What is Legionella?

Legionella are bacteria that live in water that can cause two types of illness in humans: Legionnaires’ disease and Pontiac fever.

Legionnaires’ disease is a serious respiratory illness that results in pneumonia. The name Legionnaires’ disease comes from an outbreak of pneumonia that killed 29 people at an American Legion Convention in Philadelphia in 1976.

In Canada, the average number of reported cases of Legionnaires’ disease is generally less than 100 per year. However the actual number of cases is thought to be much higher, as many people with pneumonia may not be tested for infection with Legionella.

Pontiac fever is a milder illness causing flu-like symptoms but not pneumonia and was first described in Pontiac, Michigan in the early 1970s. People with Pontiac fever generally recover in two to five days without treatment.

How common is Legionnaires’ Disease?

Legionnaires’ disease occurs throughout North America, as well as in Asia, Africa, Australia, Europe and South America. Sporadic cases and outbreaks most commonly occur in summer and autumn, although cases can occur throughout the year.

Legionnaires’ disease is not a common disease, and the risk of getting it is generally quite low. Cases of Legionnaire’s disease may be difficult to detect because very few of the people exposed to the bacteria get infected. For instance, if 100 people are exposed to Legionella, fewer than 5 of them will get Legionnaire’s disease. On the other hand, in an epidemic of Pontiac fever, if 100 people are exposed, 95 of them are likely to become sick.

What are the symptoms of Legionnaires’ disease?

In addition to pneumonia, symptoms can include fever, cough, muscle pain and headache. Symptoms start within 2 to 14 days of infection, and can last for several months.

If symptoms persist, consult your health care provider. Most cases can be treated successfully with antibiotics. Effectiveness of the treatment varies with age and overall health, and can also depend on how quickly the person receives the right medical treatment. 

Who’s at risk for Legionnaires’ disease?

The following people are at greater risk of developing the disease:

  • individuals over 40 years of age;
  • smokers;
  • alcoholics;
  • people with chronic lung or kidney disease;
  • people with diabetes; and
  • people with weakened immune systems due to conditions such as cancer or an organ transplant.

People in certain occupations, such as those who do maintenance work on large air-conditioning systems, may also be at increased risk of exposure to Legionella bacteria.
Generally, more men than women contract Legionnaires’ disease. It is uncommon for people younger than 20 years of age to get Legionnaires’ disease.

How do you get Legionnaires’ Disease?

Legionella bacteria are found in natural water sources such as lakes, rivers, ponds, and streams generally at levels that are too low to cause disease in people. Cases of Legionnaires’ disease have occurred in many different settings, including homes, commercial buildings, spas, cruise ships and health care facilities. Legionnaires’ disease cannot be spread from one person to another. The number of bacteria determines the risk for humans.

Certain conditions that may be present in buildings and homes promote the growth of the bacteria. These include stagnant water, warm water temperatures (especially between 20°C and 50°C) and the presence of biofilm, scale and sediment. These conditions may be found in:

  • cooling towers, such as those used with the air conditioning systems of large buildings;
  • whirlpool bathtubs, hot tubs and public spas;
  • plumbing systems (including water heaters, faucets and showers) either in the home or in larger buildings; and
  • humidifiers.

When water contaminated with larger quantities of Legionella is released into air in the form of droplets or mist, people may be exposed to the bacteria by breathing in the contaminated air.

Cooling towers can be a good environment for Legionella to grow and these towers can release large quantities of water droplets to the air. This is why they are often linked to outbreaks of Legionnaires’ disease. Droplets with high concentrations of Legionella may then enter a building if, for example, the ventilation system intake is near the cooling tower. In some outbreaks however, Legionella bacteria from cooling towers appear to have survived and spread in the air over a distance of several kilometers. In these outbreaks, cases were not linked to a particular building or public space.

How can I reduce my risk?

The risk of getting Legionnaires’ disease is generally quite low.

In your home, you can reduce the risks through proper maintenance of all mist-producing devices, such as shower heads, hot tubs, whirlpool bathtubs, and humidifiers. Make sure you clean and disinfect these devices regularly according to manufacturer directions. Keeping your home water heater at a suitable temperature (a minimum of 60°C) will also help prevent the growth of Legionella. However, to reduce the risk of scalding, it is important that temperature of the water be no higher than 49°C at the tap. A qualified plumber should be contacted to install mixing valves to control the tap water temperature.

Consult the national case definition for additional information.

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