Control of Legionella in the Healthcare Environment: Focus on Cooling Towers

Richard D. Miller, Ph.D.

Environmental Safety Technologies, Inc.

What is Legionnaires’ disease?

Legionnaires’ disease is a serious, life-threatening pneumonia caused by the environmental bacterium Legionella pneumophila and related species.  The disease is transmitted via aerosols from environmental water sources and can be seen as both a community-acquired pneumonia, as well as a hospital acquired (nosocomial) pneumonia.  While single isolated cases (transmitted from domestic hot water) are more common in the hospital setting, large multiple-case outbreaks have been reported.  The largest reported nosocomial outbreak occurred in 1985 in England and had 158 cases with 36 deaths.  Several large community-acquired outbreaks have occurred world-wide over the past two years, including a contaminated cooling tower at the Aquarium in Melbourne, Australia (113 cases, 2 deaths), a whirlpool spa at a home products/flower show in the Netherlands (250 cases, 28 deaths), and a cooling tower outbreak in Murcia, Spain (315 cases, 2 deaths).

All Legionella bacteria are normal environmental organisms that are part of virtually every fresh-water aquatic habitat (although at low concentrations).  They have special abilities to grow inside of free-living amoebae and other protozoa in these environments.  Disease occurs when conditions in building water stimulate the growth of Legionella to high numbers. 

Legionella are then transmitted to humans via contaminated aerosols from these aquatic habitats associated with building water.  The three most common sources of transmission are domestic hot water systems, cooling towers, and heated whirlpool spas.  However, other sources have been observed, such as indoor decorative fountains, grocery store vegetable misters, and even potting soil.

Legionella in the Healthcare Environment

Because of their population of variably immuno-compromised patients, the risk of Legionnaires’ disease is higher in hospitals and other healthcare facilities than in the general population.  As mentioned above, the domestic hot water system in hospitals is clearly the greatest risk of infection for most resident patients.  For advice on managing the risk from domestic hot water, the 1997 document from Pittsburgh/Allegheny County (PA), “Approaches to Prevention and Control of Legionella Infection in Allegheny County Health Care Facilities,” (www.Legionella.org) is highly recommended.

Cooling towers, because of the extensive aerosolization as part of their normal function, are a risk for large outbreaks in the community as well as healthcare facilities.  These latter outbreaks are often more likely to have cases in visitors, employees, and patients entering or leaving, than in the resident patient population.  However, because of the risk to immunocompromised patients, the evaluation of risk from a cooling tower associated with a healthcare facility always has a low tolerance for Legionella colonization.

In recognition of this risk (effective January 1, 2001) the Joint Commission on Accreditation of Healthcare Organizations (JCAHO) added significantly new language to the Utility Systems Management, Standard EC.1.9, mandating facilities to “reduce the potential for organizational-acquired illness” by developing a specific management plan providing processes for “managing pathogenic biological agents in cooling towers, domestic hot water, and other aerosolizing water systems.”  While broadly written, the intent of this standard is clearly to control Legionella and Legionnaires’ disease.

Legionella Control in Cooling Towers

In the years following the 1976 outbreak of Legionnaires’ disease in Philadelphia, and the understanding of its transmission from environment habitats, the maintenance of cooling towers had acquired an entirely new focus.  Historically, chemical biocides were added to cooling tower water in order to control biofouling, which could lead to performance problems such as slime and microbial corrosion problems in the tower structure, heat exchanger, and plumbing.  With the new concern about Legionella and Legionnaires’ disease, the control of this specific bacterium in cooling towers became a public health issue. 

The general operating procedures related to Legionella control in cooling towers are addressed in several excellent guidelines on the topic, including the ASHRAE Guideline 12-2000: “Minimizing the Risk of Legionellosis Associated with Building Water Systems,” (www.ashrae.org) and the Association of Water Technologies’ document, “Legionella 2003.”  (www.awt.org)  

However, there is still no agreement on which biocides work best to inhibit Legionella while continuing to control general biofouling conditions.  Opinions from OSHA (www.osha.gov) and the Cooling Technology Institute (www.cti.org) recommend using an oxidizing biocide (bromine most commonly) with periodic addition of a non-oxidizing biocide.  However, a wide variety of biocides (oxidizers and non-oxidizers) continue to be used in cooling towers with apparent success. 

In order to help answer these questions, Environmental Safety Technologies, Inc., in collaboration with the University of Louisville, carried out a study on a large population of cooling towers (2,590 samples) throughout the U.S., correlating the presence and numbers of Legionella pneumophila in each tower with the 28 different biocide treatment combinations of eight different biocides. 

The results of this study found that, on average, Legionella were detected in 13% of the cooling towers, and 18% of these positive towers had high levels of Legionella (>1,000 cfu/ml; 2% of the total towers).  No biocide combinations were 100% effective in preventing Legionella colonization, and also none always failed.  Oxidizing biocide (primarily bromine) combinations with non-oxidizers were not significantly better than two non-oxidizers and in many cases were significantly worse.  The non-oxidizing biocide THPS was particularly effective in all combinations, while glutaraldehyde, DBNPA, and hydroperoxide were less effective than average in most combinations.  Monitoring of Legionella levels in cooling towers is required in order to assess accurately the effectiveness of any biocide regimen.

An Ounce of Prevention

The prevention of Legionnaires’ disease in healthcare facilities begins with the implementation of an effective management plan, as mandated by JCAHO.  Since prevention of Legionnaires’ disease is a risk management issue, persons with an expertise in this area should be utilized in the preparation of the Management Plan.  Additionally, both the engineering and infection control departments of the hospital should be represented and involved in a coordinated effort.

The Management Plan should clearly identify all potential water sources for Legionella in the hospital, such as the domestic hot water, cooling towers, indoor decorative fountains, etc.  Using existing guidelines, recommendations, and other information (such as the current cooling tower biocide study) as a guide, specific procedures forcontrollingLegionella growth in each water system should be described.  Procedures for environmental surveillance of the water systems are important for monitoring the effectiveness of the control procedures.  As indicated above, monitoring of Legionella levels in cooling towers is required in order to assess accurately the effectiveness of any biocide regimen.  It is equally important to describe the procedures and criteria used to identify cases of Legionnaires’ disease in the hospital.

Finally, the Management Plan must have clearly outlined criteria for interpretation of the results (environmental and clinical) and specific steps to be taken if Legionella are found in the water or cases of nosocomial Legionnaires’ disease are detected.

Environmental Safety Technologies, Inc. (EST)

EST specializes in working with hospitals on a variety of different areas of infection control, including construction-related mold and indoor air quality issues, as well as Legionnaires’ disease monitoring and risk assessments.  EST has performed thousands of risk assessments for Legionnaires’ disease over the past eleven years for hospitals, hotels and resorts, the cruise industry, office buildings, and other places of concern for this disease.  This comprehensive service includes onsite testing (including emergency response), development of test kits and training for self-testing, complete laboratory analysis, and personalized risk assessments.