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Top 10 things clinicians need to know - legionellosis.

Top 10 Things Every Clinician Needs to Know About Legionellosis

#1 – Diagnosing Legionellosis

 Legionnaires' diseasePontiac fever
Clinical featuresPneumonia, cough, feverFlu-like illness (fever, chills, malaise) without pneumonia
Radiographic pneumoniaYesNo
Incubation period2-14 days after exposure24-72 hours after exposure
Etiologic agentLegionella speciesLegionella species
Attack rate[1]< 5%> 90%
Isolation of organismPossibleNever
OutcomeHospitalization common
Case-fatality rate: 5-30%[2]
Hospitalization uncommon
Case-fatality rate: 0%

#2 – Who to Test for Legionnaires' Disease

View guidelines from IDSA

  • Patients who have failed outpatient antibiotic therapy
  • Patients with severe pneumonia, in particular those requiring intensive care
  • Immunocompromised host with pneumonia
  • Patients with pneumonia in the setting of a legionellosis outbreak
  • Patients with a travel history [Patients that have traveled away from their home within two weeks before the onset of illness.]
  • Patients suspected of healthcare-associated pneumonia

#3 – How to Test for Legionnaires' Disease

  • Urinary antigen assay AND culture of respiratory secretions on selective media are the preferred diagnostic tests for Legionnaires' disease
  • Sensitivity varies depending on the quality and timing of specimen collection as well as technical skill of the laboratory performing the test

Sensitivity and specificity of diagnostic tests

TestSensitivity (%)Specificity (%)
Urine antigen70-100100
Paired serology80-90>99
Direct fluorescent antibody stain25-75≥95

See Diagnostic Testing for more information.

#4 – Importance of Obtaining a Respiratory Specimen for Culture of Possible Legionella Infection

Isolation of Legionella from respiratory secretions, lung tissue, pleural fluid, or a normally sterile site is still an important method for diagnosis, despite the convenience and specificity of urinary antigen testing. Investigations of outbreaks of Legionnaires' disease rely on both clinical and environmental isolates. Clinical and environmental isolates can be compared using monoclonal antibody and molecular techniques. Because Legionella are commonly found in the environment, clinical isolates are necessary to interpret the findings of an environmental investigation.

#5 – Preferred Treatment for Legionnaires' Disease

If your patient has Legionnaires' disease, please see the most recent guidelines from IDSA for treatment of community-acquired pneumonia.

If your patient has Pontiac fever, antibiotic therapy should not be prescribed. It is a self-limited illness that does not benefit from antibiotic treatment. Complete recovery usually occurs within 1 week.

#6 – Ask Patients about Travel in the 14 Days before Onset of Disease

More than 20% of all cases are thought to be associated with recent travel. Outbreaks of Legionnaires' disease among travelers can be difficult to detect because of the low attack rate, long incubation period, and the dispersal of persons from the source of the outbreak. Timely reporting of travel-associated cases could allow early identification and control of known sources of infection.

#7 – Frequency of Legionnaires' Disease

Each year an estimated 8,000-18,000 hospitalized cases occur in the U.S. However, accurate data reflecting the true incidence of disease are not available because of under-utilization of diagnostic testing and under-reporting. It is a common cause of severe pneumonia requiring hospitalization. The majority of reported cases are sporadic. Travel-associated outbreaks, outbreaks in community settings, and healthcare and occupational outbreaks are common.

#8 – Sources of Legionella

Legionella can be found in natural, freshwater environments, but they are present in insufficient numbers to cause disease. Potable (drinking) water systems, whirlpool spas, and cooling towers provide the 3 conditions needed for Legionella transmission-heat, stasis, and aerosolization; therefore, these are common sources of outbreaks.

#9 – Epidemiologic Risk Factors for Legionellosis

  • Recent travel with an overnight stay outside of the home
  • Exposure to whirlpool spas
  • Recent repairs or maintenance work on domestic plumbing
  • Renal or hepatic failure
  • Diabetes
  • Systemic malignancy
  • Smoking
  • Immune system disorders
  • Age > 50 years

See also People at Risk

#10 – How to Report Legionellosis

Legionellosis is a nationally notifiable disease.

  • Report cases of legionellosis to your local or state health department.
    1. Find your state health department.
    2. Submit case report form [2 pages]
  • Call, fax, or mail this information to your local or state health department within 7 days of diagnosis. Prompt reporting could allow early identification and control of known sources of infection.



  1. Percent of persons who, when exposed to the source of an outbreak, become ill.
  2. Percent of persons who die from Legionnaires' disease or Pontiac fever.
  3. A single antibody titer of any level is not diagnostic of legionellosis.
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