What is Legionella

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. rivers and ponds. However, the conditions are rarely right for people to catch the disease from these sources. Outbreaks of the illness occur from exposure to legionella growing in purpose-built systems where water is maintained at a temperature high enough to encourage growth, e.g. cooling towers, evaporative condensers, spa pools, and hot water systems used in all sorts of premises (work and domestic).

How do people get it?

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

While most cases of legionnaires’ disease are the result of infections caught in the UK, a number of cases occur abroad. Useful advice on travel can be found from the European Working Group for Legionella Infections (EWGLI).

What are the symptoms?

The symptoms are similar to those of flu, i.e. high temperature, fever and chills, cough, muscle pains and headache. In a severe case, there may also be pneumonia, and occasionally diarrhoea, as well as signs of mental confusion. Legionnaires’ disease is not known to spread from person to person.

What measures are there to control legionella?

To prevent exposure to the legionella bacteria, you as a duty holder must comply with legislation that requires you to manage, maintain and treat water systems in your premises properly. This will include, but not be limited to, appropriate water treatment and cleaning regimes.

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