■ Legionella is the only genus in the family Legionellaceae. The species Legionella pneumophila is responsible for most legionelloses in humans. Legionellae are difficult to stain. They are Gram-negative, aerobic rod bacteria. Special mediums must be used to grow them in cultures. Infections with Legionella occur when droplets containing the pathogens are inhaled. Two clinically distinct forms are on record: legionnaire's disease leading to a multifocal pneumonia and nonpneumonic legionellosis or Pontiac fever. The persons most likely to contract legionnaire's disease are those with a primary cardiopulmonary disease and generally weakened immune defenses. Laboratory diagnostic methods include microscopy with direct immuno-fluorescence, culturing on special mediums and antibody assays. The antibiotics of choice are the macrolides. The natural habitat of legionellae is damp biotopes. The sources of infection listed in the literature include hot and cold water supply systems, cooling towers, moisturizing units in air conditioners, and whirlpool baths. Legionelloses can occur both sporadically and in epidemics. ■
Classification. Legionella bacteria were discovered in 1976, occasioned by an epidemic among those attending a conference of American Legionnaires (former professional soldiers). They are now classified in the family Legionella-ceae, which to date comprises only the genus Legionella. This genus contains numerous species not listed here. Most human infections are caused by L. pneumophila, which species is subdivided into 12 serogroups. Human infections are caused mainly by serogroup 1.
Morphology and culture. L. pneumophila is a rod bacterium 0.3-1 im wide and 2-20 im long. Its cell wall structure is of the Gram-negative type, but gram staining hardly "takes" with these bacteria at all. They can be rendered visible by means of direct immunofluorescence.
Legionella grow only on special mediums in an atmosphere containing 5% CO2.
Pathogenesis and clinical picture. The pathomechanisms employed by legio-nellae are not yet fully clarified. These organisms are facultative intracellular bacteria that can survive in professional phagocytes and in alveolar macrophages. They are capable of preventing the phagosome from fusing with ly-sosomes. They also produce a toxin that blocks the oxidative burst. Two clinical forms of legionellosis have been described:
■ Legionnaire's disease. Infection results from inhalation of droplets containing the pathogens. The incubation period is two to 10 days. The clinical picture is characterized by a multifocal, sometimes necrotizing pneumonia. Occurrence is more likely in patients with cardiopulmonary primary diseases or other immunocompromising conditions. Lethality >20%.
■ Pontiac fever. Named after an epidemic in Michigan. Incubation period one to two days. Nonpneumonic, febrile infection. Self-limiting. Rare.
Diagnosis. Specific antibodies marked with fluorescein are used to detect the pathogens in material from the lower respiratory tract. For cultures, special culture mediums must be used containing selective supplements to exclude contaminants. The mediums must be incubated for three to five days. Legionella antigen can be identified in urine with an EIA. A gene probe can also be used for direct detection of the nucleic acid (rDNA) specific to the genus Legionella in the material. Antibodies can be assessed using the indirect immu-nofluorescence technique.
Therapy. Macrolide antibiotics are now the agent of choice, having demonstrated clinical efficacy. Alternatively, 4-quinolones can be used.
Epidemiology and prevention. Legionellosis can occur in epidemic form or in sporadic infections. It is estimated that one third of all pneumonias requiring hospitalization are legionelloses. Soil and damp biotopes are the natural habitat of Legionella. Sources of infection include hot and cold water supply systems, cooling towers, air moisturizing units in air conditioners, and whirlpool baths. Human-to-human transmission has not been confirmed. Legionella bacteria tolerate water temperatures as high as 50 °C and are not killed until the water is briefly heated to 70 °C.
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