Background
  • Lab Tests
  • References
  • Related Topics

Indications for Testing

Laboratory Testing

  • Legionellosis resources (CDC)
  • Routine microbiology testing – only for pathogens that would alter empirical decisions (Infectious Diseases Society of America [IDSA] and American Thoracic Society [ATS], 2007)
  • Nonspecific testing – may reveal
  • Testing for other common agents of community-acquired pneumonia – Streptococcus pneumoniae, Chlamydophila pneumoniae, Mycoplasma pneumoniae, Haemophilus influenzae
  • Rapid testing – aid in early initiation of treatment
    • Urine antigen testing (enzyme-linked immunosorbent assay [ELISA])
      • Relatively short turnaround time (TAT) – point of care for some assays
      • Detects only serogroup 1 (most common disease-causing serogroup)
      • More sensitive than direct fluorescent antibody (DFA) testing – sensitivity depends on severity of disease, presence of serogroup 1
      • Antigens or antibodies may be detected as early as 1 day after symptom onset; positivity of test may persist >60 days
        • Cannot be used for test of cure
      • For negative test early in illness and high pretest probability – repeat test in 2-3 days
    • Polymerase chain reaction (PCR) – good sensitivity; low rate of false positives
    • Respiratory specimen (DFA)
      • Highly insensitive compared to PCR and culture
      • Can detect various serogroups
      • Technically demanding; requires large respiratory specimen
  • Culture – gold standard
    • Requires sputum or bronchoalveolar lavage (BAL)
    • May be used to confirm PCR results
    • Minimum of 48 hours TAT – fastidious with average culture taking 2-5 days for growth
    • 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
    • Cavitation may occur – most common in immunocompromised patients

Differential Diagnosis


  • Incidence
  • Age
    • Usually 45-64 years
    • Rare cause of pneumonia in children
  • Sex – M>F; 2-3:1
  • Transmission
    • Majority are sporadic
    • 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


  • Fastidious gram-negative coccobacillus
    • Widespread in nature but live mainly in freshwater bodies
    • Parasitizes freshwater free-living 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)
    • Tobacco use
    • Underlying chronic disease (eg, chronic obstructive pulmonary disease, diabetesrenal failure)
    • Immunocompromised state (immunosuppressive therapy or T-cell dysfunction, including cancer)
    • Recent surgery
    • Alcoholism
    • Long-term use of breathing machines (ventilators)
  • Children
    • Prematurity
    • Immunocompromised state
    • Bronchopulmonary dysplasia

Clinical Presentation

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

Legionella pneumophila Antigen, Urine 0070322
Method: Qualitative Enzyme-Linked Immunosorbent Assay


Detect antigens to L. pneumophila serogroup 1; a negative test result does not rule out the possibility of Legionella infection due to other serogroups or species of Legionella

Legionella Species by Qualitative PCR 2010125
Method: Qualitative Polymerase Chain Reaction


Nucleic acid from other Legionella species will be detected by this test but cannot be differentiated

Negative result does not rule out the presence of polymerase chain reaction (PCR) inhibitors in patient specimen or test-specific nucleic acid in concentrations below the level of detection by this test

Legionella Species, Culture 0060113
Method: Culture/Identification

Explify Respiratory Pathogens by Next Generation Sequencing 2013694
Method: Massively Parallel Sequencing


Mandell LA, Wunderink RG, Anzueto A, Bartlett JG, Campbell D, Dean NC, Dowell SF, File TM, Musher DM, Niederman MS, Torres A, Whitney CG, Infectious Diseases Society of America, American Thoracic Society. Infectious Diseases Society of America/American Thoracic Society consensus guidelines on the management of community-acquired pneumonia in adults. Clin Infect Dis. 2007; 44 Suppl 2: S27-72. PubMed

General References

Bartlett JG. Diagnostic tests for agents of community-acquired pneumonia. Clin Infect Dis. 2011; 52 Suppl 4: S296-304. PubMed

Blyth CC, Adams N, Chen SC. Diagnostic and typing methods for investigating Legionella infection. N S W Public Health Bull. 2009; 20(9-10): 157-61. PubMed

Carratalà J, Garcia-Vidal C. An update on Legionella. Curr Opin Infect Dis. 2010; 23(2): 152-7. PubMed

Charles PG. Early diagnosis of lower respiratory tract infections (point-of-care tests). Curr Opin Pulm Med. 2008; 14(3): 176-82. PubMed

Cunha BA. Atypical pneumonias: current clinical concepts focusing on Legionnaires' disease. Curr Opin Pulm Med. 2008; 14(3): 183-94. PubMed

Darby J, Buising K. Could it be Legionella? Aust Fam Physician. 2008; 37(10): 812-5. PubMed

Diederen BM. Legionella spp. and Legionnaires' disease. J Infect. 2008; 56(1): 1-12. PubMed

Yu VL, Lee TC. Neonatal legionellosis: the tip of the iceberg for pediatric hospital-acquired pneumonia? Pediatr Infect Dis J. 2010; 29(3): 282-4. PubMed

References from the ARUP Institute for Clinical and Experimental Pathology®

Medical Reviewers

Content Reviewed: 
November 2017

Last Update: December 2017