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Legionella

Technical Document

Introduction

Legionella are aerobic, non-spore forming, typically flagellated, gram-negative bacteria. The genus was named after a severe epidemic of pneumonia at an American Legion convention in 1976, which led to the isolation and characterization of L. pneumophila. Since then, thirty (30) species of the genus Legionella have been identified.

In humans, L. pneumophila can cause Legionnaires’ disease and Pontiac Fever. Legionnaires’ disease is manifested by severe pneumonia, gastrointestinal symp-toms and, in some cases, death. Pontaic fever is a nonfatal, nonpneumonic, influenza-like syndrome typified by headache, fever, and myalgia. The Centers for Disease Control (CDC) estimates that between 10,000 and 25,000 cases of Legionnaires’ disease occur annually in the United States.

Habitats

Legionella bacteria occur naturally in aquatic habitats and are routinely recovered from municipal water supplies in the United States¹. Low levels of Legionella in municipal water may seed industrial potable water systems and cooling water networks. Legionella may then colonize and amplify in hot water tanks, humidifiers, water spray irrigation systems, cooling towers, ice machines, dead legs in distribution systems, and other areas where bioflora are able to flourish.

Correlations between the presence of scale and sediment in distribution systems and the presence of Legionella have been noted in the literature^1,2,3. Sediment and scale create hospitable environments for a variety of microflora, including bacteria, algae, protozoa, and amoebae. Legionella has been shown to have some resistance to low levels of chlorine, and survives in municipal distribution systems because typical chlorine residuals are insufficient to kill it⁴. Large distribution systems may provide nearly optimal conditions for Legionella growth, including warm water temperatures (45-50^oC) and abundant nutrients contributed by sediments and biofilms. In addition, the presence of commensal microorganisms (amoebae and protozoa) which have been shown to harbor Legionella bacteria may create a shielding effect, further reducing the effect of biocides².

Monitoring

The primary route of Legionella infection in humans is inhalation of aerosolized bacteria. Therefore, showers, faucets, evaporative condensers, respiratory therapy machines, cooling towers, vegetable misters and other aerosolizing agents are of paramount concern^5,6.

Regular, proactive monitoring for municipal water suppliers using surface water sources would allow relevant information to be passed on to consumers^1,4. In addition, regular Legionella monitoring of industrial systems where potential exposure of susceptible individuals exists is advisable^3,7. Water suppliers and industrial system managers should collect samples at a variety of locations throughout the distributions system, similar to coliform monitoring. In addition to first draw water samples, Legionella monitoring should include swab samples as a primary means of sample collection⁵. Biofilm research has shown that in most habitats, bacteria grow preferentially on surfaces rather than in the aqueous phase⁶. Swabs collected from the inner walls of faucets have demonstrated equivalent sensitivity to bulk water samples, while resulting in higher recovery of Legionella⁷. This is significant because outbreaks of Legionnaires' disease have been linked to exposure to elevated levels of Legionella and suggested remedial actions have been based on Legionella concentration^8,9.

Nosocomial (hospital acquired) Legionnaires' disease is of particular concern due to the presence of immunocompromised individuals. The Allegheny County (PA) Health Department, with the assistance of the Association for Professionals in Infection Control and Epidemiology (APIC) developed guidelines for the prevention and control of Legionella infection in health care facilities. These guidelines involve routine monitoring for Legionella, including a minimum of one annual survey with at least ten distal sites (faucets, showerheads, etc.) sampled^8,10.

A comprehensive cooling tower monitoring program includes swab and bulk water samples collected from the incoming or make-up water, the header tank, tower pond, and the water returning from the circulation system (at the point of entry to the tower). Swab or scraping samples should be collected of representative sludge, slimes and sediments in the header tank or tower pond. Our experience indicates monthly monitoring of cooling towers and systems during the cooling season is prudent to track Legionella occurrence.

Water samples should be collected in sterile, one-liter (1L) plastic bottles, using sodium thiosulfate for chlorine neutralization. Swab samples should be collected using swabs formulated with transport media to prevent desiccation. Samples should be shipped, unrefrigerated, to the laboratory in an insulated cooler by overnight delivery. All samples are analyzed within forty-eight hours of collection.

Analytical Techniques

A variety of analytical methods are available for the detection of Legionella. Screening tests include direct and Indirect Fluorescent Antibody (IFA) techniques and Polymerase Chain Reaction (PCR) techniques, although these methods may detect non-viable Legionella. The culture method, which involves plating samples on Buffered Charcoal Yeast Extract (BCYE) agar and incubating them for up to 10 days, detects only viable organisms, and is the generally accepted standard procedure for Legionella testing.

Disinfection Techniques

Several disinfection techniques have been used in conjunction with contaminated water systems, including hyperchlorination, ultraviolet (UV) light, ozone, thermal eradication, instantaneous superheating systems, copper-silver ionization and other techniques. For these or other methods to be successful, Legionella in the water column, inside commensal microbes and in biofilms must all be killed. The effectiveness and costs (direct, financial and associated labor) can vary in practice and information is readily available in the literature^1,4,9. A thorough review is beyond the scope of this document.

Summary

ASI can assist managers to develop programs to generate baseline Legionella occurrence data and provide regular monitoring updates. These data, together with consulting and data interpretation services from ASI’s senior staff, will give managers confidence regarding the extent of Legionella contamination in their systems and their ability to provide appropriate responses.

References

1. States, S.J., et al. 1990. Legionella in Drinking Water. Drinking Water Microbiology. Springer- Verlag, NY.

2. Sutherland, E.E. and Burk, S.G. 1996. Survival of protozoa in cooling tower biocides. JIM, 16:73-78.

3. Bollin, G.Y., et al. 1985. Aerosols containing Legionella pneumophila generated by shower heads and hot-water faucets. AEM, 50:1128-1131.

4. Lin, Y.E., et al. 1998. Legionella in water distribution systems. JAWWA, 90:112-121.

5. Barbaree, J.M., et al. 1987. Protocol for sampling environmental sites for Legionellae. AEM, 53:1454-1458.

6. Costerton, J.W. and H.M. Lappin-Scott. 1989. Behavior of Bacteria in Biofims. ASM News, 55:650-654.

7. Ta, A.C., et al. 1995. Comparison of Culture Methods for Monitoring Legionella species in Hospital Potable Water Systems and Recommendations for Standardization of Such Methods. JCM, 33:2118-2123.

8. Stout, J.E. and Yu, V.L. 1997. Legionellosis. NEJM, 337:682-687.

9. Yu, V.L., et al. 1993. Legionella disinfection of water distribution systems; principles, problems, and practice. Infect Control Hosp Epidemiol, 14:567-570.

10. Allegheny County Health Dept. 1997. Approaches to prevention and control of Legionella infection in Allegheny County health care facilities. 2^nd ed. Pittsburg: ACHD, p. 1-15.

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