Legionella transmission is thought to occur via inhalation of aerosolized mist from water sources, such as the following, that have been contaminated with the bacterium [2, 3] :
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Cooling systems
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Showers
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Decorative fountains
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Humidifiers
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Respiratory therapy equipment
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Whirlpool spas
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Ice machines
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Potting soil
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Compost
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Roadside puddles
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Tubs used for water births [4]
Legionnaires disease may be travel associated from exposure in aircraft or hotel facilities. Person-to-person transmission, however, has not been documented.
The highest incidence of Legionnaires disease occurs during late spring and early fall, when air-conditioning systems are used more frequently. [5, 6] Nosocomial acquisition likely occurs via aspiration, respiratory therapy equipment, [3] 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 human-made water environments:
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Temperature of 25-42°C
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Stagnation
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Scale and sediment
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Presence of certain free-living aquatic amoebae capable of supporting intracellular growth of legionellae
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This electron micrograph depicts an amoeba, Hartmannella vermiformis (orange), as it entraps a Legionella pneumophila bacterium (green) with an extended pseudopod. After it is ingested, the bacterium can survive as a symbiont within what then becomes its protozoan host. The amoeba then becomes a so-called "Trojan horse," since, by harboring the pathogenic bacterium, the amoeba can afford it protection. In fact, in times of adverse environmental conditions, the amoeba can metamorphose into a cystic stage, enabling it, and its symbiotic resident, to withstand the environmental stress. Image courtesy of the Centers for Disease Control and Prevention and Dr. Barry S Fields.
