By Kelly R. Reynolds, MSPH, Ph.D.

Following a mysterious pneumonia-like outbreak at the 1976 American Legion Convention in Philadelphia, scientists identified a new bacterial agent, known now to us all as Legionella. More than three decades later, the mysteries of this bacterial pathogen are still being uncovered. Although cooling towers are most commonly identified as the suspected source in outbreaks, researchers have recently reported that this exposure route has been historically overemphasized and that potable water sources are the more commonly identified exposure route. Understanding the primary source of Legionella transmission is important for proper implementation of infection control procedures and reducing the risk of disease.

What is Legionella?
Legionella are known as water-based pathogens, occuring naturally in water and are not transmitted from person-to-person. Legionella is the causative agent of Legionnaires’ disease and Pontiac fever. Both syndromes are characterized initially by anorexia, malaise, myalgia and headache. Within 24 hours a fever ensues and chills. A nonproductive cough may occur and abdominal pain and diarrhea are seen with many patients. Respiratory failure is also possible, with a 15 percent fatality rate in hospitalized cases. Pontiac fever most commonly occurs in healthy persons and is a much milder form of the disease with no pneumonia or death. The infection is self-limiting in 2-5 days.

Over 28 species of Legionella cause disease, with L. pneumophila being the most prominent pathogen of the group. The disease is found worldwide with most cases in the summer and fall months. According to serological surveys, 1-20 percent of the general population has been exposed, as evidenced by the presence of antibodies to the organism, but certain people are much more susceptible to infection and disease.

The general population is relatively resistant to Legionella infection, however, in addition to hospital patients, people over the age of 65, smokers and those chronic health conditions (i.e., chronic obstructive pulmonary disease, emphysema, or a weakened immune system) are at higher risk of an adverse outcome following exposure to Legionella. Therefore, an individual assessment of risk is critical to determine feasible and necessary protective limits.

Transmission via aerosolized water droplets is the most commonly reported exposure route, however recent studies indicate that aspiration is the most likely mechanism of spreading the organism. Aspiration occurs when liquids accidentally enter the lungs, usually while coughing or choking while drinking. There is no current evidence that drinking water, even when contaminated with Legionella, causes illness (i.e, via the stomach route) but there is growing evidence of the importance of lung infections via aspirated drinking water contaminated with Legionella (Sabria and Yu, 2002).

Although prompt treatment with antibiotics is usually curative of Legionella infections, there have been reports of chronic effects (fatigue, neurologic symptoms, neuromuscular symptoms and respiratory symptoms) even after treatment (Lettinga et al., 2002).

Waterborne routes
Outbreaks of Legionella occur worldwide and have been linked to whirlpool spas, cooling towers, decorative fountains, misters, swimming pools, therapy pools and equipment and water systems in large buildings, hotels, hospitals and nursing homes. The organism has been isolated from water and soil environments.

The United States Center for Disease Control and Prevention (CDC) estimates that between 8,000 and 18,000 people are hospitalized with Legionnaires’ disease in the U.S. and that many more infections likely go undiagnosed or unreported. The largest recorded outbreak of Legionella occurred in 2001 in Spain. More than 800 suspected cases were recorded and over 400 were confirmed as having the disease. Six people died as a result of the outbreak. Epidemiological surveys implicated the cooling towers at a city hospital and molecular identification tools indicated that the bacteria isolated from the cooling tower was genetically related to the isolates from those infected.

Legionella outbreak data associated with drinking water is now available since the organism was added to the CDC waterborne outbreak surveillance list in 2001. During the first survey period (2001-2001) six Legionella outbreaks were documented, all from institutional or large building environments, making it now the single most common causative agent in drinking water outbreaks identified. Researchers have isolated Legionella from up to 33 percent of tap water samples surveyed.

Controlling Legionella in water
Little is known about the prevalence of Legionella in household water systems and the risk to those exposed. Most studies, to date, have focused on hospital or health-care facilities. Various studies of hospital water supplies found a prevalence of Legionella colonized in the water systems ranging from 12-85 percent (Rutala and Weber, 1997 and others). A survey of whirlpools for Legionella found the bacterium in nearly 66 percent of the 32 samples collected (Kuroki et al, 1998).

Legionella can survive wide temperature and pH fluctuations in water but multiply best in stagnant water and at temperatures between 35oC to 50oC (95oF to 122oF). To further complicate matters, Legionella growth and survival is supported by the presence of other organisms in water, including algae, ameobae and protozoa. If contained within protozoan cysts, the bacteria are further protected against disinfectants and harmful environmental conditions.

Various studies have shown up to 60 percent of cooling towers and 70 percent of hospital water distribution systems test positive for Legionella. Guidelines are widely available (i.e., from US Occupational Safety and Health Administration, CDC, American Society of Heating Refrigeration and Air-Conditioning Engineers) on how to control Legionella in cooling towers. For hospital water supplies, guidances issued in certain US states and abroad have recommended infrequent monitoring cooling systems for Legionella. The CDC recommends culturing hospital water supplies only following identification of nosocomial infection or in hospitals performing organ transplantation.

Many scientists are calling for a more proactive approach of monitoring and treating water, as indicated, in health care facilities. Focused studies found significant decreases in the incidence of Legionnaires’ disease following a routine monitoring and disinfecting regimen. This study found 71 percent of facilities colonized with Legionella and 44 percent initiated disinfection of the water distribution system. Researchers estimate that more than 39,000 lives have been lost from 1983-2005 due to Legionnaires’ disease acquired from health care facilities and that a proactive approach is warranted (Squier et al., 2005).

Legionella are ubiquitous in aqueous environments and very well adapted to survival in water supply and distribution systems. Controlling their presence in water supplies and on colonized surfaces has proven difficult. While they may not be completely eliminated in water systems, the CDC recommends thermal eradication by heating the water to 65oC and flushing water outlets and both shock and continuous hyperchlorination (1-2 ppm) as appropriate disinfection measures. A recent, long-term study in hospitals has shown copper-silver ionization to be an effective control measure (Stout et al., 2003). Other common water disinfection methods, such as ozone and ultraviolet can be effective at killing Legionella but cannot maintain a disinfection residual and thus additional disinfectants are typically needed for long-term control.

There are many unanswered questions related to Legionnella and the primary exposure routes of humans to this disease-causing agent. One thing we do know, however, is that immunocompromised populations are increasing and more people are potentially at risk of becoming infected with Legionella following exposure. There is compelling evidence in the literature that we should be targeting a wider range of exposure routes of this ubiquitous organism and taking a more proactive approach to controlling the growth of Legionella in our potable water supplies.


  • Lettinga, et al., (2002) Follow-up after Legionnaires’s disease. Clinical Infectious Diseases 35:11-17.
  • Squier, C. L. et al., (2005) A proactive approach to prevention of health care-acquired Legionnaires’ disease. The Allegheny County (Pittsburgh) experience. American Journal of Infection Control 33: 360-367.
  • Rusin, P. A. et al., (1997) Risk assessment of opportunistic bacterial pathogens in drinking water. Reviews in Environmental Contamination and Toxicology 152: 57-83.
  • Rutala, W. A. and Weber, D. J. (1997) Uses of inorganic hypochlorite (bleach) in health-care facilities. Clinical Microbiological Reviews 10:597-610.
  • Yu, V. L. (1993) Could Aspiration by the major mode of transmission for Legionella? American Journal of Medicine 95:13-15.
  • Sabria, M. and Yu, V. L. (2002) Hospital-acquired legionellosis: solutions for a preventable infection. The Lancet Infectious Diseases 2: -373.
  • Stout, J. E. et al., (2003). Infection Control and Hospital Epidemiology. 24: 563-568.

About the author:
Dr. Kelly A. Reynolds is an associate professor at the University of Arizona College of Public Health. She holds a Master of Science Degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds has been a member of the WC&P Technical Review Committee since 1997. She can be reached via email at [email protected]


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