• Gram negative aerobe
  • L. pneumophila: associated with two distinct syndromes: pneumonia (legionnaires disease) and Pontiac Fever
  • Infection typically occurs via inhalation of aerosolized contaminated water or aspiration of contaminated water
    • Outbreaks often associated with cooling towers, hot tubs, medical equipment
    • Increased incidence with hot, humid, weather and thundershowers
  • Most commonly found in patients age > 50 years, smokers, immunocompromised.[1]
    • However, likely underestimated in children[2]

Clinical Features

Gastrointestinal symptoms Neurologic symptoms Can cause tremor, seizures, Guillain-Barre, and chorea. Has been linked to cases of Acute Cerebellar Ataxia. [Nigro, 1983] Respiratory failure occurs in ~10% of all cases.

Differential Diagnosis

Causes of Pneumonia






Severe case of Legionellosis on CXR
  • Sputum sample for culture
  • Urine legionella antigen
  • Na: Low in CAP but mostly frequently associated with Legionella
  • AST/ALT: Mildly increased 2-5x normal
  • Phosphate: Decreased
  • CK: Increased
  • CRP: > 35
  • Ferritin: Increased >2x normal


Antibiotic Sensitivities[10]

Category Antibiotic Sensitivity
Penicillins Penicillin G R
Penicillin V R
Anti-Staphylocccal Penicillins Methicillin R
Nafcillin/Oxacillin R
Cloxacillin/Diclox. R
Amino-Penicillins AMP/Amox R
Amox-Clav R
AMP-Sulb R
Anti-Pseudomonal Penicillins Ticarcillin R
Ticar-Clav R
Pip-Tazo R
Piperacillin R
Carbapenems Doripenem R
Ertapenem R
Imipenem R
Meropenem R
Aztreonam R
Fluroquinolones Ciprofloxacin S
Ofloxacin S
Pefloxacin S
Levofloxacin S
Moxifloxacin S
Gemifloxacin S
Gatifloxacin S
1st G Cephalo Cefazolin R
2nd G. Cephalo Cefotetan R
Cefoxitin R
Cefuroxime R
3rd/4th G. Cephalo Cefotaxime R
Cefizoxime R
Ceftaroline R
CefTAZidime R
Cefepime R
Oral 1st G. Cephalo Cefadroxil R
Cephalexin R
Oral 2nd G. Cephalo Cefaclor/Loracarbef R
Cefproxil R
Cefuroxime axetil R
Oral 3rd G. Cephalo Cefixime R
Ceftibuten R
Cefpodox/Cefdinir/Cefditoren R
Aminoglycosides Gentamicin X1
Tobramycin X1
Amikacin X1
Chloramphenicol X1
Clindamycin X1
Macrolides Erythromycin S
Azithromycin S
Clarithromycin S
Ketolide Telithromycin X2
Tetracyclines Doxycycline X2
Minocycline X2
Glycylcycline Tigecycline X2
Daptomycin R
Glyco/Lipoclycopeptides Vancomycin X1
Teicoplanin X1
Telavancin X1
Fusidic Acid I
Trimethoprim S
Urinary Agents Nitrofurantoin X1
Fosfomycin X1
Other Rifampin X1
Metronidazole R
Quinupristin dalfoppristin X1
Linezolid X1
Colistimethate X1


  • S susceptible/sensitive (usually)
  • I intermediate (variably susceptible/resistant)
  • R resistant (or not effective clinically)
  • S+ synergistic with cell wall antibiotics
  • U sensitive for UTI only (non systemic infection)
  • X1 no data
  • X2 active in vitro, but not used clinically
  • X3 active in vitro, but not clinically effective for Group A strep pharyngitis or infections due to E. faecalis
  • X4 active in vitro, but not clinically effective for strep pneumonia

Table Overview

See Also


  1. Cunha BA. Legionnaires' disease: clinical differentiation from typical and other atypical pneumonias. Infect Dis Clin N Am. 2010;24(1):73-105.
  2. Yu VL1, Lee TC. Neonatal legionellosis: the tip of the iceberg for pediatric hospital-acquired pneumonia? Pediatr Infect Dis J. 2010 Mar;29(3):282-4.
  3. https://wwwnc.cdc.gov/travel/yellowbook/2020/travel-related-infectious-diseases/legionellosis-legionnaires-disease-and-pontiac-fever
  4. https://wwwnc.cdc.gov/travel/yellowbook/2020/travel-related-infectious-diseases/legionellosis-legionnaires-disease-and-pontiac-fever
  5. Eison, R. Legionella Pneumonia: When to Suspect, Diagnostic Considerations, and Treatment Strategies for Hospital-Based Clinicians. Curr Emerg Hosp Med Rep (2014) 2: 205.
  6. https://pedemmorsels.com/legionellosis-in-children/
  7. Cunha BA. Legionnaires' disease: clinical differentiation from typical and other atypical pneumonias. Infect Dis Clin N Am. 2010;24(1):73-105.
  8. https://www.cdc.gov/legionella/downloads/case-report-form.pdf
  9. Burke et Al. Legionnaires Disease Treatment & Management. Aug 18, 2014. http://emedicine.medscape.com/article/220163-treatment#d11
  10. Sanford Guide to Antimicrobial Therapy 2014