Legionella are ubiquitous bacteria in nature. However, they are most commonly found in freshwater systems and man-made structures, such as cooling towers, hot water systems and air-conditiong systems. These bacteria are known to originate Legionnaire's Disease, a form of pneumonia, that has been associated with several relevant outbreaks, as well as Pontiac Fever.
The first outbreak to be reported as caused by Legionella happened in 1976, at a conference in Philadelphia. However, researchers have been able to identify previous outbreaks, which had been misclassified as pneumonia outbreaks. Ever since the number of outbreaks has been incresing.
Currently, there are reports of more than 61 Legionella species, of which Legionella pneumophila is recognized to be the leading cause of Legionnaire's Disease, accounting for 90% of the total number of cases (Springston, 2017). Nonetheless, L. micdadei, L. bozemanii, L. dumoffii, and L. longbeachae are also known to be associated with this disease.
Engineered structures, mainly hot water systems, cooling towers and air-conditioning systems, are recognized as a source of Legionella cultures, as they provide the optimal conditions for their development. These include temperatures between 20 and 50ºC, pH between 5.5 and 9.2, and the presence of iron salts.
Dead legs or other places with stagnant water that favour the presence of biofilms also present a favourable environment for the development of Legionella bacteria. Biofilms provide a stable environment and protection against hazardous conditions and some of the commonly used treatments for water disinfection. Besides, the existence of protozoa in biofilms also significantly enhances Legionella persistence, as these bacteria can use protozoa for intracellular replication.
The source of Legionella is not easily identifiable in many cases. However, when researchers are able to identify where the outbreak originated, they usually encounter places that present inadequate maintenance. Besides, it is apparent that when outbreaks originate in cooling towers and air treatment systems, the number of affected people significantly increases, as these structures enhance the propagation of aerosols.
Outbreaks with identified sources
Identified sources presented biofilm formation
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