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Chapter 79 : Legionellae and Legionnaires’ Disease
Legionellae are now associated with two forms of respiratory illness, collectively referred to as legionellosis. Inhalation of legionellae in aerosolized droplets is the primary means of transmission for legionellosis. The number and types of sites that should be tested to detect legionellae must be determined on an individual basis. Two primary sample types should be collected when sampling for legionellae: water samples and swabs of point-of-use devices or system surfaces. Examination of water samples is the most efficient microbiologic method for identifying sources of legionellae. Most investigations of epidemic legionellosis have used culture to detect legionellae in the environment. Practical information concerning treatment processes that effectively control legionellae is limited. Practices to control legionellae in amplifying reservoirs can be divided into two categories, routine maintenance and emergency decontamination procedures. Some control strategies are intended to prevent exposure of susceptible individuals to aerosols which may contain legionellae. Early investigations of Legionnaires’ disease associated with cooling towers resulted in a recommendation for relocation of cooling towers or air intake vents so that cooling tower exhaust would not be carried directly into the heating, ventilation, and air-conditioning systems of buildings. Effective prevention strategies require more-effective decontamination techniques and approaches to prevent amplification of legionellae in reservoirs.
Gimenez stain of the ciliated protozoan T. pyriformis infected with L. pneumophila. Chains of multiplying Legionella cells are contained within vesicles, as observed in human phagocytic cells infected with L. pneumophila. Magnification, ×1,650.
Overview of procedures for the culture of water samples to detect legionellae.
Two Legionella colonies and a non-Legionella bacterial colony as seen through a dissecting microscope upon primary isolation (4 days of incubation). Note the white “cut-glass” appearance of the center of the colony and the purple iridescence which borders it. The iridescence can be one of several colors; the significance of the colors is unknown.
Characteristics of Legionnaries’ disease and Pontiac fever
Sources known to transmit legionellae via aerosols a
Components and supplements of BCYE agar for culturing legionellae from the environment