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Microbiological control |
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Legionnaires Disease Legionnaires Disease first came into prominence in 1976, the result of a pneumonia-like epidemic which occurred among American Legionnaires attending a convention held at the Bellevue Stratford Hotel in Philadelphia. Additional cases were discovered among persons who had not been directly associated with the convention but had been near the hotel. All told, 221 persons became ill and 34 died. An investigation by the Center for Disease Control (CDC) determined the causative agent to be a bacterial organism disseminated by the hotel's air conditioning-industrial system. Since that time, subsequent outbreaks and sporadic cases of Legionellosis have occurred all over the world. While published reports indicate that in many of the outbreaks industrial systems were not involved, in a number of cases industrial water systems again played a key role. As a result, today Legionnaires Disease is a topic of concern among personnel responsible for the treatment, operation and maintenance of industrial water systems. Legionnaires Disease is a form of pneumonia, albeit an uncommon one. The causitive agent is a unique bacterial organism, Legionella pneumophilia. Over 20 sero groups or strains of Legionella have been identified, with the first four groups associated with 95% of the disease outbreaks. Infection is caused by inhaling mist or airborne dust which is fine enough to pass deep into the lungs. There is no evidence that the disease is transmitted by ingestion or direct contact. Legionnaires Disease affects all age groups with the elderly and those individuals with impaired respiratory disease or who suffer some form of immunosuppression affected to a greater degree. Legionella pneumophilia is not receptive to standard bacterial staining techniques and requires unusual concentrations of amino acids and iron for cultivation on laboratory media. Although its cultivation on laboratory media requires special nutritional characteristics, Legionella pneumophilia is widespread in the natural environment, having been isolated from almost every ground and surface water source. There appears to be no specific preference for industrial systems — wherever the environment is conducive to bacterial growth, the presence of Legionella pneumophilia exists. Frequency has been noted near sites of excavation or construction where mud and airborne dust are prevalent. Its occurrence in industrial systems is most likely due to transmission through windborne droplets or particles as the industrial water "washes" the air. There is some evidence that the organism may be washed into pipework during construction operations or enter the system via the makeup water supply. Whether the organisms proliferate and concentrate in the industrial system is not known, but they are able to survive in the industrial water environment. Once a system is colonized, detectable concentrations can be found, particularly when water stagnates or flows intermittently. In industrial water systems, the presence of sludge, scale, corrosion products, algae, biofilm and organic particulates are thought to contribute to the proliferation of Legionella pneumophilia. Further, water temperature is a factor. The organism can multiply in the temperature range of 20-50°C, and while it can be grown in the lab and has been found in the environment at temperatures outside this range, in industrial systems it is less common for it to multiply below 20°C and it does not survive constant temperatures of 60°C and above. Industrial towers readily generate fine water droplets by spraying water onto a packing material through which there is a countercurrent flow of air. Transmission of the organism occurs in the "drift" of the tower as the water droplets are blown upward by the fan. In large modern buildings, such as hospitals, universities, offices and recreational facilities, the industrial tower and the air inlet to the air conditioning system are often situated close together at roof level. When this occurs, the wind may lift the water droplets and carry them toward the air intake vents and open windows of nearby buildings. Laboratory studies conducted by the CDC indicate Legionella pneumophilia does not demonstrate any unusual pattern of resistance to commercial microbiocides. Additional studies conducted by several universities support this fact. However, there is no known practical level of chemical treatment that effectively keeps Legionella pneumophilia out of industrial water systems. The disease need only be initiated from a single organism finding its way into a susceptible individual. Thus, even if a chemical control program were to produce a significant decrease in Legionella pneumophilia organisms present in a industrial water system, the probability of Legionnaires Disease occurring can never be eliminated. A misconception commonly held is that the addition of a microbiocide product which controls the organism is sufficient. This is not so. To remove all risk, complete elimination of the organism must be achieved. This is an impossible task because of the constant reinocculation of industrial systems and the fact that the organism is widespread in the environment. Some water treatment companies and testing laboratories market a service wherein industrial water samples will be evaluated for the presence of Legionella pneumophilia for a fee. Since transmission of the disease is via aerosols into the respiratory tract, testing water samples does not mean the occurrence of the disease has been prevented since Legionella pneumophilia could still exist in the aerosol picked up in the fan stack, drift eliminators or from the fan blades. For controlling Legionnaires Disease in industrial systems, the CDC and the Industrial Tower Institute advise adherence to the policy of implementing an effective microbiological control program to reduce the risk. Emphasis should be placed on selection of the correct nonoxidizing microbiocide product or products that will provide the most effective slime and algae control for a particular system along with an effective oxidizing biocide such as chlorine, bromine or chlorine dioxide. Another helpful suggestion is the minimization of tower plume or drift into air intake to buildings or air conditioning systems whenever possible. |
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