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Legionella[Last Updated: Feb 24, 2021]

Legionella bacteria are opportunistic premise plumbing pathogens (OPPP) that can cause illnesses collectively referred to as legionellosis. Legionellosis includes Pontiac fever, extrapulmonary legionellosis and Legionnaires’ disease, a severe pneumonia named after an outbreak that resulted in 29 deaths following an American Legion Convention in Philadelphia in 1976. Legionella is naturally occurring waterborne bacteria that can survive within building water systems and multiply under ideal conditions. These conditions include optimum temperatures for growth (e.g., 25-45°C), stagnant water, lack of disinfectant residual and the presence of biofilms that form in storage containers, pipes or outlets where Legionella can be sheltered from disinfectants and survive or multiply.

Infection occurs via inhalation or aspiration of Legionella contaminated water droplets or aerosols into the lungs. When contaminated water in cooling towers, shower heads, decorative fountains, hot tubs, flushed toilets, or other mists or sprays, becomes aerosolized, there is a risk of exposure and subsequent illness among susceptible individuals.  Legionellosis is not caused by ingestion but cases of nosocomial legionellosis have been reported in patients drinking ice water or chewing on ice chips from hospital ice machines contaminated by Legionella, followed by aspiration of the bacteria into the lungs. Some of the symptoms of Legionnaires’ diseases are similar to those experienced due to COVID-19 infection, which has raised concerns about the need to maintain surveillance for possible cases of legionellosis during the pandemic. (See more from the NCCEH on Building shutdown and re-opening during the COVID-19 pandemic).

The reported occurrence of legionellosis is on the rise in Canada with the rate of cases per 100,000 persons increasing from an average of 0.29 before 2010 to an average of 1.02 between 2010 and 2018. In 2018 there were over 600 cases in Canada. Smokers, over-50s, and those with compromised immunity are the most at risk of infection and cases occur more often in men than women. The majority of outbreak-related deaths associated with Legionella originate in infected cooling towers.  Major outbreaks in Canada include Toronto in 2005 affecting 135 people and causing 23 deaths and Quebec City in 2012 affecting over 180 people and causing 14 deaths. Other outbreaks in Moncton New Brunswick and London, Ontario in 2019 were also associated with cooling towers. The source of infection can be difficult to trace as shown with a cluster of six cases in New Westminster, B.C., and seven cases in Montreal, Quebec, both in September 2020, for which the possible sources of infection are still under investigation.

The most effective measures to control Legionella risks include actions to reduce survival and growth of the bacteria in building water systems and to reduce opportunities for exposure. Temperature controls, preventing accumulation of stagnant water, adequate disinfection, and regular maintenance and monitoring of water systems can be incorporated into building water management plans to reduce occurrence and improve responses to outbreaks when they occur. It is difficult to say how climate change may impact the occurrence of legionellosis in the future. Increased use of air conditioning systems or rising ambient temperatures in a building’s cold-water systems may increase opportunities for Legionella to survive and grow in those systems. This emphasizes the importance of regular monitoring and having water safety management plans in place for buildings.

The resources listed here are intended to assist environmental and public health professionals and those responsible for managing water systems in buildings to:

  • Understand the health risks associated with exposure to Legionella bacteria;
  • Understand the factors that can lead to Legionella survival and growth in building water systems;
  • Identify the key approaches to managing Legionella risks in buildings;
  • Understand the steps needed to respond to Legionella outbreaks and the corrective actions required.


Selected External Resources

About Legionella

  • Legionella and Legionnaires’ disease (legionellosis) (Walkerton Clean Water Centre, 2020)
    This resource collection includes a broad range of information on legionellosis, prevention of Legionella exposure through effective water management, sampling, standards, surveillance, and outbreak investigations.
  • Legionella (Legionnaires’ disease and Pontiac fever) (Centers for Disease Control and Prevention (CDC), 2019)
    This webpage provides a suitable starting point for general information on legionellosis including causes, spread and risks, signs and symptoms, diagnosis, treatment and complications, prevention, and history of the illnesses. Resources specific to clinicianshealth departments and laboratories can also be accessed through this link.
  • Legionellosis fact sheet (World Health Organisation, 2018)
    This fact sheet provides an overview of the causes, symptoms and suggested public health response to legionellosis. The WHO emphasize that addressing the public health threat posed by legionellosis includes prevention measures such as water safety plans in building water systems to control growth of Legionella and dispersion of aerosols.

Managing Legionella in building water systems

  • Toolkit for controlling Legionella in common sources of exposure (CDC, 2021)
    This toolkit provides actionable information for public health and building owners and managers to control Legionella in common sources of disease outbreaks including potable water systems, cooling towers, hot tubs, decorative water features. Advice for other devices that contain non-sterile water such as irrigation systems, fire suppression systems, safety showers, ice machines, humidifiers etc. is also provided.

  • Legionella quick facts (Public Services and Procurement Canada (PSPC), 2019)
    This fact sheet sets out the five steps for developing a programme to control the growth of Legionella in building water systems applied by PSPC, who operate and maintain a large and diverse portfolio of federal buildings. More detailed information on PSPC’s approach to Legionella control can be found here.

  • Legionnaires’ disease from water systems left idle during the COVID-19 pandemic (Worksafe BC, 2020).
    This risk advisory sets out potential sources of Legionella exposure in the workplace, and steps to reduce risks arising from idle water systems.
  • Public water system characteristics that may affect Legionella occurrence in building water systems. (US EPA, 2020)
    This webinar recording from the US EPA highlights public health impacts and challenges to managing Legionella in drinking water, and potential risk factors in public water supplies including inadequate disinfectant residual, high water age, nutrient availability, corrosion, and poor pipework condition.
  • Turning the Tide: The role of water management to prevent Legionnaires’ disease. (CDC, 2019)
    This webinar recording of the CDC Public Health Grand Rounds from May 2019 includes presentations that examine the connection between Legionella and the built environment, emphasizing the key prevention message that water management programmes can reduce the risk of Legionnaires’ disease and reverse the increase in cases being observed. Approaches to addressing the higher proportion of healthcare acquired cases of legionellosis are discussed.
  • Legionella. Who’s addressing the risks in Canada. (National Research Council of Canada, Health Canada and Public Services and Procurement Canada, 2019)
    This report provides a general overview of Legionella, risk factors, symptoms, and treatment and sets out the roles and responsibilities for managing the risks of Legionella before and during construction of a building, during occupancy and operation of a building and for Legionella case management. This document also lists Provincial and Territorial protocols for Legionella related outbreak prevention, investigation and control.
  • Water quality challenges: Morbidity and mortality associated with building water systems (Environmental Science, Policy & Research Institute, 2018)
    This presentation to the Occupational and Environmental Health Seminar Series outlines challenges in controlling OPPPs including Legionella and key steps for effective building water management. Case studies of different facilities types including healthcare facilities are examined which may be of interest to public health and managers/operators of large or complex buildings.
  • Toolkit: Developing a water management program to reduce Legionella growth and spread in buildings(CDC, 2018)
    This toolkit is a plain language translation of the ASHRAE Standard 188 providing a public health perspective to assist building owners and managers to evaluate whether their buildings require a Legionella water management plan and to identify the necessary process to develop one. The ASHRAE Standard 188 is currently the most widely used industry standard for developing water management programs.
  • Control of Legionella in mechanical systems Standard MD – 15161. (Public Services and Procurement Canada, 2016)
    This document sets out the design, operation, maintenance and testing requirements used by the PSPC to prevent legionellosis in building water systems in federal facilities, which can be adopted by other building managers, maintenance personnel and property owners.
  • Residential hot tubs and pools: Safe water quality (Health Link BC, Feb 2018)
    This fact sheet outlines some key tips for achieving and maintaining disinfection in a hot tub or pool to prevent exposure to bacteria such as Legionella.
  • Preventing Legionnaires’ disease: A training on Legionella water management programs (PreventLD Training) (US CDC and partners, 2018)
    This free online training course is designed to assist public health professionals, building managers and others involved in supply of clean drinking water to understand the steps involved in assessing Legionella hazards and developing a Legionella management program.

Outbreak investigation and control

Peer-reviewed Articles

This list is not intended to be exhaustive. Omission of a resource does not preclude it from having value.


Last updated Feb 24, 2021