Legionellosis overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Legionellosis is an infectious disease caused by bacteria belonging to the genus Legionella.[1] Over 90% of legionellosis cases are caused by Legionella pneumophila, a ubiquitous aquatic organism that thrives in warm environments (25 to 45 °C with an optimum around 35 °C).
Historical Perspective
Legionnaires' disease acquired its name in 1976 when an outbreak of pneumonia occurred among people attending a convention of the American Legion in Philadelphia. On January 18, 1977 the causative agent was identified as a previously unknown bacterium, subsequently named Legionella. Outbreaks of Legionnelosis are listed below in chronological order.
Classification
Legionellosis may be classified into three types based upon clinical presentations or affected organ systems: pulmonary infection (Legionnaires' disease), extrapulmonary infection, and Pontiac fever.
Pathophysiology
Legionellosis is acquired by inhalation and micro aspiration of Legionella into the lungs. Possible virulence factors include aerosol stability, ability to grow within macrophages, possession of eukaryotic gene homologues, and surface hydrophobicity.
Causes
Legionella pneumophila is a thin, pleomorphic, flagellated Gram-negative bacterium of the genus Legionella. L. pneumophila is the primary human pathogen in this group and is the causative agent of legionellosis or Legionnaires' disease.
Differentiating Legionellosis from other Diseases
Legionellosis should be differentiated from other types of atypical pneumonia.
Epidemiology and Demographics
Legionellosis is ubiquitous worldwide, is generally caused by Legionella pneumophila, tends to affect people over the age of 65 who are smokers with lung disease, and occurs after exposure to aquatic settings that promote bacterial growth where three conditions are met:
- The aquatic environment is somewhat stagnant (e.g. water towers)
- The water is warm (77°F–108°F [25°C–42°C]) (e.g. hot tubs), and
- The water must be aerosolized so that the bacteria can be inhaled into the lungs.
These 3 conditions are met almost exclusively in developed or industrialized settings. Legionellosis does not occur in association with natural settings such as waterfalls, lakes, or streams.
Risk Factors
Common risk factors for Legionellosis include old age (usually 65 years of age or older), smokers, chronic lung disease, alcoholism, diabetes, and renal failure.
Natural History, Complications and Prognosis
The fatality rate of Legionnaires' disease has ranged from 5 to 30% during various outbreaks. It should be noted that some people can be infected with the Legionella bacterium and have only mild symptoms or no illness at all.
Diagnosis
History and Symptoms
Legionnaires’ disease typically presents with pneumonia, which usually requires hospitalization and can be fatal in 10%–15% of cases. Symptom onset occurs 2–14 days after exposure. In outbreak settings, <5% of people exposed to the source of the outbreak develop Legionnaires’ disease.
Pontiac fever is milder than Legionnaires’ disease and presents as an influenza like illness, with fever, headache, and myalgias, but no signs of pneumonia. Pontiac fever can affect healthy people, as well as those with underlying illnesses, and symptoms occur within 72 hours of exposure. Most patients fully recover. Up to 95% of people exposed in outbreak settings can develop symptoms of Pontiac fever.
Physical Examination
Possible physical findings include elevated body temperature, blood pressure, hypotension, tachypnea, reduced breath sounds, rales, weakness, confusion, and ataxia.
Laboratory Findings
Laboratory findings of Legionellosis include lymphocytopenia, positive urinary antigen test, increased antibody titers, hyponatremia, hyperbilirubinemia, and proteinuria.
Chest X ray
Chest X-ray findings in Legionellosis include consolidation, pleural effusion, cavitation, bullous emphysema, and interstitial infiltrates.
CT
Chest CT findings in Legionellosis include bilateral, multiple affected segments and peripheral lung consolidation with ground glass opacity.
Other Diagnostic Studies
Other diagnostic studies for Legionellosis include PCR, bronchoalveolar lavage, and thoracocentesis.
Treatment
Medical Therapy
Primary Prevention
Secondary Prevention
Cost-Effectiveness of Therapy
Future or Investigational Therapies
Sources
- CDC Legionellosis [2]
References
- ↑ Ryan KJ, Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed. ed.). McGraw Hill. ISBN 0838585299.