eMedicine Specialties > Infectious Diseases > Lower Respiratory Tract Infections

Legionnaires Disease

Author: 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
Coauthor(s): Lynn E Sullivan, MD, Assistant Professor of Medicine, Yale University School of Medicine
Contributor Information and Disclosures

Updated: Feb 5, 2008

Introduction

Background

Legionella pneumophila is an important cause of both nosocomial and community-acquired pneumonia (CAP) and must be considered a possible causative pathogen in any patient who presents with pneumonia.

The Legionella bacterium was first identified in the summer of 1976 during the 58th annual convention of the American Legion, which was held at the Bellevue-Stratford Hotel in Philadelphia. Infection was presumed to be spread by contamination of the water in the hotel's air conditioning system. The presentation ranged from mild flulike symptoms to multisystem organ failure. Of the 182 people infected, 29 died. A bacterium that would later be named L pneumophila was isolated from different organ tissues of guinea pigs inoculated with lung tissue samples from 4 individuals who died. Although this pathogen was not identified until 1976, retrospective analysis suggests that L pneumophila may have been responsible for previous pneumonia epidemics in Philadelphia; Washington, DC; and Minnesota. L pneumophila was identified in a clinical specimen dating to 1943.

Legionellosis is the term that collectively describes infections caused by members of the Legionellaceae family. Legionnaires disease (LD) is the pneumonia caused by L pneumophila. LD also refers to a more benign, self-limited, acute febrile illness known as Pontiac fever, which has been linked serologically to L pneumophila, although it presents without pneumonia.

Pathophysiology

The Legionella bacterium is a small, aerobic, waterborne, gram-negative, unencapsulated bacillus that is nonmotile, catalase-positive, and weakly oxidase-positive. Legionella is a fastidious organism and does not grow anaerobically or on standard media. Buffered charcoal yeast extract (CYE) agar is the primary medium used for isolation of the bacterium.

The Legionellaceae family consists of more than 42 species constituting 64 serogroups. L pneumophila is the most common species, causing up to 90% of the cases of legionellosis, followed by Legionella micdadei (otherwise known as the Pittsburgh pneumonia agent), Legionella bozemanii, Legionella dumoffii, and Legionella longbeachae. Fifteen serogroups of L pneumophila have been identified, with serogroups 1, 4, and 6 being the primary causes of human disease. Serogroup 1 is thought to be responsible for 80% of the reported cases of legionellosis caused by L pneumophila.

Legionella species are obligate or facultative intracellular parasites. Water is the major environmental reservoir for Legionella. The bacterium can infect and replicate within protozoa such as Acanthamoeba and Hartmannella species, which are free-living amoebae found in both natural and manufactured water systems. The Legionella species within the amebic cells can avoid the endosomal-lysosomal pathway and can replicate within the phagosome. Legionella can survive and grow in the amebic cells, thereby enabling the organism to persist in nature.

Legionella species infect human macrophages and monocytes, and intracellular replication of the bacterium is observed within these cells in the alveoli. The intracellular infections of protozoa and macrophages have many similarities.

Transmission is thought to occur via inhalation of aerosolized mist from water sources (eg, whirlpools, showers, cooling towers¹ ) contaminated with either the bacterium or amebic cells infected with the bacterium. Direct inhalation is the most likely method of transmission, with aerosol-generating systems playing a crucial role.² Person-to-person transmission has not been documented. The highest incidence occurs during the warmer months, when air-conditioning systems are used more frequently. Nosocomial acquisition likely occurs via aspiration³ , respiratory therapy equipment² , or contaminated water. In addition, transmission has been linked to the use of humidifiers, nebulizers, and items that were rinsed with contaminated tap water.

The following features increase the likelihood of colonization and amplification of legionellae in man-made water environments: (1) temperature of 25-42°C, (2) stagnation, (3) scale and sediment, and (4) presence of certain free-living aquatic amoebae capable of supporting intracellular growth of legionellae. Legionellae can resist low levels of chlorine used in water distribution systems.

Activated T cells produce lymphokines that stimulate increased antimicrobial activity of macrophages. This cell-mediated activation is key to halting the intracellular growth of legionellae. The significant role of cellular immunity explains why legionellae are observed more frequently in immunocompromised patients. Humoral immunity is thought to play a secondary role in the host response to legionellae infection.

Frequency

United States

LD has a reported incidence of 8000-18,000 cases per year. In certain geographic areas, community-acquired LD is more common.

LD is reportable in all 50 states. Only 5-10% of cases are estimated to be reported. While most cases of LD are sporadic, 10-20% are linked to outbreaks. LD is among the top 3-4 microbial causes of CAP, constituting approximately 1-9% of patients with CAP who require hospitalization. LD is an even more common cause of severe pneumonia in patients who require admission to an intensive care unit (ICU). LD ranks second, after pneumococcal pneumonia, as the etiology of pneumonia severe enough to require ICU admission.

Some LD cases are acquired in the hospital; they usually occur as outbreaks. Legionellae in the hospital setting is usually due to its presence in water sources and on surfaces (eg, pipes, rubber, plastics). The organism is also found in water sediment, which may explain its ability to persist despite flushing of hospital water systems.

International

LD is thought to occur worldwide and to be the cause of 2-15% of all CAP cases that require hospitalization.

Mortality/Morbidity

The mortality rate may approach 100% in patients with underlying disease. In untreated patients, the mortality rate may be as high as 80%.

Sex

Men have a greater risk of acquiring L pneumophila infection.

Age

Elderly persons have a greater risk of acquiring infection with Legionella species.

Clinical

History

L pneumophila causes 2 distinct disease entities. Legionnaires disease (LD) is characterized by pneumonia. Pontiac fever is a milder illness than LD and is not characterized by pneumonia; Pontiac fever manifests as fever and myalgias that resolve without treatment.

• Legionnaires disease
  • The incubation period 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.
  • Individuals with LD can present with a broad spectrum of symptoms.
• Symptoms of legionnaires disease
  • Fever greater than 40 ^º C (>102 º F)
  • Chills
  • Cough - Dry or productive; hemoptysis rare
  • Pleuritic or nonpleuritic chest pain
  • Neurologic symptoms
    • Headache
    • Lethargy
    • Encephalopathy
    • Mental status changes - The most common neurologic symptom
  • GI symptoms
    • Diarrhea - Watery, not bloody
    • Nausea, vomiting, and abdominal pain
  • Myalgias

Physical

• Manifestations of LD may include the following:
  • Mental status changes
  • Fever greater than 40°C (range, 38.8-40.5°C)
  • Hypotension
  • Relative bradycardia in all (excluding patients with pacemakers or arrhythmias or those receiving beta-blockers, diltiazem, or verapamil)
  • Tachypnea
  • Localized rales
  • Depressed mental status or agitation
• Extrapulmonary manifestations
  • In addition to relative bradycardia, cardiac manifestations are common findings and include myocarditis, pericarditis, and prosthetic valve endocarditis.
  • Pancreatitis
  • Peritonitis
  • Acute renal failure

Modified Winthrop-University Hospital Infection Disease Division's Point System for Diagnosing Legionnaires Disease in Adults

Open table in new window

*Otherwise unexplained (acute and associated with pneumonia)

Adapted from Cunha BA. Antibiotic Essentials. 5^th ed. Royal Oak, Mich: Physicians Press; 2006.

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

Causes

The risk of infection increases with the type and intensity of the exposure, as well as the health status of the exposed individual. Numerous factors increase the risk of acquiring legionellae infections.

• Risk factors for infection
  • Advanced age
  • Smoking
  • Chronic heart or lung disease
  • Immunocompromised state or immunosuppressive medication use (especially corticosteroids)
  • Recent exposure to water or soil
• Pediatric cases of Legionella pneumonia are less common. Most of these cases occur in children who are immunosuppressed or in immunocompetent children who have undergone surgery or who are on a ventilator.

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