Legionellosis (Legionnaires' disease and Pontiac fever)

Legionellosis is a group of infections caused by Legionella bacteria. The bacteria can cause 2 types of illness in humans: Legionnaires’ disease and Pontiac fever.

On this page

• Symptoms of Legionellosis
• Diagnosing Legionellosis
• Treating Legionellosis
• How Legionellosis spreads
• Preventing Legionellosis
• Risks of getting Legionellosis
• Who is most at risk

Symptoms of Legionellosis

Legionnaires’ disease is a serious form of pneumonia, whereas Pontiac fever is a milder illness with flu-like symptoms, but not pneumonia.

Symptoms may start within 2 to 14 days of exposure. General symptoms include:

• fever
• chills
• headache
• confusion
• muscle pain
• loss of appetite
• symptoms of pneumonia, such as:
  • cough
  • shortness of breath
• abdominal pain with or without diarrhea

The severity can range from mild to severe. In some cases, symptoms can progress quickly to organ failure and death.

Untreated Legionnaires’ disease usually worsens during the first week. Once symptoms appear, it’s important to consult your health care provider.

Diagnosing Legionellosis

Health care providers can order different types of laboratory tests to confirm if your infection is caused by Legionella bacteria.

Treating Legionellosis

Most cases of Legionnaires’ disease can be treated successfully with antibiotics. Effectiveness of the treatment depends on:

• age
• overall health
• how quickly you receive the right medical treatment 

People with Pontiac fever generally heal on their own within a week.

How Legionellosis spreads

While Legionella bacteria levels are usually too low to cause disease in people, they appear in natural water sources, such as:

• lakes
• rivers
• ponds
• streams

Cases of legionellosis have occurred in and around many different settings, including:

• spas
• homes
• cruise ships
• cooling towers
• health care facilities
• commercial buildings

The higher the number of bacteria, the higher the risk for humans.

There have been a small number of cases of legionellosis after exposure to the bacteria in soil, like:

• potting mixes
• gardening compost

Legionella cannot be spread from human to human.

How bacteria grows in water systems

Certain conditions may be present in buildings and homes that promote the growth of the Legionella bacteria. These conditions include:

• the presence of:
  • sediment (small pieces of material that settle at the bottom of water)
  • biofilm (a group of small microbes that stick together on the surface of water)
• not enough disinfectant in the water
• stagnant water (slow or no water movement)
• warm water temperatures (especially between 20°C and 50°C)

Such conditions may be found in:

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

Water contaminated with larger quantities of Legionella may be released into the air in the form of tiny droplets or mist. People are exposed to the bacteria by breathing in the contaminated air. This is why cooling towers are often linked to outbreaks of legionellosis.

In some outbreaks, Legionella spread through the air over several kilometres. In those situations, people got sick even though they hadn’t been inside a specific building or public place.

Preventing Legionellosis

In your home, you can reduce the chance of illness through proper maintenance of all mist-producing devices, such as:

• hot tubs
• humidifiers
• shower heads
• whirlpool bathtubs

Make sure you clean and disinfect these devices regularly according to manufacturer directions. Keeping your home water heater at a suitable temperature (minimum of 60°C) will also help prevent the growth of Legionella.

Learn more:

• Water temperature and burns or scalds

Risks of getting Legionellosis

The risk of getting Legionnaires’ disease or Pontiac fever is generally low.

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. This is because many people with pneumonia may not be tested for infection with Legionella.

Legionnaires’ disease occurs worldwide. Cases and outbreaks most commonly occur in summer and autumn, although they can occur throughout the year.

Who is most at risk

Those at greater risk of developing the disease include people:

• who smoke
• who are over 50 years of age
• with a history of high-risk alcohol use
• with chronic diseases, such as:
  • diabetes
  • liver failure
  • kidney failure
  • chronic lung disease like chronic obstructive pulmonary disease (COPD) and emphysema
• with weakened immune systems due to medical conditions, such as:
  • cancer
  • organ transplant
• with weakened immune systems due to medications, such as chemotherapy, immunomodulators or immunosuppressants
  • these are medications that control or suppress the immune system

Occupations involving tasks like maintenance on large air conditioning systems may also increase people's risk of exposure to Legionella bacteria.