Legionnaire's Disease

Introduction

In summer 1976, participants of an American Legions conference in Philadelphia began to fall ill with a mysterious pneumonia. The illness, dubbed Legionnaire's disease, led to 34 deaths among the 221 people affected. Ultimately, Legionella pneumophilia, a novel gram-negative bacterium, was isolated from harvested lung tissue of the epidemic's victims.

Over the last three decades, over 40 species and 64 serotypes of Legionella have been identified. Most have been isolated from the environment and nearly half have been implicated with human disease. L. pneumophilia remains the most clinically significant species accounting for over 80 percent of Legionellosis cases in the United States and is being increasingly recognized as a fairly common cause of pneumonia in adults. Legionella infections may be sporadic or epidemic, community or hospital acquired.

Epidemiology

Epidemiologically, there are three main requirements for a Legionella infection to occur: a contaminated source, a means of dissemination, and host susceptibility.

Water is the only documented source of Legionella species, particularly the surface waters of rivers, lakes, and reservoirs, and drinking water. Man-made sources include cooling towers and potable water distributions systems. Freshwater amoebas serve as a reservoir and have been found to facilitate the growth of the bacteria. Another factor that supports the survival of Legionella in natural or treated water is its relative resistance to heat and chlorination, which is why it can thrive in normally inhospitable environments such as hot-water tanks.

Transmission occurs through inhalation of aerosols or aspiration of contaminated water. Documented sources of dissemination include cooling towers, humidifiers, respiratory therapy equipment, whirlpool spas, evaporative condensers, hot-water supplies, and potable water distributors (e.g., showers, faucets). Because numerous activities, including taking a shower and flushing a toilet, can produce aerosols in the environment, it is often difficult to identify the source of infection. Direct infection of wounds after contact with contaminated water has also been documented. No human-to-human transmission has ever been documented.

Host susceptibility to Legionella depends on both specific and nonspecific defenses. Processes that compromise the mucociliary action of the upper respiratory tract, such as smoking, can leave a person more vulnerable to infection. Other significant risk factors include chronic lung disease, end-stage renal disease, diabetes mellitus, and advanced age. Immunosuppression (e.g., malignancy, corticosteroid therapy, human immunodeficiency virus) has been shown to be the most important factor in the development and severity of infection. Surgery (especially head/neck, transplant procedures) has also been shown to predispose a patient to hospital-acquired infections. Infections are more common in the summer months, presumably because there is an increased use of aero-sol-generating cooling systems and improved growth conditions for the organism.

...