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CSTE Position Statement

Strengthening Surveillance for Travel-Associated Legionellosis and Case Definitions for Legionellosis

Submission Date: 6/08/2005
ID: 05-ID-01
Committee: Infectious Disease
Title: Strengthening surveillance for travel-associated legionellosis and revised case definitions for legionellosis
Council of State and Territorial Epidemiologists (CSTE)


Statement of the Problem

Legionellosis is a nationally notifiable disease. Cases are reported electronically through the National Notifiable Disease Surveillance System (NNDSS), which collects basic demographic information, or via a paper-based supplementary case report form which, in addition, collects diagnostic and travel-related information. Since the legionellosis case definition was last revised in 1996, it has become apparent that several changes would improve national legionellosis surveillance. First, increased experience with diagnostic testing has led to a need to update the laboratory criteria for the diagnosis of legionellosis. For example, for the serologic criteria, the 1996 case definition required a 4-fold increase in antibody titer to = 1:128 against Legionella pneumophila serogroup 1. With the availability of newer serologic assays, this requirement means that some cases are not captured by the 1996 definition. Second, outbreaks of travel-associated legionellosis are infrequently identified, even though more than 20% of all cases are thought to be associated with recent travel. In 2004 alone, CDC was contacted regarding over 150 cases of confirmed legionellosis among travelers. Many of these cases occurred among cruise ship passengers or persons staying overnight in large hotels. Like other travel-related infectious diseases, the identification of any given outbreak is hindered by the difficulties inherent in detecting clusters of infections among persons who have recently dispersed from a point source and returned to their home states. Current surveillance for legionellosis lacks the timeliness and sensitivity to detect outbreaks of travel-associated cases. Timely reporting of travel-associated cases with complete travel information could allow early identification and control of known sources of infection.

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Statement of the Desired Action(s) to be Taken

  1. Modify case definitions for all cases of legionellosis (i.e., travel-associated and not travel-associated) from 1996 Nationally Notifiable Diseases to read:

    Clinical description:
    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

    Laboratory criteria for diagnosis:


    • By culture: isolation of any Legionella organism from respiratory secretions, lung tissue, pleural fluid, or other normally sterile fluid.
    • By detection of specific Legionella pneumophila serogroup 1 antigen in urine using validated reagents.
    • By seroconversion: fourfold or greater rise in specific serum antibody titer to Legionella pneumophilia serogroup 1 using validated reagents.


    • By seroconversion: fourfold or greater rise in antibody titer to specific species or serogroups of Legionella other than L. pneumophila serogroup 1 (e.g. L. mcdadei, L. pneumophila serogroup 6).
    • By seroconversion: fourfold or greater rise in antibody titer to multiple species of Legionella using pooled antigen and validated reagents.
    • By the detection of specific Legionella antigen or staining of the organism in respiratory secretions, lung tissue, or pleural fluid by direct fluorescent antibody (DFA) staining, immunohistochemistry (IHC), or other similar method, using validated reagents.

    Case classification:

    • Confirmed: a clinically compatible case that meets at least one of the confirmatory laboratory criteria
      • Travel-associated: a case that has a history of spending at least one night away from home, either in the same country of residence or abroad, in the ten days before onset of illness.
    • Suspect: a clinically compatible case that meets at least one of the suspect laboratory criteria
      • Travel-associated: a case that has a history of spending at least one night away from home, either in the same country of residence or abroad, in the ten days before onset of illness.
  2. Set goals for the timeline for reporting of legionellosis cases:
    • Within 7 days of notification of legionellosis case, the investigating health department will ascertain whether the case-patient spent at least one night away from home in the 10 days before onset of illness.
    • If history of travel is present in the 10 days before onset of illness, the state health department will, within 7 days of the initial notification, report travel destination (city and state or country) and dates of travel to CDC and to the state of travel.
    • If there is no history of travel in the 10 days before onset of illness, the state health department will complete the legionellosis case report and send to CDC within 30 days of notification.
    • If there are epidemiologically linked travel-associated legionellosis cases, CDC will notify within one day and work with state health departments to investigate further.
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Goals of Surveillance

  • Monitor and describe incidence and trends of legionellosis cases
  • Rapidly recognize cases that occur in similar locations or with similar exposures
  • Understand risk factors for infection
  • Identify opportunities for control and prevention


Methods of Surveillance

All confirmed cases (travel-associated and not travel-associated) should be reported. Suspect cases need not be reported, but should be retained by the state health department to assist with investigations of possible outbreaks.


Public Health Impact

Because the majority of cases are now confirmed under the 1996 case definition by urinary antigen and culture, CDC does not expect a substantial change in the number of reported legionellosis cases under the proposed case definition (table). Outbreaks of legionellosis that occur among travelers are difficult to detect. Although the burden is not well defined, more than 20% of legionellosis cases are thought to be associated with recent travel. Strengthening surveillance would facilitate timely identification of epidemiological links, prompt outbreak investigations, and allow early detection of sources of transmission and implementation of control measures. Travel-associated legionellosis is highly preventable; disease prevention would have a significant impact on public health.

Laboratory Test1996 Definition2005 Definition (Proposed)
Culture of respiratory secretions or tissueConfirmedConfirmed
Urinary antigen for L. pneumophila serogroup 1ConfirmedConfirmed
Serology - L. pneumophila serogroup 1 using validated reagentsConfirmed - 4-fold increase to =1:128Confirmed - 4-fold increase
Serology - species-specific antigen other than L. pneumophila serogroup 1Not usedSuspect
Serology - multiple species (pooled antigen)Not usedSuspect
Detection of Legionella antigens or staining of the organismConfirmed - DFA onlySuspect-DFA, IHC, or other similar methods
Validated nucleic acid assayNot usedSuspect
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  • Joseph CA. Legionnaires' disease in Europe 2000-2002. Epidemiol Infect 2004;132:417-24.

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