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, over thirty (30)species of the genus Legionella have been
identified.
In humans, L. pneumophila can cause Legionnaires disease and Pontiac
Fever. Legionnaires disease is difficult to diagnose as it produces symptoms very similar to a number of other respiratory infections. Symptoms may include dry coughing, high fever, chills, diarrhea, shortness of breath, chest pains, headaches, excessive sweating, nausea, vomiting, and abdominal pain. It can be fatal, particularly if the onset of treatment is delayed. Pontiac 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 have been recovered from some 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 literature1,2,3. Legionella has been shown to have some resistance to low levels of chlorine, and survives in municipal distribution systems because typical chlorine residuals may be insufficient to kill it4. Large distribution systems and building water systems may provide near optimal conditions for Legionella growth, including warm water temperatures and abundant nutrients contributed by sediments and biofilms. In addition, the presence of amoebae and protozoa, which have been shown to harbor Legionella, 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
concern5,6.
While Legionella have been found in potable water distribution systems, the concentration at which they are found is usually insufficient to be of public health concern. In cold water Legionella may be out-competed by other organisms, while in hot water systems Legionella proliferate. Regular monitoring of municipal supplies can determine the concentration of organisms present. This information can be used to assess if any additional flushing or disinfection should be undertaken and can also be extremely useful if a Legionella outbreak should occur in the general area. Demonstrably low Legionella counts within the municipal system indicates that infection is extremely unlikely to have occurred as a result of Legionella growing in the municipal system.
Building water systems, particularly those with susceptible populations, should also be monitored to ensure that system operation and disinfection procedures are adequate to prevent Legionella colonization and amplification. In addition to disinfectant residual and standard bacterial parameters, periodic Legionella monitoring is recommended.
OSHA suggests prompt remedial action be implemented when concentrations of Legionella in water samples are >10 CFU/mL (potable water) or >100 CFU/mL (cooling tower water)10. Ta, et.al. have suggested that for swabs, a Legionella action trigger of >1,000 CFU/swab may be appropriate for hospital potable water systems7. However, a recently published World Health Organization document shows that in Europe, the action levels are much lower (up to three orders of magnitude lower).
Nosocomial (hospital acquired) Legionnaires' disease is of particular concern due to
the presence of immunocompromised individuals. The Allegheny County (PA) Health Department and 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.) sampled8,11. International experience indicates the OSHA guidelines should not apply to these healthcare facilities and that more conservative action limits would be prudent.
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 for Legionella during the cooling season is appropriate.
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. Samples are analyzed within 96 hours of collection.
Analytical Techniques
Although a variety of Legionella methods exist (including direct and indirect fluorescent antibody tests, polymerase chain reaction, etc.) the standard procedure is a traditional culture method, based on the CDC procedure, which uses Buffered Charcoal Yeast Extract media with additives. After initial incubation (7 days), colonies with morphologies similar to Legionella are patch-plated to distinguish non-Legionella isolates. At the end of this 48 hour incubation, isolates meeting Legionella criteria are referred to as “Presumptive Positives.” ASI provides Legionella confirmation, if requested, by identifying representative isolates using cellular fatty acid analysis. Confirmed L. pneumophila isolates can be serotyped by latex agglutination to differentiate L. pneumophila Serogroup 1(which may account for up to 80% of legionellosis cases) from Serogroups 2-14.
Disinfection Techniques
Several disinfection techniques have been used to treat contaminated 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 of these treatments can vary in practice.(4, 9) ASI can provide assistance in determining the most appropriate treatment for your application and monitor treatment efficacy.
Summary
ASI can help water system managers by developing and interpreting Legionella occurrence data and assessing system operation. This approach instills confidence regarding the microbial quality of the system and the manager’s ability to provide appropriate responses, particularly when litigation is threatened.
References
States, S.J., et al. 1990. Legionella in Drinking Water. In Drinking
Water Microbiology. Springer- Verlag, NY.
Sutherland, E. and Burk, S. 1996.. JIM, 16:73-78.
Bollin, G.Y., et al. 1985. AEM, 50:1128-1131.
Lin, Y.E., et al. 1998. JAWWA, 90:112-121.
Barbaree, J.M., et al. 1987. AEM, 53:1454-1458.
Costerton, J.W. and H.M. Lappin-Scott. 1989. ASM News, 55:650-654.
Ta, A.C., et al. 1995. JCM, 33:2118-2123.
Stout, J.E. and Yu, V.L. 1997. NEJM,
337:682-687.
Yu, V.L., et al. 1993. Infect Control Hosp Epidemiol,
14:567-570.
OSHA Tech. Manual, Sec. III, Ch 7: Legionnaires' Disease. http://www.osha-slc.gov/dts/osta/otm/otm_iii/otm_iii_7.html
Allegheny County Health Dept. 1997. Approaches to prevention and
control of Legionella infection in Allegheny County health care facilities. 2nd
ed. Pittsburg: ACHD, p. 1-15.
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