Legionellosis overview

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Epidemiology and Demographics
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Natural History, Complications and Prognosis
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History and Symptoms
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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. 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:

1. The aquatic environment is somewhat stagnant (e.g. water towers)
2. The water is warm (77°F–108°F [25°C–42°C]) (e.g. hot tubs), and
3. 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

Most cases of Legionella can be treated successfully with antibiotics. The most effective antibiotics are those that have excellent intracellular penetration where Legionella resides. The preferred regimen for non-immunocompromised patients with Legionellosis is either Azithromycin 500 mg PO qd for 3-5 days or Levofloxacin 500 mg PO qd for 7-10 days. Patients who are severely ill or immunocompromised may required intervenous regimens with the preferred regimens being Azithromycin 500 mg PO/IV q24h for 5-7 days OR Levofloxacin 500 mg PO/IV q24h for 7-10 days (or 750 mg PO/IV q24h for 5-7 days). Early initiation of antibiotics is associated with lower mortality (<5%). Pontiac fever requires no specific antibiotic treatment, and the treatment is supportive.

Primary Prevention

There is no vaccine for legionellosis, and antibiotic prophylaxis is not effective. Travelers at increased risk for infection, such as the elderly or those with immunocompromising conditions such as cancer or diabetes, may choose to avoid high-risk areas, such as whirlpool spas. If exposure cannot be avoided, travelers should be advised to seek medical attention promptly if they develop symptoms of Legionnaires’ disease or Pontiac fever.

Sources

• CDC Legionellosis [2]