LEGIONELLA: FRIEND OR FOE?
(REDUCING THE RISKS OF LEGIONNAIRES' DISEASE)

Author: Richard D. Miller, Ph.D.

It seems that Legionella suffers from an identity crisis.  How can a bacterium cause such fear and panic as a result of devastating illness and death from Legionnaires' Disease, while at the same time be found all around us as a common inhabitant of most freshwater environmental ecosystems?  Perhaps within the answer to this question lies the solution to reducing our risks for this disease.  Four important points need to be considered in the exploration of this question:  1) the number of Legionella in the water at a particular site, 2) a mechanism for aerosolization of the Legionella in association with human work, recreation, healthcare, or home-living spaces, 3) the susceptibility of the exposed population, and 4) the presence of free-living amoebae and other protozoa.

Legionella Concentrations

Legionnaires' Disease, like all infectious diseases, requires a minimum infectious dose in order to cause disease. While the exact number required for humans varies depending on the susceptibility (i.e. immune status) of the individual, it is likely that the number for most individuals is relatively large.  It should be apparent from the ubiquitous nature of this bacterium in the environment, that in order to cause disease, the number of Legionella in the water  would need to be much higher than that found in most normal aquatic habitats.

Thus, it can not be overemphasized, that in most situations, the mere presence or absence of Legionella at a particular site is not sufficient for making an accurate risk assessment. Rather it is the numbers (i.e. concentration) of Legionella in the water that is most important.

Fortunately, most of the common techniques for detecting Legionella in the water samples are also capable of providing quantitative results. The most widely accepted technique (i.e. "gold standard") for detecting Legionella in environmental samples is the culture technique, where the bacteria are isolated as colonies growing on agar Petri dishes. This technique is somewhat involved and labor intensive to carry out, and requires experienced laboratory personnel. However, it does provide good quantitative data for making risk assessments.  The other common technique used by some individuals is the so-called DFA test (Direct Fluorescent-Antibody test) which detects fluorescing bacteria under the microscope.  While the DFA procedure is quantitative, it has the disadvantages of detecting both live (i.e. infectious) and dead (i.e. non-infectious) bacteria, it depends on the somewhat questionable specificity of various antibody reagents, and it obviously does not provide a living culture for use in confirmatory tests, DNA fingerprinting, etc.

How many Legionella is too many? While it is difficult to come up with an exact number to cover all circumstances, there is data from past outbreaks of Legionnaires' Disease that would suggest that the number required would depend on the relative intimacy of the exposure. With cooling towers, for example, the distances can be as much as from the top of a ten-story building to the street below. Thus, the concentration of Legionella in cooling towers generally must exceed 1,000 bacteria/ml of water in order to be classified in a high-risk category. For sites such as a whirlpool spa, on the other hand, the exposure would be more intimate, with a significantly lower exposure limit. In fact, because of the disinfection systems routinely used in these spas, the presence of any Legionella could be interpreted as unacceptable.

Mechanisms for Aerosolization

Since Legionnaires' Disease is a pneumonia, the bacteria must be inhaled into the lungs for optimal transmission.  Thus, the Legionella in the water must be aerosolized in order to cause disease, and the mere presence of Legionella in the water at a particular site would result in little risk for disease if there were no mechanism for aerosolization.  Unfortunately for humans, sites such as cooling towers create a huge aerosol as part their operation.  Other sites that have been documented as sources for Legionnaires' Disease (hot tubs and whirlpools, faucets and showers, decorative fountains, supermarket vegetable misters, etc.) all have obvious mechanisms for getting Legionella from the water into the air.  Based on this knowledge, the risk assessment of a building should not necessarily sample all locations with water, but rather focus on those sites where documented aerosolization and disease transmission can occur.

Susceptible Populations

The immune status of an individual is known to have a significant impact on that person's susceptibility to infectious diseases in general.  While many outbreaks of Legionnaires' Disease are considered to be community-acquired (i.e. normal individuals), cases are also very commonly seen as hospital-acquired infections.  In many instances the outbreaks become prolonged due to colonization of the entire potable water system in hospitals, particularly the hot water system.  The numbers of Legionella present in these water systems that cause disease is difficult to determine accurately since these bacteria tend to colonize the interior surfaces of the faucets and showerheads, leading to a burst of Legionella when the fixtures are turned on.  Thus, because of the unique susceptibility of the hospital population, some healthcare institutions have monitored their fixtures and hot water tanks, basing their risk assessment not on the number of Legionella in any one sample, but rather the number (i.e. percentage) of positive sites within the building. The reader should refer to the document provided by the Allegheny County (Pennsylvania) Health Department, which outlines the details of this approach to risk assessment in the healthcare environment.

Other water sources, such as cooling towers, have also been associated with outbreaks of disease in hospitals.  Thus, it should be obvious that the regular monitoring and risk assessments of all relevant sites in and around these buildings can provide important information for disease prevention.

Legionella and Amoebae

It has been well documented that Legionella in environmental water habitats multiply as intracellular parasites within amoebae and other free-living protozoa.  It is indeed a strange quirk of fate that the phagocytic cells in the human lung appear to be "close cousins" to the amoebae, such that the Legionella inhaled into the lung multiply in these phagocytic cells much the same way they do within amoebae in the environment.  Actually, they appear to grow better because the warm temperature of the human body also provides a more ideal environment for rapid growth of these bacteria.  The relevance of this observation is that amoebae are probably essential participants in the Legionnaires' Disease phenomenon.  Thus, the presence of amoebae in the water at particular sites of concern could be used as an indicator of the risk for Legionella replication.  Unfortunately, the amoebae seem to be so common that correlations with types and numbers of amoebae or other protozoa have not been established to date.

Summary

Many of the sites in buildings that have been identified as sources of Legionella proliferation already utilize some type of chemical disinfection for purposes of controlling microbial populations (particularly human pathogens), such as the biocides added to cooling towers, and the chlorine or bromine added to whirlpool spas and drinking water.  Thus, the first concern for a building owner or facilities manager must be the optimal operation of the disinfection and other maintenance procedures.  For a more in depth discussion of the disinfection procedures utilized for control of Legionella, the reader is referred to the recent article by Rosa in the November, 1994 issue of this magazine.

Nevertheless, failure of the disinfection systems may occur for a variety of reasons, occasionally leading to uncontrolled proliferation of Legionella at these sites.  The only way to reliably determine the risk for Legionnaires' disease is to analyze the water directly for the numbers of Legionella.  With a regular monitoring program coupled to good maintenance procedures, Legionnaires' Disease is a preventable illness. This principle of prevention has been adopted by OSHA for work place exposure to Legionella, and they are actively pursuing violations under the "general duty" clause.   Hospitals, nursing homes, and other healthcare facilities obviously have a special concern because of the susceptible population.  Other locations such as hotels, resorts, and cruise ships are also concerned about Legionella because of past outbreaks of disease and the accompanying bad publicity, loss of business, etc.

Based on our knowledge of Legionella and Legionnaires disease, it is surprising, that there are currently no guidelines from federal public health officials with regard to monitoring for Legionella or otherwise controlling the risk of Legionnaires' disease (although there are in some other countries).  Nevertheless, there are still very real and very large financial and legal implications associated with an outbreak of Legionnaires' Disease.  Having a large baseline of data on the status of Legionella in a building can not only help prevent disease in the first place, but also reduce the liability if cases of disease do occur.   In an attempt to provide some guidance in this area, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. (ASHRAE) has been actively working on a position statement and proposed guidelines for control of Legionnaires' Disease, and it is likely that they will be addressing many of these issues in the near future.