Santé Montréal

Legionnaires' Disease

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Legionella bacteria are very widespread and occur naturally in fresh water and wet soils, as well as in water sources in a variety of artificial environments, especially where hot water is involved, such as household faucets, water heaters, shower heads (inside surfaces) and cooling towers.

Legionellosis is not a common disease and your risk of contracting it is quite low if you are in good health. People become infected by breathing in mist or steam containing the bacteria. Legionellosis is NOT transmitted by person-to-person contact. Exposure can occur in the community (e.g., at home), in the workplace (e.g., during maintenance work on air-conditioning systems), on trips and in health care settings. 

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Legionnaires' Disease is the most frequent form of Legionellosis, and the most serious, because it causes an acute lung infection (pneumonia). As is the case for other types of pneumonia, it is generally accompanied by:

  • high fever
  • shivers
  • coughing
  • fatigue
  • muscle pain
  • loss of appetite

The diagnosis is made on the basis of lab tests. Legionnaires' Disease can be treated with antibiotics.

The groups who run a higher risk of developing Legionnaires' Disease are:

  • people aged 50 and over;
  • smokers;
  • heavy drinkers of alcohol; 
  • people suffering from a chronic disease (e.g.,  kidney,  lung or  heart disease or diabetes);
  • immune-suppressed individuals (e.g., as a result of cancer or a cancer treatment,  following a tissue or organ transplant); and
  • people who have recently undergone surgery.

Sources of infection

Exposure to a water source containing Legionella in the community (e.g., cooling tower, water heater), at work (e.g., spa, dentist's office), on trips (e.g., whirlpool baths, hot tubs) or in a healthcare setting (e.g., inhalation therapy equipment, humidifiers).

Illness and complications

Legionnaires' Disease can cause serious complications and even death. While the proportion may vary depending on age and state of health, the mortality rate is one to two out of every ten people infected. However, in hospitalized patients, Legionnaires' Disease can kill up to four to eight people out of ten.

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When to Consult

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Protection and prevention

  • At home, keeping the water heater at an adequate temperature helps maintain the quality of domestic hot water and reduce the risk of Legionellosis, especially for vulnerable individuals (see the Feuille d’information pour le public) (in French). The risks of infection can be further reduced by properly maintaining any devices that cause water droplets to be sprayed into the air (e.g., shower heads, whirlpool baths, spas, humidifiers) in accordance with the manufacturer's instructions or the advice of a qualified professional.
  • In large buildings, the key to controlling the risk of infection from these sources is a combination of good operating, maintenance and housekeeping practices (e.g., in hotels) and good infection control policies and guidelines  (e.g., in hospitals).
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Risk factors

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Help and Resources

Communiquez avec Info-Santé au 811 ou consultez un médecin.

Plus d'information :

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People at Risk

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Dernière mise à jour le : 2015.05.22