Disease
L. pneumophila (and other legionellae) causes pneumonia, both in the community and in hospitalized immunocompromised patients. The genus is named after the famous outbreak of pneumonia among people attending the American Legion convention in Philadelphia in 1976 (Legionnaires’ disease).
Important Properties
Legionellae are gram-negative rods that stain faintly with the standard Gram stain. They do, however, have a gram-negative type of cell wall, and increasing the time of the safranin counter stain enhances visibility. Legionellae in lung biopsy sections do not stain by the standard hematoxylin-and-eosin (H&E) procedure; therefore, special methods, such as the Dieterle silver impregnation stain, are used to visualize the organisms.
L. pneumophila causes approximately 90% of pneumonia attributed to legionellae. There are 16 serogroups of L. pneumophila, with most cases caused by serogroup 1 organisms. There are about 30 other Legionella species that cause pneumonia, but most of the remaining 10% of cases are caused by two species, Legionella micdadei and Legionella bozemanii.
Pathogenesis & Epidemiology
Legionellae are associated chiefly with environmental water sources such as air conditioners, hot tubs, and water-cooling towers. Outbreaks of pneumonia in hospitals have been attributed to the presence of the organism in water taps, sinks, and showers.
Legionellae can replicate to large numbers in free-living amebas in these water sources. The amebas also enhance the survival of legionellae. Under adverse environmental conditions, the amebas encyst, ensuring both their own survival and the survival of the intracellular legionellae as well.
The portal of entry is the respiratory tract, and pathologic changes occur primarily in the lung. However, in severe cases, bacteremia occurs, accompanied by damage to the vascular endothelium in multiple organs, especially the brain and kidneys. The major virulence factor of the organism is lipopolysaccharide (endotoxin). No exotoxins are produced.
The typical candidate for Legionnaires’ disease is an older man who smokes and consumes substantial amounts of alcohol. Patients with acquired immunodeficiency syndrome (AIDS), cancer, or transplants (especially renal transplants) or patients being treated with corticosteroids are predisposed to Legionella pneumonia, which indicates that cell-mediated immunity is the most important defense mechanism. Despite airborne transmission of the organism, person-to-person spread does not occur, as shown by the failure of secondary cases to occur in close contacts of patients.
Clinical Findings
The clinical picture can vary from a mild influenza-like illness to a severe pneumonia accompanied by mental confusion, nonbloody diarrhea, proteinuria, and microscopic hematuria. Although cough is a prominent symptom, sputum is frequently scanty and nonpurulent. Hyponatremia (serum sodium ≤130 mEq/L) is an important laboratory finding that occurs more often in Legionella pneumonia than in pneumonia caused by other bacteria. Most cases resolve spontaneously in 7 to 10 days, but in older or immunocompromised patients, the infection can be fatal.
Pontiac fever is a mild, flulike form of Legionella infection that does not result in pneumonia. The name “Pontiac” is derived from the city in Michigan that was the site of an out break in 1968.
Laboratory Diagnosis
Sputum Gram stains reveal many neutrophils but no bacteria. The organism fails to grow on ordinary media such as blood agar, but it will grow on charcoal-yeast agar, a special medium supplemented with iron and cysteine. The preferred diagnostic test is the polymerase chain (PCR) assay because it detects the bacterial nucleic acids and therefore will detect all serogroups of Legionella.
Detection of L. pneumophila antigens in the urine is a rapid means of making a diagnosis. The urinary antigen test is available only for serogroup 1 organisms. If tissue is available, it is possible to demonstrate Legionella antigens in infected lung tis sue by using fluorescent antibody staining. The cold-agglutinin titer does not rise in Legionella pneumonia, in contrast to pneumonia caused by Mycoplasma.
Treatment & Prevention
Azithromycin and levofloxacin are the drugs of choice for this infection. Prevention involves eliminating aerosols from water sources that are colonized with this organism. Once can reduce the incidence of Legionella in hospital water supplies by using high temperatures and hyperchlorination. There is no vaccine.
