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Legionellosis (Legionnaires' Disease and Pontiac Fever)

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Legionellosis Menu

Outbreak Response

Defining an Outbreak

  • CDC defines a Legionella outbreak as two or more cases of confirmed Legionnaires' disease occurring in one location.
  • Among Legionellosis cases reported to CDC, less than 20 percent are described as an outbreak.
  • Outbreaks may occur in hospitals, on cruise ships, in hotels, and in buildings and workplaces, often from cooling towers and water used in drinking and showering.

Source: Legionella (Legionnaires' Disease and Pontiac Fever), Centers for Disease Control and Prevention (CDC).

Defining an Outbreak

  • CDC defines a Legionella outbreak as two or more cases of confirmed Legionnaires' disease occurring in one location.
  • Among Legionellosis cases reported to CDC, less than 20 percent are described as an outbreak.
  • Outbreaks may occur in hospitals, on cruise ships, in hotels, and in buildings and workplaces, often from cooling towers and water used in drinking and showering.

Source: Legionella (Legionnaires' Disease and Pontiac Fever), Centers for Disease Control and Prevention (CDC).

Introduction

A Legionellosis outbreak exists in a workplace when medically confirmed Legionellosis cases, either Legionnaires' disease or Pontiac Fever, are associated with a common exposure. This usually means confirming two or more Legionellosis cases within a six week period at a particular site or in close proximity to it. Cases may include individuals working at a certain company or facility or at a job site near a cooling tower, for example. They may also include a combination of different people, including non-workers (e.g., visitors, customers) with a common—although not always readily apparent—exposure. If at least one case occurred within the most recent 30 days, assume that the outbreak is still in progress.

Legionellosis is a nationally reportable disease. State and local health departments follow Centers for Disease Control and Prevention (CDC) guidelines, including the Council of State and Territorial Epidemiologists (CSTE) case definition, for identifying Legionellosis cases. Legionellosis cases may be identified in different ways. Usually, an ill worker is diagnosed by his or her own healthcare provider (e.g., doctor, nurse practitioner), who reports the case to the Health Department. However, employers may learn that one or more of their employees has been diagnosed with Legionellosis before the health department knows. In these cases, the employer should report cases to the state or local health department; and some jurisdictions may require reporting Legionellosis cases under their state laws or local ordinances. It is extremely important to maintain confidentiality and protect workers' health information in all communications between workers, employers, healthcare providers, and health authorities. If employers have any questions about confidentiality procedures, they should obtain advice from their medical department or consultant. See HHS's Health Information Privacy page for more information.

Upon learning of an outbreak, the state or local health department will conduct an investigation to identify additional Legionellosis cases. Employers should cooperate with the health department to identify employee cases by reviewing sick leave records and conducting a Worker Awareness Program. A short Health Surveillance Questionnaire may be helpful in identifying cases. This questionnaire must be administered confidentially by medical department personnel or a medical consultant.

An employer’s medical department or medical consultant may also consider restricting workers at increased risk of Legionellosis infection. Employers may also receive work restrictions from workers’ healthcare providers. Once a potential source has been identified, the process may need to be shut down and/or cordoned off to prevent the spread of infection. Employers should seek advice from industrial hygiene consultants, medical consultants, the health department, and/or OSHA, especially if the investigation requires environmental sampling beyond the employer’s capabilities.

Employers should take prompt action to provide maximum protection to workers and eliminate the hazard. Actions include a worker awareness program, immediate investigation of potential outbreak sources (including water system sampling) and implementation of control measures to prevent additional exposures from all potentially contaminated water systems. Employers should continue to cooperate with their state and/or local health departments and OSHA (in the event that an OSHA inspection is conducted) throughout the outbreak response.

Worker Awareness Program

A worker awareness program is intended to inform workers about the potential outbreak and to educate them about Legionnaires' disease. Worker notification and awareness is critical to:

  • Ensuring workers understand early symptoms and seek medical attention promptly.
  • Identifying potential new cases as early as possible.
  • Addressing worker concerns about the disease.

Employers should implement a worker awareness program whenever there is a Legionellosis outbreak. The local or state health department and/or a medical consultant can help employers design and implement such a program. Elements of a worker awareness program include:

  • Conducting an initial worker training session to provide basic information about the disease along with information about planned and ongoing actions to investigate the problem.
  • Implementing an ongoing general information service to provide updates and answer workers’ questions.
  • Providing access to confidential medical and psychological counseling services to assist workers after an outbreak has occurred.

The following links are to a sample letter and supplemental information on Legionnaires' disease that employers may find helpful for informing workers about a Legionellosis outbreak. The supplemental information is in a question and answer format, easily adaptable by employers for their situation.

Facility Inspection and Water System Sampling

Conduct a thorough building inspection to identify all potential Legionella sources. Consider seeking additional expertise from environmental health and safety professionals who can help identify when, where, and how people with suspected or confirmed Legionellosis may have been exposed.

Potential contamination sources include:

Inspecting domestic water systems, including water heaters and any water sources maintained above 20°C (68°F) that could be aerosolized, is a key part of an outbreak response following a Legionellosis outbreak. Ongoing maintenance practices to prevent such events from occurring are also essential. OSHA. Inspecting domestic water systems, including water heaters and any water sources maintained above 20°C (68°F) that could be aerosolized, is a key part of an outbreak response following a Legionellosis outbreak. Ongoing maintenance practices to prevent such events from occurring are also essential. OSHA.

Photo Credit: OSHA

Inspecting domestic water systems, including water heaters and any water sources maintained above 20°C (68°F) that could be aerosolized, is a key part of an outbreak response following a Legionellosis outbreak. Ongoing maintenance practices to prevent such events from occurring are also essential.

  • Water-cooled, heat-transfer systems;
  • Domestic water systems;
  • Humidifiers and misters; and
  • Any water sources maintained above 20°C (68°F) and that could be aerosolized.

Focus inspection efforts in areas where the workers with confirmed Legionellosis were most likely exposed.

Assume all potential contamination sources are contaminated and treat them accordingly. This includes hot and cold domestic water systems, cooling towers and humidifiers.

Workers performing water system sampling must wear appropriate personal protective equipment (PPE), as described on the Control and Prevention page.

Before flushing or disinfecting water in these suspected sources, ensure water system samples are collected for analysis. These samples are important in confirming that a Legionella hazard exists in the workplace, and, if so, what water systems or parts of water systems are contaminated. Depending on outbreak, these samples also can help determine:

  1. The predominant L. pneumophila strains (i.e., serogroups and subtypes) in each water source.
  2. The amount or concentration of Legionella in the system, often measured in colony forming units (CFU) per milliliter (ml) or other water unit.
  3. Whether the the serogroup and subtype of bacteria found in individuals with Legionellosis match the bacteria found in a water source (a strong evidence supporting an associated epidemiological link). This information helps identify Legionellosis outbreak sources if the L. pneumophila serogroup is identified in the afflicted worker population.

Consider seeking additional expertise from environmental health and safety professionals and/or the local or state health department. Most employers are not equipped to take their own water system samples. The Hazard Recognition page provides additional information and resources on sampling protocols.

Ensure water system sampling, testing, disinfection, and other outbreak response activities are conducted safely and in accordance with currently accepted procedures. Always follow the hierarchy of controls—starting with elimination/substitution and engineering controls before implementing administrative controls and safe work practices and personal protective equipment (PPE)—to prevent or reduce worker exposures to Legionella during these activities. The following section discusses control measures and other actions for responding to outbreaks related to several different types of water systems.

Implement Controls to Prevent Additional Exposure

Controlling exposure to Legionella during an outbreak might not require facility shutdown. Temporary provisions to isolate the affected water system can allow work to continue, and may include:

  • Supplying bottled water for drinking instead of drinking from the domestic cold water system.
  • Shutting down water heaters to eliminate access to hot water in the domestic system.
  • Using temporary cooling towers.
  • Using portable heaters, air conditioners, and fans instead of the contaminated HVAC system.

Building maintenance, engineering staff, or a third-party expert consultant who has working knowledge about a water system's design and current operation can explain how the system operates and the proper procedure for a controlled shutdown.

If the hazard assessment indicates there may be Legionella in the system, it is critical to take control actions immediately, even if the sampling results are negative. Water system sampling can produce false negatives and the sampling protocol can miss a contaminated portion within the system. Just because Legionella was not found during pre sampling does not ensure that the facility’s water system will remain negative.

Once samples are analyzed, the laboratory will provide colony forming units (CFU) counts per either volume or area for the water and/or biofilm samples. Results indicating Legionella is present are the most reliable and can lead to definitive statements about the source. However, negative results are not reliable since there is a high probability for Legionella to die, thus not counted, in the sampling process. A negative Legionella result from a water system sample should not lead to the assumption that the water system is no longer a potential source for Legionella in the workplace. The analyzing laboratory may also provide CFU counts per either volume or area for water and/or biofilm samples.

Most outbreaks from cooling towers and evaporative condensers are associated with high Legionella concentrations, at least 1,000 CFU/mL or more in the implicated source.1 Typical Legionella concentrations average 160 CFU/mL (range <1 to 1,500) in a potable water system associated with outbreaks. Legionella concentrations as few as 10 CFU/mL in fogger reservoir water may have caused disease in people in immediate direct contact with the mist.

The expandable boxes below contain operating system-specific water treatment protocols for potential contamination sources following inspection and water system sampling.

Use the decontamination procedure below to treat all cooling towers or evaporative condensers that have been tested and found to be contaminated with Legionella at detectable levels, or when Legionella contamination is suspected.

Step 1. Clean and disinfect the entire cooling system, including attached chillers and/or storage tanks (sumps), following the Cooling Technology Institute's emergency disinfection procedure:

  • Remove heat load from the cooling system, if possible.
  • Shut off fans associated with the cooling equipment.
  • Shut off the system blowdown.
  • Keep make-up water valves open and operating.
  • Close building air intake vents in the cooling tower’s vicinity (especially those downwind) until after the decontamination procedure is complete.
  • Continue to operate the recirculating water pumps.
  • Add a biocide sufficient to achieve free residual halogen at 25 to 50 ppm.
  • Add an appropriate bio-dispersant (and antifoam, if needed).
  • Maintain 10 ppm free residual halogen for 24 hours. Add more biocide, as needed, to maintain the 10 ppm residual.
  • Monitor the system pH. Since the halogen disinfection rate slows at higher pH values, add acid, if needed, and/or reduce cycles to achieve and maintain a pH less than 8.0 (for chlorine-based biocides) or 8.5 (for bromine-based biocides).
  • Drain the system to a sanitary sewer. If the unit discharges to a surface water source under a permit, dehalogenation is necessary prior to any discharges.
  • Refill the system and repeat from beginning of Step 1.
  • Inspect after the second drain-off. If a biofilm is evident, repeat the procedure.
  • When no biofilm is obvious, mechanically clean the tower fill, tower supports, cell partitions and sump.
  • Refill and recharge the system to achieve a 10 ppm free halogen residual. Hold this residual for one hour and then drain the system until no turbidity exists.
  • Refill the system and charge with appropriate corrosion and deposit control chemicals, re-establish normal biocontrol residuals and put the cooling tower back into service.

Step 2. Identify and eliminate all water leaks into the cooling water system.

Step 3. Institute maintenance to ensure continued safe and proper operation, as follows:

  • Inspect equipment monthly.
  • Drain and clean quarterly.
  • Treat circulating water to control microorganisms, scale, and corrosion. This should include systematically using biocides and rust inhibitors, preferably supplied by continuous feed, and monthly microbiologic analysis to ensure bacteria control.
  • Document operation and maintenance in a log or maintenance records book.

If any water system sample contains Legionella at 10 or more CFU/mL, take immediate steps to clean the system. These steps may include more frequent biocide application or increased biocide concentration, pH adjustment, additional “shock” water treatments, or any other action to reduce Legionella levels. Take new water system samples and begin the cooling system water testing schedule again.

Use the following decontamination procedure to treat all domestic hot water systems either tested and found contaminated with Legionella at detectable levels or when Legionella contamination is suspected.

Step 1. Disinfect the system using any effective chemical, thermal, or other water treatment method. For example:

  • Heat the water to at least 70°C (158°F) and maintain this temperature for a 24-hour minimum. While maintaining the temperature at 70°C (158°F), continuously flush each faucet in the system with hot water for 20 minutes.
  • Use an accepted chemical disinfectant such as chlorine or an acceptable biocide treatment to decontaminate the water system. Thoroughly flush the water system after treatment to remove all corrosive and possibly hazardous chemicals.
  • Follow any other technique with demonstrated effectiveness and safety.

Step 2. Maintain domestic water heaters at 60°C (140°F) minimum and water delivered at the faucet at 50°C (122°F) minimum. Where maintaining these temperatures is not feasible, control Legionella growth with a safe and effective alternative method (See the Control and Prevention page).

Use the following decontamination procedure if cold water systems are found contaminated with Legionella at detectable levels or when Legionella contamination is suspected.

Step 1. Clean and disinfect all cold water systems, including storage tanks, drinking fountains, water lines, and water outlets, as follows:

  • Use an accepted chemical disinfectant, such as chlorine or other acceptable biocide (see the Control and Prevention page).
  • Use any other technology demonstrated as safe and effective.

Step 2. Ensure that cold water systems are maintained so that conditions do not promote Legionella growth. Maintain temperatures at 20°C (68°F) and keep residual chlorine in the 1-2 ppm range. However, this chlorination level may also corrode metal pipes and containers.

Step 3. Flush all cold water outlets and fountains for four minutes, 12 hours before re-entry.

The most probable condition where a Legionellosis outbreak might link directly with an air handling system would include Legionella-contaminated water continuously entering the system and aerosolizing before delivery to workers or building occupants. Air handling systems may be ruled out as sources after the following:

  • Inspect the entire air distribution system, including return and exhaust systems, for visible water accumulation (e.g., leaks, stagnant water, water damage).
  • Eliminate all water leaks and remove any standing water found in the system. Replace or eliminate any water-damaged insulation in the system.
  • Operate the HVAC system using 100-percent outside air for eight hours before returning the building to normal operation.

There is no reliable detection of Legionella in the air, and Legionella can live only in water. If the ducts are dry, they cannot serve as a Legionella contamination source.

After returning the building to normal operation, keep outside air supply rates as high as possible for one month. At a minimum, operate the system to meet the outdoor air requirements from American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Standard 62.1, Ventilation for Acceptable Indoor Air Quality.

Cold water humidifiers require rigorous maintenance to ensure that the water source does not contribute to potential microbe problems. Since humidifiers discharge into HVAC air distribution systems, inspect for standing water and treat according to the HVAC Air Distribution Systems protocol found on the Control and Prevention page.

Use the following decontamination procedure if Legionella contamination is suspected in water in humidifiers.

Step 1. Disinfect water in piping or reservoirs feeding the humidifier with chlorine or other effective biocides.

Step 2. Implement an adequate maintenance program to reduce Legionella growth. Ensure that water storage temperatures are kept above or below the 20° to 50°C (68° - 122°F) range, and keep the system clean.

Step 3. Before using the humidifier, flush the piping and/or reservoir thoroughly to remove biocides.


1 Shelton, B.G., Flanders, W.D. and Morris, G.K. “Legionnaires' Disease Outbreaks and Cooling Towers with Amplified Legionella Concentrations," Curr. Microbiol., 28, 359-363 (1994).

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