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Disease Name: Legionnaires' disease

Quick Links

Please review the Idaho Reportable Disease Rules (IDAPA 16.02.10) for the most up-to-date information.

Legionella: http://www.cdc.gov/legionella/index.html

Overview / Case Definition

Legionellosis is associated with two clinically and epidemiologically distinct illnesses: Legionnaires’ disease, which is characterized by fever, myalgia, cough, and clinical or radiographic pneumonia; and Pontiac fever, a milder illness without pneumonia.

Causative agents and clinical features

Legionella spp. Legionella are atypical gram-negative bacilli that are intracellular parasites of free-living protozoa primarily found in fresh water. There are at least 60 different species of Legionella; most are considered pathogenic. The majority of reported cases are caused by the Legionella pneumophila, serogroup 1. Fourteen other L. pneumophila serogroups are known.

Pneumonic (Legionnaires’ disease)

Legionella spp. invade and grow within alveolar macrophages, mistaking them for their natural host. Clinical features are non-specific. Anorexia, malaise, myalgia, fever, loss of strength. Abdominal pain, nausea and vomiting (10%–30%), and diarrhea (25%–50%) may be seen. Pneumonia, non-productive cough, fever, chest pain. Radiographic findings variable, patchy or focal areas or bilateral consolidation. CNS manifestations such as confusion and delirium (50%), renal failure. Attack proportion <5%, hospitalization common, case-fatality 5%–40%.

Non-pneumonic (Pontiac fever)

Anorexia, malaise, myalgia, fever, loss of strength, joint pain. Diarrhea. Nausea and vomiting in a small proportion of people. Cough may or may not be present, pneumonia absent. Attack proportion >90%, hospitalization uncommon, case-fatality 0%. Patients typically recover without intervention. May represent a reaction to inhaled antigen rather than bacterial invasion.

Epidemiologic risk factors for legionellosis include recent travel with an overnight stay outside of the home, including stay in a healthcare facility, exposure to hot tubs, recent repairs or maintenance work on domestic plumbing, increasing age (>50 yrs), cigarette smoking (current or historical), diabetes mellitus, chronic lung disease, renal or hepatic failure, systemic malignancy, and compromised immunity, particularly patients who are receiving corticosteroids or who have had an organ transplant. Some studies have found that driving vehicles and not adding windshield washer to windshield wiper fluid are risk factors for legionellosis.




Within 3 working days

Reportable by Healthcare and Labs:

Reportable by Food Service Facility:

Suspect Reportable:

Reporting Timeframe: Within 3 working days

Diagnosis / Testing

Diagnostic Tests for Legionellosis

                                          Sensitivity    Specificity                                                            

                                               (%)                (%)          Advantages                              Disadvantages




• Clinical and environmental isolates can be compared

• Detects all species and serogroups

• Technically difficult

• Slow (>5 days to grow)

• Sensitivity highly dependent on technical skill

• May be affected by antibiotic treatment

Urinary antigen for L. pneumophila serogroup 1 (Lp1)



• Rapid (same day)

• Can only be used to identify L. pneumophila serogroup 1 (LP1)

• Does not allow for molecular comparison to environmental isolates

Paired serology



• Less affected by antibiotic treatment

• Must have paired sera

• ~5%–10% of population has titer 1:≥256

Direct Fluorescent Antibody Stain



• Can be performed on pathologic specimens

• Technically difficult

Polymerase Chain Reaction



• Can be performed on pathologic specimens

• Rapid

• Assays vary by laboratory and are not FDA-approved


Environmental assessment to identify sources of infection will be instituted on a case-by case basis.

Patients who indicate recent travel should be reported as soon as possible.  Investigation of the case will be referred to the appropriate jurisdiction.

Additional Information

Legionella: http://www.cdc.gov/legionella/index.html

Click to Call South Central Public Health District

Click to Call the Idaho State Epidemiologist

Click to Call Idaho State Communications