Legionella and Legionnaires’ disease

What is Legionnaires’ disease?

There are on average about 10 cases of Legionnaires Disease reported each year in Ireland. Legionellosis is the collective name given to the pneumonia-like illness caused by legionella bacteria. This includes the most serious legionnaires’ disease, as well as the similar but less serious conditions of Pontiac fever and Lochgoilhead fever. Legionnaires’ disease is a potentially fatal form of pneumonia and everyone is susceptible to infection. However, some people are at higher risk, including:

  • people over 45 years of age
  • smokers and heavy drinkers
  • people suffering from chronic respiratory or kidney disease
  • anyone with an impaired immune system

The bacterium Legionella pneumophila and related bacteria are common in natural water sources such as rivers, lakes and reservoirs, but usually in low numbers. They may also be found in purpose-built water systems such as cooling towers, evaporative condensers and whirlpool spas.

If conditions are favourable, the bacteria may grow increasing the risks of legionnaires’ disease. Therefore, it is important to control the risks by introducing measures outlined in Legionnaires’ disease – The Control of Legionella bacteria in water systems (L8).

Where does it come from?

Legionella bacteria are widespread in natural water systems, e.g. hot and cold water distribution systems, air conditioning systems, condensers, humidifiers, water taps, showerheads, water fountains, water features, whirlpool baths, Jacuzzis, spas, respiratory therapy equipment, fire sprinkler systems, misting systems, vehicle washes, etc (work and domestic).

How do people get infected?

People can catch legionnaires’ disease by inhaling small droplets of water, suspended in the air, containing the bacteria. Certain conditions increase the risk from legionella, including:

  • water temperature between 20–45 °C, which is suitable for growth
  • creating and spreading breathable droplets of water, e.g. aerosol created by a cooling tower, or water outlets
  • stored and/or re-circulated water
  • a source of nutrients for the organism e.g. presence of sludge, scale or fouling


What are the symptoms?

The illness usually starts with flu-like symptoms including fever, tiredness, headache, and muscle pains. This is followed by a dry cough and breathing difficulties that may progress to a severe pneumonia. Some people also develop diarrhoea or may become confused. Death occurs in 10-15% of otherwise healthy people and may be higher in some groups of patients.

The incubation period for Legionnaire Disease is usually about 2-10 days with symptoms appearing 5-6 days after infection but may take longer.


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