© 2003 BY CRC PRESS LLC CHAPTER 11 Legionella and Cooling Towers Martha J. Boss and Dennis W. Day CONTENTS 11.1 Legionella Pneumophila 11.2 Legionnaires’ Disease 11.3 Pontiac Fever 11.4 Legionellosis: Probable vs. Confirmed 11.4.1 Culture 11.4.2 Urine Antigen Test 11.4.3 Direct Fluorescent Antibody Staining 11.4.4 Serology (Antibody Titers) 11.5 Transmission 11.6 Source Identification 11.7 Contaminated Water Sources 11.8 Monitoring Air 11.9 Water 11.10 Physical Survey and Water Sampling Protocol 11.10.1 Water Sampling Procedure 11.10.2 Cooling System Sampling Sites 11.10.3 Hospital Sampling Sites 11.10.4 Swabs 11.10.5 Sample Transportation 11.10.6 Water Sampling Guidelines 11.10.7 Microbiological Analysis 11.11 Interpreting Sample Results 11.12 Community Health Concerns 11.13 Investigations 11.13.1 Level One Investigation 11.13.2 Level Two Investigation 11.13.3 Ongoing Outbreak 11.14 Cooling Towers, Evaporative Condensers, and Fluid Coolers 11.14.1 Inspection and Maintenance 11.14.2 Biocide 11.14.3 Sump Treatment © 2003 BY CRC PRESS LLC 11.14.4 Drift Eliminators and Other Design Features 11.14.5 Cleaning Frequency 11.14.6 Wisconsin Cleaning Protocol 11.14.7 Recordkeeping 11.15 Domestic Hotwater Systems 11.15.1 Maintenance 11.15.2 Control 11.16 Coldwater Systems 11.16.1 Plumbing Lines 11.16.2 Dental Water Lines 11.16.3 Water Tanks 11.17 Heating, Ventilation, and Air Conditioning Systems 11.17.1 External Sources 11.17.2 Internal Sources 11.17.3 Design 11.17.4 Operation and Maintenance 11.18 Employee Awareness Program 11.18.1 Sample Letter to Employees 11.18.2 Sample Interview with Employees Calling in on Sick Leave 11.18.3 Sample Information Sheets for Legionnaires’ Disease 11.18.4 Legionnaires’ Disease Case Identification 11.18.5 Sample Health Surveillance Questionnaire for Legionellosis 11.18.6 Sample Physician Survey Questionnaire for Legionellosis 11.18.7 Sample Epidemiological Questionnaire 11.19 Water Treatment Protocols for Facilities with Legionnaires’ Outbreak Issues 11.19.1 Cooling Towers and Evaporative Condensers 11.19.2 Domestic Water Systems 11.19.3 Tepid Water Systems 11.19.4 Domestic Coldwater Systems 11.19.5 Heating, Ventilation, and Air-Conditioning Air-Distribution Systems 11.19.6 Humidifiers and Misters Resources This case study illustrates the investigative and remediation principles for dealing with Legionella in cooling towers and associated systems. The OSHA technical manual describes the Legionella threat as follows: Legionella pneumophila is often present in hot water tanks, washing systems, and pools of stagnant water, but health effects are not observed until the contaminants become aerosolized within the building confinements. 11.1 LEGIONELLA PNEUMOPHILA Legionella pneumophila was first identified in 1977 by the CDC as the cause of an outbreak of pneumonia that caused 34 deaths at a 1976 American Legion Convention in Philadelphia. L. pneumophila had undoubtedly caused previous pneumonia outbreaks, but the slow growth and special growth requirements of the organism prevented earlier discovery. The diseases produced by Legionella are called legionellosis. More than 34 species of Legionella have been identified, and more than 20 are linked with human diseases. © 2003 BY CRC PRESS LLC Legionella pneumophila causes the pneumonia known as Legionnaires’ disease and the flu-like Pontiac fever and has also been implicated in wound infections, pericarditis, and endocarditis without the presence of pneumonia. The factors that cause the same organism to produce two illnesses with major differences in attack rate and severity are not known. The L. pneumophila bacteria are Gram-negative rods that exist in a number of distinguishable serogroups. Each sero - group contains further subtypes that have different surface structures on the cell membrane and can be distinguished by special tests. Evidence indicates that some Legionella serogroups are more virulent than others. L. pneumophila serogroup 1 is the most frequently identified form of the bacterium isolated from patients with Legionnaires’ disease. Other serogroups and subtypes of the bacterium are frequently isolated from water sources. Serogroups 4 and 6 are the next most frequently linked with disease. 11.2 LEGIONNAIRES’ DISEASE Legionnaires’ disease has an incubation period of 2 to 10 days. Severity ranges from a mild cough and low fever to rapidly progressive pneumonia and coma. Early symptoms include malaise, muscle aches, and slight headache, while later symptoms include high fever (up to 105°F), a dry cough, and shortness of breath; gastrointestinal symptoms, including vomiting, diarrhea, nausea, and abdominal pain, are common. The disease is treated with erythromycin or a combination of erythromycin and rifampin. Legionnaires’ disease is frequently characterized as an opportunistic disease that most frequently attacks individuals who have an underlying illness or weakened immune system. The most suscep - tible include: • The elderly •Smokers • Immunosuppressed patients • Patients with chronic obstructive pulmonary disease (COPD) • Organ transplant patients • Persons taking corticosteroid therapy 11.3 PONTIAC FEVER Pontiac fever is a nonpneumonia, flu-like disease associated with, and likely caused by, the Legionella bacterium. Pontiac fever has an attack rate of 90% or higher among those exposed and a short incubation period of 1 to 3 days. Complete recovery usually occurs in 2 to 5 days without medical intervention. 11.4 LEGIONELLOSIS: PROBABLE VS. CONFIRMED A probable case of Legionnaires’ disease is a person who has experienced an illness clinically compatible with Legionnaires’, has a single antibody titer of 256 or higher, and can be associated with a population of individuals who have experienced confirmed cases of the disease (outbreak). A confirmed case of Legionnaires’ disease requires a physician’s diagnosis of pneumonia based on a chest x-ray and/or positive laboratory test results. A laboratory test is necessary for confirmation because the symptoms and x-ray evidence of Legionnaires’ disease resemble those of other types of pneumonia. Various methods are used to confirm the presence of the disease. © 2003 BY CRC PRESS LLC 11.4.1 Culture The definitive laboratory methods of confirming the disease presence include culturing viable Legionella cells from sputum, bronchial washing, or autopsy on special media. Further cultured cell identification can be used to identify the species and serogroup. Special tests may determine isolate subtypes. Test sensitivity to detect the disease is reported to be about 70%. 11.4.2 Urine Antigen Test The detection of antigen from L. pneumophila in the urine is considered a reliable measure of the disease. Antigenic materials may include L. pneumophila cells or portions of these cells in the urine during and after the disease. Presence of antigen in the urine is a strong legionellosis disease indicator. A patient may have a positive response for several months following the disease. Test sensitivity is limited because the only commercially available urinary antigen test detects only serogroup 1 forms of L. pneumophila. Fortunately, 80 to 90% of the clinically diagnosed cases are caused by serogroup 1. The Centers for Disease Control and Prevention (CDC) recommends only the radioimmunoassay (RIA) test because the latex antigen (LA) test has a high false-positive rate. The absence of a positive urinary test is not proof that a patient did not have Legionnaires’ disease but merely indicates the absence of antigen in the urine at the time of the test. 11.4.3 Direct Fluorescent Antibody Staining Direct fluorescent antibody (DFA) staining of lung aspirates can detect L. pneumophila. This test is frequently negative during the initial stages of the disease, as few organisms are present in the aspirate or sputum, and it requires an antigen-specific reagent. Due to the multitude of serogroups and subtypes of L. pneumophila, a test will be negative if the exact antigen-specific reagent is not included. 11.4.4 Serology (Antibody Titers) An increase in the antibody level in the infected person’s serum occurs several weeks after the onset of the disease. Pontiac fever also produces an elevated antibody titer, but the flu-like symptoms do not match those of Legionnaires’ disease. A fourfold increase in the antibody titer coupled with a physician’s diagnosis of pneumonia is considered a reliable disease indicator. The titer is measured by comparing the antibody level 4 to 8 weeks after onset (convalescent titer) to an initial (acute) titer at the beginning of the disease. Frequently, only convalescent titers have been measured from individuals who have had symptoms of the disease. For situations in which these cases are associated with an outbreak of Legionnaires’ disease, a single titer of 256 to 1 or higher is generally used as a presumptive indication of disease (probable case). Antibody strength is determined by the number of serum dilutions that elicit a positive antibody response and the reciprocal value of the number of dilutions is the antibody titer. For example, an antibody titer of 256 means a positive antibody test of the patient’s serum following serial dilutions of 1:2, then 1:4, then 1:16, etc., until the 1:256 dilution point is reached. The indirect fluorescent antibody (IFA) test is the accepted diagnostic tool for dem - onstrating L. pneumophila exposure. Another widely used antibody response test is the enzyme- linked immunosorbent assay method (ELISA). The CDC believes that direct comparison of the results of IFA and ELISA is not reliable, as insufficient data are available to compare the two. The ELISA method has gained wide medical acceptance as a useful means of demonstrating exposure to Legionella. © 2003 BY CRC PRESS LLC 11.5 TRANSMISSION The relative likelihood of contracting Legionnaires’ disease is dependent on: • Water source contamination levels • Susceptibility of the person exposed • Intensity of exposure to the contaminated water Disease transmission usually occurs via inhalation of a water aerosol contaminated with the organism. Aspiration of contaminated water into the lungs may also causes the disease. In the Philadelphia Legionnaires’ disease outbreak, the cooling tower of the hotel was identified as the likely source of the disease, although domestic water sources were not evaluated. The disease has been associated with domestic hotwater systems in a number of outbreaks. 11.6 SOURCE IDENTIFICATION L. pneumophila bacteria are widely distributed in water systems; tend to grow in biofilms or slime on the surfaces of lakes, rivers, and streams; and are not eradicated by the chlorination levels normally used to purify domestic water systems. Low and even nondetectable levels of the organism can colonize a water source and grow to high concentrations under the proper conditions. Conditions that promote growth of the organism include: •Heat •Sediment •Scale • Supporting (commensal) microflora in water •Algae •Amoebae •Protozoa • Other bacteria Support occurs as these organisms provide nutrients (algae, flavobacteria, and Pseudomonas) or harbor the L. pneumophila bacteria (amebae and protozoa). Because of L. pneumophila bacteria’s ability to remain viable in domestic water systems, this bacteria is capable of rapid multiplication under these conditions: • Stagnation • Temperatures between 20 and 50°C (68 to 122°F), with an optimal growth range of 35 to 46°C (95 to 115°F) • pH between 5.0 and 8.5 • Sediment, which tends to promote growth of commensal microflora • Microorganisms 11.7 CONTAMINATED WATER SOURCES Water sources that frequently provide optimal conditions for growth include: • Cooling towers • Evaporative condensers • Fluid coolers that use evaporation to reject heat • Industrial processes that use water to remove excess heat © 2003 BY CRC PRESS LLC • Domestic hotwater systems with water heaters that operate below 60°C (140°F) and deliver water to taps below 50°C (122°F) • Humidifiers and decorative fountains that create a water spray and use water at temperatures favorable to growth • Spas and whirlpools • Dental water lines, which are frequently maintained at temperature above 20°C (68°F) and some- times as warm as 37°C (98.6°F) for patient comfort • Stagnant water in fire sprinkler systems • Warm water for eye washes and safety showers Water stored below 20°C (68°F) is generally not a source for amplified L. pneumophila levels; however, high levels of bacteria have been measured in the water supplying ice machines. The amplification source was thought to be heat from the icemaker condenser. No cases of Legionnaires’ disease have been linked to consumption of ice made from contaminated water. 11.8 MONITORING AIR An air sample applied to special culture plates by a sampler sometimes demonstrates the presence of the organism in the air; however, negative results are frequent because of the difficulty in maintaining the viability of the organism on the culture plates. Special culture plate material and sample handling must occur in order to increase the air sampling reliability. 11.9 WATER Analysis of water samples from a source suspected of being contaminated with L. pneumophila is a valuable means of identifying potential disease sources. A qualified microbiological laboratory experienced in Legionella detection can determine the number of organisms present in colony forming units (CFU) per volume of water and identify the different serogroups. 11.10 PHYSICAL SURVEY AND WATER SAMPLING PROTOCOL • Obtain or prepare a simple schematic diagram of the water services. • Record the following locations: • Incoming supply and/or private source • Storage tanks, water treatment systems, and pumps • Water heaters and boilers • All cooling towers, evaporative condensers, and fluid coolers • Any evaporative cooling systems or humidifiers • Ornamental fountains, whirlpools, eyewashes, safety showers, or other water sources within or near the facility • Record the type and locations of: • Fittings used (e.g., taps, showers, valves) • Pipework materials • All systems served by the cooling tower water, including sump tanks, condensers, and indirect evaporative cooling coils in air handling units • Trace the service route from the point of entry of the water supply. • Assess the condition of: •Pipes • Jointing methods • Insulation • Heat sources © 2003 BY CRC PRESS LLC • Insulation in water storage tanks • Disconnected fittings • Dead legs • Check for cross-connections with other services. Once you have identified these features, take water samples from: • The incoming water supply • Each storage tank and water heater • A representative number of faucets for each of the hot and cold water systems in the facility • All cooling towers, evaporative condensers, humidifiers, spas, showers • Water entering or leaving any other type of fitting or piece of equipment under particular suspicion Do not overlook any potential water sources in the building. Water should be sampled from: • Ice machines • Hand spray bottles • Decorative fountains • Plastic injection-molding equipment 11.10.1 Water Sampling Procedure Wear appropriate personal protective equipment (PPE), including respiratory protection. Do not flush the system to be sampled before collecting samples. Use sterile sampling containers (provided by the analytical laboratory) that have been autoclaved at 121°C for 15 minutes and are made of polypropylene. 11.10.1.1 Water A 1-L sample is usually preferable. The minimum sample amount is 250 mL. Sampling bottles that contain sodium thiosulfate at a concentration of 0.5 cc of 0.1-N solution of sample water are preferred. Sodium thiosulfate inactivates any residual halogen biocide. 11.10.1.2 Temperature Measure the temperature of the sampled water. Do not measure the temperature by placing the thermometer in the sample container. When measuring the temperature from faucets, showers, and water fountains, measure the water stream flowing from the water source. Record the initial water temperature, the amount of time necessary to run the water for the temperature to stabilize, and the final temperature. To avoid cross-contamination of the samples, sanitize the thermometer with isopropyl alcohol before measuring the temperature of each sample. 11.10.1.3 Transportation As soon as possible after collection, water samples and swabs should be transported to and processed in a laboratory proficient at culturing water specimens for Legionella species. Samples may be transported at room temperature but must be protected from temperature extremes. 11.10.1.4 Analysis Test samples for the presence of Legionella species by using semiselective culture media. Use standard laboratory procedures. Detection of Legionella species antigen by the DFA technique is not suitable for environmental samples. Use of the polymerase chain reaction (PCR) for identifi - cation of Legionella species is recommended as a screening tool. © 2003 BY CRC PRESS LLC 11.10.2 Cooling System Sampling Sites Collect samples of sludge, slime, or sediments, particularly where accumulations occur. Sam- pling sites include: • Cooling towers • Make-up water (water added to system to replace water lost by evaporation, drift, and leakage) • Basin (area under tower for collection of cooled water) • Sump (section of basin from which cooled water returns to heat source) • Heat sources • Chillers • Humidifiers • Swamp coolers • Building water services • Evaporative condensers 11.10.3 Hospital Sampling Sites Hospital sampling sites include: • Potable water systems • Incoming water mains • Water softeners • Holding tanks/cisterns • Water heater tanks (inflow and outflow sites) • Potable water outlets (faucets or taps, showers), especially outlets located in or near patients’ rooms • Humidifiers (nebulizers) • Bubblers for oxygen • Water used for respiratory therapy equipment • Decorative fountains • Irrigation equipment • Fire sprinkler system (if recently used) • Whirlpools/spas 11.10.4 Swabs When obtaining swab samples always used prepackaged sterile swabs. Collect culture-swabs of the internal surfaces of faucets, aerators, and showerheads. Use sterile, screw-top container, such as a 50-cc plastic centrifuge tube, submerge each swab in 5–10 cc of sample water taken from the sampling location. 11.10.4.1 Swab Sampling Sites Swab samples should be obtained from the following locations: • Potable water systems • Faucets (proximal to aerators) • Faucet aerators • Shower heads • Internal components of cooling towers (e.g., splash bars and other fill surfaces) • Areas with visible biofilm accumulation © 2003 BY CRC PRESS LLC 11.10.4.2 Domestic Water Heaters Take a sample of water from the bottom drain. Collect a sample of water from the outlet pipe if the plumbing provides for access. 11.10.4.3 Faucets and Showers Collect a before-flush, initial-flow sample of water. This sample is intended to indicate the contamination level at the sample point or fitting. Collect an after-flush sample of water when the maximum temperature has been reached. The final sample should reveal the quality of the water being supplied to the sample point or fitting. Collect sterile swab samples from faucets or shower heads: • Remove the fitting. • Vigorously swab the interior. • Swab samples may be positive for Legionella even when water samples from the source are negative. 11.10.4.4 Cooling Towers Take a sample from the incoming supply to the tower and from any storage tanks or reservoirs in the system (e.g., chilled-water return tanks or header tanks). Take a sample from the basin of the cooling tower at a location distant from the incoming make-up water and another sample from the water returning from the circulation system at the point of entry to the tower. Take a sample of any standing water in the condensate trays or from the cooling coils. 11.10.4.5 Humidifiers, Swamp Coolers, and Spas Take a sample from the water reservoirs. Sample the incoming water supply if it is accessible. Take swabs of showerheads, pipes, and faucets and rehydrate from water taken from the sampling site. Swab areas of scale build-up (e.g., remove showerheads, faucet screens, and aerators). 11.10.5 Sample Transportation Prepare samples for shipment carefully: • Wrap vinyl tape clockwise around the neck of each bottle to hold the screw cap firmly in place. • Seal the interface between the cap and the bottle. • Wrap absorbent paper around bottles. • Place the bottles in resealable plastic bags. • Place the sealed plastic bags in an insulated container (styrofoam chest or box). Samples should be stored at room temperature (20 ± 5°C) and processed within 2 days. Samples should not be refrigerated or shipped at reduced temperature and should be protected from tem - perature extremes such as sunlight or other external heat or cold sources. Ship samples to the laboratory using overnight delivery. If shipping samples on a Friday, make arrangements for weekend receipt. 11.10.6 Water Sampling Guidelines The contaminant levels requiring action vary depending upon the source of exposure, based on the assumption that some routes or exposure result in a greater dose to the lung. Humidifiers and © 2003 BY CRC PRESS LLC similar devices such as misters and evaporative condensers, which produce an aerosol mist that can be directly inhaled, should be controlled to lower levels of contaminant. The numbers provided in Table 11.1 are only guidelines, and the goal is zero detectable Legionella in a water source. Levels of Legionella equal to or greater than the values in the table constitute a need for the action described. 11.10.7 Microbiological Analysis 11.10.7.1 Cultured Samples The process of growth and isolation can be time consuming, and results typically require 7 to 14 days from the time of submission. Water samples are cultured on special buffered charcoal yeast extract (BCYE) culture media. Selective isolation processes to eliminate other microbial overgrowth can determine the number of colony-forming units of L. pneumophila per milliliter of water. Cultured samples can also be analyzed to identify specific serogroups. Matching the serogroup and subtype of organism in the patient to that found in a water source is considered strong evidence of an associated link. 11.10.7.2 Direct Fluorescent Antibody Direct fluorescent antibody (DFA) conjugate tests stain the organism with a fluorescent dye and can be useful in screening water samples. DFA tests, however, are unable to distinguish between live and dead bacteria. The DFA test may also have some cross-reactivity with other bacteria. Results can be available in one or two days. Use caution in interpreting the results, because the potential exists for both false-positive and false-negative results. 11.10.7.3 DNA Amplification A relatively new method for rapid, specific organism detection employs a PCR process to amplify and then detect portions of DNA unique to L. pneumophila. Results can be produced in one day. Preliminary evidence indicates that sensitivity and specificity are comparable to those of cell culture. 11.11 INTERPRETING SAMPLE RESULTS Because total eradication of Legionella may not be possible, an acceptable control strategy is to minimize the number of organisms present in a water source. A private consulting firm and microbiological laboratory (PathCon, Inc., Norcross, GA) has introduced suggested guidelines for control based on the number of colony-forming units of L. pneumophila per milliliter of water. These guidelines vary depending on the water source, a recognition by the authors of the PathCon guidelines that dose is related both to the potential for exposure and to concentration. For example, Table 11.1 Levels of Legionella (CFU/mL water) Location Action 1 a Action 2 b Cooling tower 100 1000 Domestic water 10 100 Humidifier 1 10 a Action 1 is prompt cleaning and/or biocide treatment of the system. b Action 2 is immediate cleaning and/or biocide treatment and taking prompt steps to prevent employee exposure. [...]... 122°F) 11. 14.1 Inspection and Maintenance Visual inspection and periodic maintenance are the best ways to control growth of Legionella and related organisms Good maintenance is necessary both to control Legionella growth and for effective operation The system should be properly monitored and maintained to prevent build-up of scale and sediment and biofouling, all of which support Legionella growth and. .. pneumophila growth 11. 19.1.1 Personal Protective Clothing Provide personal protective equipment to workers who will be performing the disinfection to prevent their exposure to chemicals used for disinfection and aerosolized water-containing Legionella Protective equipment may include full-length protective clothing, boots, gloves, goggles, and a full- or half-face mask that combines a HEPA filter and chemical... the value of operating with low sump-water temperatures Sumpwater temperatures depend on: • • • • Tower design Heat load Flow rate Ambient dry-bulb and wet-bulb temperatures Under ideal conditions, sump-water temperatures in evaporative devices approach the ambient wet-bulb temperature © 2003 BY CRC PRESS LLC 11. 14.4 Drift Eliminators and Other Design Features High-efficiency drift eliminators are essential... of the make-up water to the system should be readily available Written procedures for proper operation and maintenance of the system should include standard operating procedures for using: • Scale and corrosion inhibitors • Antifoaming agents • Biocides or chlorine Logbooks should list dates of inspections and cleanings, water-quality test results, and maintenance © 2003 BY CRC PRESS LLC 11. 15 DOMESTIC... system humidifiers can be hazards 11. 17.2.1 Heated-Pan Humidifiers Heated-pan humidifiers use a heat source to evaporate water from a pan open to the airstream Intermittent use of the device coupled with a warm pan of water may support Legionella growth Contaminant-free water is essential 11. 17.2.2 Direct Steam-Type Humidifiers Direct steam-type humidifiers inject boiler-generated steam directly into... with the air-distribution systems These designs should also include fail-safe measures and control or monitoring devices Use steam or atomizing humidifiers instead of units that use recirculated water (atomizing humidifiers must have contaminant-free water) Do not use raw steam from the central heating boiler that contains corrosion inhibitors and antiscaling chemicals 11. 17.4 Operation and Maintenance... should always consider operation and maintenance during design; inadequate access ways, portals, clean-out devices, and water or chemical sources can defeat the best intentions 11. 18 EMPLOYEE AWARENESS PROGRAM An employee awareness program informs employees of a potential outbreak, educates employees about the disease risk and consequences, and should be part of a level-two investigation or response... supply and shipping of the required number of sterile sample containers with the appropriate laboratory 11. 13.2.2 Step 2: Second Walkthrough Survey and from Step 1 Water Sampling During this step, visual assessments are verified and sampling completed 11. 13.2.3 Step 3: Employee Awareness Program Development and Sick Leave Monitoring Ensure that employees understand the early disease symptoms and seek... make-up water source, and recirculated water from the HVAC system at its point of return to the unit 11. 19.1.3 Cleaning Mobilization • • • • Shut off cooling-tower If possible, shut off the heat source Shut off fans, if present, on the cooling tower and evaporative condenser (CT/EC) Shut off the system blowdown (purge) valve Shut off the automated blowdown controller, if present, and set the system controller... ensure continued safe and proper operation Inspect the equipment monthly, and drain and clean quarterly Treat circulating water to control microorganisms, scale, and corrosion Maintenance should include systematic use of biocides and rust inhibitors, preferably supplied by continuous feed Monthly microbiologic analyses should be performed to ensure control of bacteria Document operation and maintenance in . 11. 10.5 Sample Transportation 11. 10.6 Water Sampling Guidelines 11. 10.7 Microbiological Analysis 11. 11 Interpreting Sample Results 11. 12 Community Health Concerns 11. 13 Investigations 11. 13.1. Maintenance 11. 15.2 Control 11. 16 Coldwater Systems 11. 16.1 Plumbing Lines 11. 16.2 Dental Water Lines 11. 16.3 Water Tanks 11. 17 Heating, Ventilation, and Air Conditioning Systems 11. 17.1 External. BY CRC PRESS LLC CHAPTER 11 Legionella and Cooling Towers Martha J. Boss and Dennis W. Day CONTENTS 11. 1 Legionella Pneumophila 11. 2 Legionnaires’ Disease 11. 3 Pontiac Fever 11. 4 Legionellosis: