History

Legionella pneumophila causes 2 distinct disease entities. Legionnaires disease (LD) is characterized by pneumonia. Pontiac fever is a short-term, milder illness than LD and is not characterized by pneumonia, instead manifesting as fever and myalgias that resolve without treatment. Extra pulmonary legionellosis such as pericarditis, endocarditis and cellulitis although not common, has been reported in both immunocompromised and immunocompetent patients and can occur as isotated cases or in the setting of legionella pneumonia. Usually species other than L pneumophila have been implicated in these cases.^{[11, 12]}

The incubation period in LD ranges from 2-10 days. Patients who develop legionellae infection and who have been hospitalized continuously for 10 or more days before the onset of illness are classified as having definite nosocomial LD. Patients with laboratory-confirmed infection that develops 2-9 days after hospitalization are classified as having possible nosocomial LD. Nosocomial LD occurs in clusters.

Symptoms of LD can occur as follows:

Patients often experience a prodrome of 1-2 days of mild headache and myalgias, followed by high fever, and chills
Cough is present in 90% of cases; cough usually is nonproductive at first but may become productive as the disease progresses
Other pulmonary manifestations include dyspnea, pleuritic chest pain, and hemoptysis, which may be present in as many as one third of cases
Gastrointestinal symptoms include nausea, vomiting, diarrhea (watery, not bloody), abdominal pain, and anorexia
Neurologic symptoms include headache, lethargy, encephalopathy, altered mental status (the most common neurologic symptom), and rarely, focal symptoms
Musculoskeletal symptoms include myalgias
Nonpulmonary symptoms are prominent early in the disease

Physical Examination

Manifestations of LD may include the following:

Headache
Mental status changes
Fever greater than 40°C, or 102°F (range, 38.8-40.5°C)
Relative bradycardia (excluding patients with pacemakers or arrhythmias or those receiving beta blockers, diltiazem, or verapamil)
Myocarditis
Prosthetic valve endocarditis
Tachypnea
Localized rales
Blood-streaked sputum
Extrapulmonary manifestations
Abdominal pain
Pancreatitis
Acute renal failure

A clinical point score may be helpful in increasing the probability of correctly diagnosing LD and prompting specific/definitive LD testing.

Table 1. Legionnaires Disease: Six Clinical Predictors and Diagnostic Eliminators in Adults Admitted with Pneumonia ^a (Open Table in a new window)

Diagnostic Predictors

Diagnostic Eliminators

Clinical Predictors

Fever (>102°F)

Clinical Eliminators

Sore throat
Severe myalgias

Laboratory Predictors ^b

Highly elevated ESR (>90 mm/h) or CRP (>180 mg/L)
Highly elevated ferritin levels (>2 X normal)
Hypophosphatemia (on admission/early)
Highly elevated CPK (>2 X normal)
Microscopic hematuria (on admission)

Laboratory Eliminators

Leukopenia
Thrombocytopenia
Negative chest radiographic findings (no infiltrates)

Legionnaire disease very likely if >3 predictors present

Legionnaires disease very unlikely if < 3 predictors or >3 diagnostic eliminators present

Abbreviations: CPK = creatinine phosphokinase test; CRP = C-reactive protein; ESR = erythrosedimentation rate.

^aPulmonary symptoms: shortness of breath, cough, and so forth with fever and a new focal/segmental infiltrate on chest radiograph.

^bOtherwise unexplained. If finding is due to an existing disorder, it should not be used as a clinical predictor.

Differential Diagnoses

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Media Gallery

This electron micrograph depicts an amoeba, Hartmannella vermiformis (orange), as it entraps a Legionella pneumophila bacterium (green) with an extended pseudopod. After it is ingested, the bacterium can survive as a symbiont within what then becomes its protozoan host. The amoeba then becomes a so-called "Trojan horse," since, by harboring the pathogenic bacterium, the amoeba can afford it protection. In fact, in times of adverse environmental conditions, the amoeba can metamorphose into a cystic stage, enabling it, and its symbiotic resident, to withstand the environmental stress. Image courtesy of the Centers for Disease Control and Prevention and Dr. Barry S Fields.

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Author

Chinelo N Animalu, MD, MPH, FIDSA Associate Professor, Division of Infectious Diseases, Department of Internal Medicine, University of Tennessee Health Science Center College of Medicine; Physician, Department of Infectious Disease, University of Tennessee Methodist Physicians (UTMP), Methodist University Hospital

Chinelo N Animalu, MD, MPH, FIDSA is a member of the following medical societies: American College of Physicians, American Medical Association, Infectious Diseases Society of America, Memphis Medical Society, Tennessee Medical Association

Disclosure: Nothing to disclose.

Specialty Editor Board

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Received salary from Medscape for employment. for: Medscape.

Chief Editor

Michael Stuart Bronze, MD David Ross Boyd Professor and Chairman, Department of Medicine, Stewart G Wolf Endowed Chair in Internal Medicine, Department of Medicine, University of Oklahoma Health Science Center; Master of the American College of Physicians; Fellow, Infectious Diseases Society of America; Fellow of the Royal College of Physicians, London

Michael Stuart Bronze, MD is a member of the following medical societies: Alpha Omega Alpha, American College of Physicians, American Medical Association, Association of Professors of Medicine, Infectious Diseases Society of America, Oklahoma State Medical Association, Southern Society for Clinical Investigation

Disclosure: Nothing to disclose.

Additional Contributors

Frank C Smeeks, III, MD Specialty Medical Director of Emergency Medicine, Applied Medico Legal Solutions

Frank C Smeeks, III, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Emergency Medicine, American College of Emergency Physicians, American Association for Physician Leadership, American Medical Association, North Carolina Medical Society

Disclosure: Nothing to disclose.

Burke A Cunha, MD Professor of Medicine, State University of New York School of Medicine at Stony Brook; Chief, Infectious Disease Division, Winthrop-University Hospital

Burke A Cunha, MD is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, Infectious Diseases Society of America

Disclosure: Nothing to disclose.

Acknowledgements

Joseph F John Jr, MD, FACP, FIDSA, FSHEA Clinical Professor of Medicine, Molecular Genetics and Microbiology, Medical University of South Carolina College of Medicine; Associate Chief of Staff for Education, Ralph H Johnson Veterans Affairs Medical Center