Legionellosis is associated with three clinically and epidemiologically distinct illnesses: Legionnaires’ disease, Pontiac fever, or extrapulmonary legionellosis.
Legionnaires’ disease (LD): LD presents as pneumonia, diagnosed clinically and/or radiographically. Evidence of clinically compatible disease can be determined several ways: a) a clinical or radiographic diagnosis of pneumonia in the medical record OR b) if “pneumonia” is not recorded explicitly, a description of clinical symptoms that are consistent with a diagnosis of pneumonia1.
Pontiac fever (PF): PF is a milder illness. While symptoms of PF2 could appear similar to those described for LD, there are distinguishing clinical features. PF does not present as pneumonia. It is less severe than LD, rarely requiring hospitalization. PF is self-limited, meaning it resolves without antibiotic treatment.
Extrapulmonary legionellosis (XPL): Legionella can cause disease at sites outside the lungs (for example, associated with endocarditis, wound infection, joint infection, graft infection). A diagnosis of extrapulmonary legionellosis is made when there is clinical evidence of disease at an extrapulmonary site and diagnostic testing indicates evidence of Legionella at that site.
Confirmatory laboratory evidence:
Presumptive laboratory evidence:
None required for case classification
Supportive laboratory evidence:
1 Clinical symptoms of pneumonia may vary but must include acute onset of lower respiratory illness with fever and/or cough. Additional symptoms could include myalgia, shortness of breath, headache, malaise, chest discomfort, confusion, nausea, diarrhea, or abdominal pain.
2 Clinical symptoms may vary but must include acute symptom onset of one or more of the following: fever, chills, myalgia, malaise, headaches, fatigue, nausea and/or vomiting.
Confirmed Legionnaires’ disease (LD): A clinically compatible case of LD with confirmatory laboratory evidence for Legionella.
Probable Legionnaires’ disease (LD): A clinically compatible case with an epidemiologic link during the 14 days before onset of symptoms.
Suspect Legionnaires’ disease (LD): A clinically compatible case of LD with supportive laboratory evidence for Legionella.
Confirmed Pontiac fever (PF): A clinically compatible case of PF with confirmatory laboratory evidence for Legionella.
Probable Pontiac fever (PF): A clinically compatible case with an epidemiologic link during the 3 days before onset of symptoms.
Suspect Pontiac fever (PF): A clinically compatible case of PF with supportive laboratory evidence for Legionella.
Confirmed Extrapulmonary legionellosis (XPL): A clinically compatible case of XPL with confirmatory laboratory evidence of Legionella at an extrapulmonary site.
Suspect Extrapulmonary legionellosis (XPL): A clinically compatible case of XPL with supportive laboratory evidence of Legionella at an extrapulmonary site.
Criteria to distinguish a new case of this disease or condition from reports or notifications which should not be enumerated as a new case for surveillance.
An individual should be considered a new case if their previous illness was followed by a period of recovery prior to acute onset of clinically compatible symptoms and subsequent laboratory evidence of infection. The recovery period for legionellosis can vary based on patient-specific factors. CDC consultation is encouraged for case classification of individuals without clear periods of recovery or subsequent acute illness onset.
S = This criterion alone is SUFFICIENT to classify a case.
N = All “N” criteria in the same column are NECESSARY to classify a case. A number following an “N” indicates that this criterion is only required for a specific disease/condition subtype (see below). If the absence of a criterion (i.e., criterion NOT present) is required for the case to meet the classification criteria, list the absence of criterion as a necessary component.
O = At least one of these “O” (ONE OR MORE) criteria in each category (categories=clinical evidence, laboratory evidence, and epidemiologic evidence) in the same column—in conjunction with all “N” criteria in the same column—is required to classify a case. A number following an “O” indicates that this criterion is only required for a specific disease/condition subtype.
| Criterion | Legionnaires’ Disease | Extrapulmonary Legionellosis | Pontiac Fever | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Suspect | Probable | Confirmed | Suspect | Confirmed | Suspect | Probable | Confirmed | ||||
| Clinical Evidence: | |||||||||||
| Patient presents with radiographic or clinical pneumonia3 | O | O | O | ||||||||
| Patient presents with symptoms of lower respiratory illness | O | O | O | ||||||||
| Patient presents with symptoms of acute illness4 | N | N | N | ||||||||
| Diagnostic testing reveals evidence of Legionella from an extrapulmonary site of disease | N | N | |||||||||
| Laboratory Evidence: | |||||||||||
| Isolation of any Legionella organism from lower respiratory secretions, lung tissue, or pleural fluid | O | O | |||||||||
| Isolation of any Legionella organism from any extrapulmonary site associated with clinical disease | O | ||||||||||
| Detection of any Legionella species from lower respiratory secretions, lung tissue, or pleural fluid by a validated nucleic acid amplification test | O | O | |||||||||
| Detection of any Legionella species from any extrapulmonary site associated with clinical disease by a validated nucleic acid amplification test | O | ||||||||||
| Detection of Legionella pneumophila serogroup 1 antigen in urine using validated reagents | O | O | |||||||||
| Fourfold or greater rise in specific serum antibody titer to Legionella pneumophila serogroup 1 using validated reagents | O | O | |||||||||
| Fourfold or greater rise in antibody titer to specific species or serogroups of Legionella other than L. pneumophila serogroup 1 (e.g., L. micdadei, L. pneumophila serogroup 6) | O | O | |||||||||
| Fourfold or greater rise in antibody titer to multiple species of Legionella using pooled antigens | O | O | |||||||||
| Detection of specific Legionella antigen or staining of the organism in lower respiratory secretions, lung tissue, or pleural fluid by direct fluorescent antibody (DFA) staining, immunohistochemistry (IHC), or other similar method, using validated reagents | O | O | |||||||||
| Detection of specific Legionella antigen or staining of the organism from any extrapulmonary site associated with clinical disease by direct fluorescent antibody (DFA) staining, immunohistochemistry (IHC), or other similar method, using validated reagents | O | ||||||||||
| Other Records Evidence: | |||||||||||
| Healthcare record contains diagnosis of Pontiac fever | S | O | |||||||||
| Healthcare record contains a diagnosis of extrapulmonary legionellosis | S | O | |||||||||
| Healthcare record contains diagnosis of Legionnaires’ disease | S | O | |||||||||
| A clinically compatible case with an epidemiologic link to a setting with either a confirmed source of Legionella or a suspected source of Legionella associated with at least one confirmed case | N | N | |||||||||
| Epidemiologic link occurred during the 14 days before onset of symptoms | N | ||||||||||
| Epidemiologic link occurred during the 3 days before onset of symptoms | N | ||||||||||
3 Clinical symptoms of pneumonia may vary but must include acute onset of lower respiratory illness with fever and/or cough. Additional symptoms could include, myalgia, shortness of breath, headache, malaise, chest discomfort, confusion, nausea, diarrhea, or abdominal pain.
4 Clinical symptoms may vary but must include acute symptom onset of one or more of the following: fever, chills, myalgia, malaise, headaches, fatigue, nausea and/or vomiting.
Appendix 1. Note: Appendices were developed as tools for health department use, but they are not binding and are independent from the case definition and case classifications.
The incubation period for legionellosis should inform the collection of data related to possible sources of exposure. We recommend that public health practitioners conduct case investigations and gather potential exposure sources across the time frames below based on incubation period data. Use of consistent time frames for case investigations facilitates cross-jurisdiction notification which is critical for travel- and healthcare-associated cluster detection. Because the incubation periods for Legionnaires’ disease and Pontiac fever are different, we recommend that exposure history collection time frames differ accordingly.
We recommend that exposure history data be collected for the 14 days prior to illness onset. This recommendation is based on observations that most cases have illness onset within 10 days of exposure, but up to 16% of cases have onset more than 10 days after exposure. 99% of cases have illness onset within 14 days of exposure. Use of a 14-day exposure history period will better enable cluster detection than use of a ten-day exposure history period.
We recommend that exposure history data be collected for the 3 days prior to illness onset. This recommendation is based on observations that most cases have onset within 2 days of exposure and all cases had onset with 3 days with only one exception noted in the literature.
Appendix 2. Note: Appendices were developed as tools for health department use, but they are not binding and are independent from the case definition and case classifications.
Cases of legionellosis may be associated with travel on a cruise ship or staying overnight in a hotel or other public accommodation. Like other travel-related infectious diseases, the identification of any given outbreak is hindered by the difficulties inherent in detecting clusters of disease among persons who have recently dispersed from a point source and returned to their home states. Outbreaks can occur in many settings but are often reported in association with travel exposures.
Timely reporting of travel-associated cases with complete travel information aids early identification and control of sources of infection.
During an outbreak these definitions may be modified.
Public health response to cases, including defining an outbreak or decisions regarding environmental investigation, will be based on the local or state jurisdiction’s assessment of the Legionella exposure risk at the identified accommodation(s) and evidence of epidemiologic links.
Standardized reporting definitions5 for travel-associated legionellosis:
Travel-associated Legionnaires’ disease: A case of Legionnaires’ disease in a patient who has a history of spending at least one night away from home (excluding healthcare settings) in the 14 days before onset of illness.
Travel-associated Pontiac fever: A case of Pontiac fever in a patient who has a history of spending at least one night away from home (excluding healthcare settings) in the 3 days before onset of illness.
The following goals for timely reporting of legionellosis cases are recommended:
5 These definitions apply to both confirmed and suspected cases
Appendix 3. Note: Appendices were developed as tools for health department use, but they are not binding and are independent from the case definition and case classifications.
Cases and outbreaks in healthcare settings may lead to investigation and preventive intervention. Thus, there is a need for a standardized definition to support identification and reporting of healthcare-associated Legionnaires’ disease.
Outbreaks can occur in many settings, but are most frequently reported in association with travel and healthcare exposure.
During an outbreak these definitions may be modified.
Public health response to cases, including defining an outbreak or decisions regarding an environmental investigation, will be based on the local or state jurisdiction’s assessment of the Legionella exposure risk at the identified facility/facilities and evidence of epidemiologic links.
Standardized reporting definitions6 for healthcare-associated Legionnaires’ disease.
Presumptive healthcare-associated Legionnaires’ disease: A case with ≥10 days7 of continuous stay at a healthcare facility8 during the 14 days before onset of symptoms.
Possible healthcare-associated Legionnaires’ disease: A case that spent a portion of the 14 days before date of symptom onset in one or more a healthcare facilities, but does not meet the criteria for presumptive HA-LD.
6 These definitions apply to both confirmed and suspected cases and to cases with multiple facility stays.
7 The majority of Legionnaires’ disease cases have illness onset within 10 days of exposure; for healthcare-associated case surveillance purposes, the goal is to capture the most likely exposure source.
8 Examples of healthcare facilities include acute care facilities, long term acute care facilities, skilled nursing facilities, and clinics
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