Etiology
Legionella species are gram-negative, intracellular, aerobic coccobacilli of the Legionellales order. They are most closely related to Coxiella burnetii, the agent of Q fever. Legionella bacteria have fastidious growth requirements and use proteins instead of carbohydrates as an energy source.
Among the 56 different Legionella species, 20 have been reported to infect humans.[13][14]Legionellapneumophila is the most common species to cause infection, specifically serogroup 1, and less commonly, serogroups 4 and 6. Infections with alternative Legionella species (L micdadei, L dumoffii, L bozemanii) are more common in patients with advanced immunosuppression, usually in the hospital setting, where microaspiration of contaminated potable water has been the predominant source.[15]
Legionella species are ubiquitous in aqueous environments such as lakes and streams, but quantities of bacteria in these environments are insufficient to cause disease.[10] Factors associated with bacterial amplification include warm water temperatures of 77°F (25°C) to 108°F (42.2°C); water stagnation; presence of scale, sediment and biofilm in pipes and fixtures; and absence of disinfectant.[10] Nearly all cases of community-acquired Legionnaires' disease are associated with contaminated aerosols produced by man-made water systems such as cooling towers, warm-water plumbing systems, and recirculating water systems.[13]
Pathophysiology
Legionnaires' disease is acquired by inhalation and, to a lesser extent, microaspiration of contaminated water. Once in the lung, the organisms are phagocytosed by alveolar macrophages, where they release virulent factors that allow them to survive and replicate. Intracellular growth is central to pathogenesis. Up to 1000 bacteria may be found in a single macrophage. The bacteria eventually kill the macrophage, which releases them to be rephagocytosed.[1] Phagocytosis may be promoted by opsonization with the C3 component of complement (the common factor between the classic and alternative pathways).[16] Alveoli become filled with the bacteria, neutrophils, additional macrophages, and erythrocytes.[17] Chemokines and cytokines, including TNF-alpha and interleukin 12, initiate an inflammatory response.
The pathogenesis of Pontiac fever is not known, but it is also caused by inhalation of Legionella.
Classification
Types of Legionella infections
1. Legionnaires' disease (pneumonic legionellosis): community-acquired or hospital-acquired. Spread from the lungs to other organs may occur in immunocompromized patients.
2. Pontiac fever (nonpneumonic legionellosis): syndrome of headache, fever, and myalgia that usually resolves spontaneously in 3-5 days.
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