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Water specialists should make Legionella reduction a top priority
This article will present an overview of Legionella bacteria, its ecology and sample collection strategies. A discussion of the pros and cons of Legionella monitoring also is included.
Control of microbial growth and biofilm development in facility water systems to reduce Legionella amplification and the risk of legionellosis is?or should be?a major focus of facility managers, safety personnel and water treatment specialists. This article will present an overview of Legionella bacteria, its ecology and sample collection strategies. A discussion of the pros and cons of Legionella monitoring also is included.
Legionella are aerobic, non-spore forming, typically flagellated, gram-negative bacilli that grow optimally at 37° C. The genus was named after an outbreak of pneumonia at an American Legion convention in 1976, which led to the isolation and characterization of L. pneumophila. Since then, more than 40 species of the genus Legionella and 61 serogroups have been identified.1, 2, 3 Approximately half the known species have been linked to human disease, but L. pneumophila (most prominently serogroup one but also four and six) accounts for 90 percent of the infections.3
Legionella causes two types of disease; Legionnaires' disease and Pontiac Fever. Legionnaires' disease has an incubation period of two to 10 days and is manifested by severe pneumonia, gastrointestinal symptoms and, in some cases, death. In contrast, Pontiac Fever has an incubation period of one to three days and, although it has an "attack rate" exceeding 90 percent among exposed individuals, it generally is self-limiting and complete recovery typically occurs within three to five days. Pontiac Fever is an influenza-like syndrome, typified by headache, fever and myalgia (nonfatal and nonpneumonic). The factors that determine whether an individual develops Legionnaires? Disease or Pontiac Fever are not fully understood but may include the virulence of the particular species or serogroup of Legionella, intensity of exposure, susceptibility (degree of immunocompetency, chronic obstructive pulmonary disease, age, smoking habits, etc.) and other factors.2
Many cases of legionellosis go undiagnosed. Legionnaires' Disease is difficult to discern from other types of pneumonia unless specifically targeted, and many cases of Pontiac Fever are attributed to typical influenzas.4 Legionella bacteria are thought to be one of the three top causes of sporadic, community-acquired pneumonia.4 The Centers for Disease Control and Prevention (CDC) has estimated that there are between 10,000 and 15,000 cases annually in the United States, while the Occupational Safety and Health Administration (OSHA) estimates there are 25,000 cases of legionellosis (68 cases per day), resulting in more than 4,000 deaths.3, 5, 6
Legionella bacteria occur naturally in freshwater aquatic habitats and are routinely recovered from source and treated municipal water supplies in the United States.7 Low levels of Legionella in municipal water may seed industrial potable and cooling water networks, where Legionella may 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.7, 8, 9 Sediment and scale create hospitable environments for a variety of microflora including bacteria, algae, protozoa and amoebae. Large distribution systems may provide nearly optimal conditions for Legionella growth including warm, stagnant water and abundant nutrients contributed by sediments and biofilms (see Table 1). Studies have shown that L. pneumophila can survive in biofilms for long intervals (20 to 40 days) at 24°C and 36°C.10 Legionella has been shown to have some resistance to low levels of chlorine and may survive in municipal distribution systems, especially in biofilms despite typical chlorine residuals.11 In addition, Legionella are intracellular parasites, meaning they can live within suitable host organisms such as amoebae and protozoa.12 The presence of these commensal microorganisms can create a shielding effect, further reducing the effect of biocides.8
The primary route of transmission of Legionella to humans is inhalation.5 Typically, infection occurs when water is contaminated with Legionella and aerosolized in respirable (1?5 microns) droplets that are deposited in the lung of a susceptible individual. In the alveolar tissue, pulmonary macrophages engulf the bacteria, but rather than being destroyed, Legionella amplify (as they do inside free-living protozoa and amoebae) resulting in overwhelming infection. Because inhalation is the most frequent mode of transmission of Legionella to humans, water-aerosolizing mechanisms such as showers, faucets, evaporative condensers, respiratory therapy machines, cooling towers and humidifiers/misters are of paramount concern as transmission agents.13, 14 Direct inhalation of the bacteria, and also drinking contaminated water with subsequent aspiration both have been theorized but not documented. There is no evidence of person-to-person transmission.
The practice of monitoring for Legionella in the absence of an outbreak remains controversial. OSHA provides guidance for investigations and sample collection, but does not specifically recommend sampling unless there is a "probable basis for suspecting that workplace water is contaminated with Legionella, or ... one or more possible or confirmed cases of Legionnaires? Disease exist(s)."6 The CDC advocates sampling after Legionnaires? Disease has been confirmed, for the purpose of source location and targeted remedial action.5 The American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) suggests that microbial monitoring is not a replacement for water chemistry testing or system maintenance and may not be predictive of risk of transmission.15 The Joint Commission on Accreditation of Healthcare Organizations (JCAHO) indicates in Standard EC 1.7 that healthcare facilities should have a documented program for "managing pathogenic biological agents in cooling towers, domestic hot water and other aerosolizing systems" but does not require routine monitoring to be part of the program.16 In addition, the culture method includes incubation for up to 10 days, so implementation of corrective measures based on these results would be delayed.17
In contrast, other experts in the United States and abroad recommend routine, proactive monitoring for Legionella. The Allegheny County (Pa.) Health Department recommends that all hospitals collect and process samples once per year.18 Researchers at the Veterans Administration Medical Center, Special Pathogens Laboratory, have long advocated Legionella monitoring and state "environmental cultural surveillance for Legionella is the foundation for a rational, long-term approach to successful disinfection."11 A Scientific Working Group convened by the state of Maryland to study Legionella reported that water systems in acute care facilities should be routinely cultured.3 However, the authors concluded that there were inadequate data to provide uniform guidelines regarding sampling frequency, or "actions levels," for environmental results and recommended analyses be qualitative (for the presence or absence of Legionella) rather than quantitative. In Routine Sampling and the control of Legionella spp. in Cooling Tower Water Systems, R.H. Bentham reviewed Legionella occurrence data from 31 cooling towers during a 16-week period. He observed relatively low average concentrations of Legionella, but high standard deviations and suggested that high concentrations are not a common occurrence but rather are sporadic within most systems.19 Australian cooling tower standards are rigorous, requiring tower registration and submission of building plans, site surveys, maintenance procedures and other documentation to regulatory authorities.20
Nosocomial (hospital acquired) Legionnaires? disease is of particular concern due to the typically high number of immunocompromised individuals present in most healthcare facilities at any given time. The Allegheny County 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 10 distal sites (faucets, showerheads, etc.) sampled.18, 21 In his book Resolving the Controversy on Environmental Cultures for Legionella: A Modest Proposal, Yu reviewed various recommendations including the CDC?s and the research that supported various positions. The author finds fault with the CDC guidance and cites several articles that suggest monitoring is indicated. Yu concluded by proposing that hospitals do their own surveillance regarding hospital acquired pneumonia and publish the results.22
The lack of consensus among experts on routine Legionella monitoring arises from both scientific and operational/legal issues. Legionella frequently is present in water systems without causing known disease outbreaks, and agreement regarding specific infectious densities has not been reached. Further, some experts argue that correlations between test results and the risk of disease transmission are inconsistent. This may be caused by a combination of
From an operational or legal perspective, there is disagreement whether Legionella monitoring results are assets or liabilities. In the event of disease, positive results may constitute a liability, but the same monitoring and maintenance records may also prevent charges of negligence.4
Routine Legionella monitoring should be performed?not to protect public health directly but to gather empirical data to determine the success of operational and maintenance practices designed to control Legionella and other microbial populations. Further, given the wealth of information available regarding Legionella and legionellosis, defending a program that includes no monitoring at all, especially in the presence of other known risk factors, would be an unenviable task.