Legionnaires' disease

Expert reviewer, Professor Robert Read, Professor of Infectious Diseases
Next review due January 2020

Legionnaires’ disease is a severe form of pneumonia caused by legionella bacteria. You don’t catch it from other people, but by breathing in tiny droplets of contaminated water.

Legionnaires’ disease first got its name in 1976, when there was an outbreak at a convention of the American Legion in Philadelphia. You may have heard about occasional outbreaks in hotels or around water cooling towers.

Legionnaires’ disease is quite rare in the UK. In 2015, 382 people in England and Wales had it. Nearly half of these people caught it abroad. Men are around three times more likely to get it than women. It can affect all ages, but most people with Legionnaires’ disease are over 50.

Legionella bacteria also cause a milder illness called Pontiac fever. See our FAQ below for more information about this form of legionella infection.

Symptoms of Legionnaires' disease

Legionnaires’ disease is a form of pneumonia. Pneumonia is an infection that causes the small air sacs in your lungs (alveoli) and the tissues around them to become inflamed. The first symptoms of Legionnaires’ disease usually appear two to 10 days after you’ve been infected. Early symptoms are similar to those of flu and may include:

  • fever (high temperature) and chills
  • a dry cough
  • muscle pains and aches
  • headaches

If your infection becomes more severe, you may start coughing up green phlegm and become short of breath. You may have chest pain when you breathe. Some people with Legionnaires’ disease get symptoms such as nausea, vomiting, diarrhoea and abdominal (tummy) pain. Your headache may get worse and you may become confused and unable to think clearly.

These symptoms aren’t always caused by Legionnaires’ disease, but if you have them, contact your GP as soon as possible.

Diagnosis of Legionnaires' disease

Your GP will ask you about your symptoms and examine you. They may also ask you about your medical history. As this infection is quite rare it’s important that you tell your GP if you’ve been abroad recently or used a spa or hot tub.

If your GP suspects a chest infection they may organise a chest X-ray and in some cases refer you for further tests. Legionnaires’ disease is usually diagnosed with a urine test. Your urine will be tested to check for legionella antigens (substances that cause your immune system to respond to the infection). You may also have a blood test to measure the amount of antibodies in your blood.

You might have an X-ray of your chest to check whether you have an infection in your lungs. You may have to go to hospital to have this done.

Treatment of Legionnaires' disease

Legionnaires’ disease is treated with antibiotics. To avoid delay, your GP will often prescribe antibiotics for a severe chest infection before the results of any tests are back. They’ll choose antibiotics that work for most chest infections. However, legionella bacteria may not respond to the usual antibiotics given for chest infections. This is why it’s important to contact your GP if your symptoms get worse or don’t improve. And it’s why, when the tests come back, your GP may need to change your antibiotics.

The sooner you start your treatment, the less likely you are to develop any serious complications. Always read the patient information leaflet that comes with your medicine carefully, and if you have any questions ask your pharmacist.

If your infection is severe or you’re more at risk of complications, your GP may admit you to hospital. Here you may receive your antibiotic treatment through a drip in your arm.

You’ll usually start to feel better within three to five days. Most people with mild or moderately severe Legionnaires’ disease make a full recovery, but how long it takes can vary from person to person. This may depend on how severe your condition is, how quickly you start treatment and whether you develop any complications. Be patient – it may be six months before you feel back to normal.

Causes of Legionnaires' disease

Legionnaires’ disease is caused by legionella bacteria infecting your lungs. The bacteria can be found naturally in freshwater lakes and streams. They usually only cause a health problem when they get into man-made water supplies where they grow and spread. They grow best in warm temperatures. You usually get infected with legionella by breathing in tiny droplets of this contaminated water, not directly from another person.

Examples of potential sources of legionella bacteria include:

  • showers and taps
  • cooling systems (air conditioning units) including cooling towers
  • spa baths and hot tubs that aren’t drained after use
  • ornamental fountains (that store or recirculate water)
  • hot water systems

Outbreaks of Legionnaires’ disease can take place when these water sources become contaminated with legionella bacteria. About half of all cases of Legionnaires’ disease are associated with travelling abroad to warmer countries and from exposure in an aeroplane or hotel. You can also catch Legionnaires’ disease if you’re in hospital and there is an outbreak.

Anyone can catch Legionnaires’ disease. However, you’re more likely to get it if:

Complications of Legionnaires' disease

Most people with Legionnaires’ disease are treated successfully.

If Legionnaires’ disease is left untreated, it can stop your lungs from working properly. If this happens, there won’t be enough oxygen passing from your lungs into your blood.

Another complication of Legionnaires’ disease is sepsis. This is a severe reaction that can happen if you have a bacterial infection which affects your whole body. You might develop septic shock – your blood pressure drops and so your organs don’t receive enough blood to work properly. You’ll need immediate hospital care.

Unfortunately, in some cases Legionnaires’ disease can be fatal.

Frequently asked questions

  • No, there’s no vaccine against the bacteria that cause Legionnaires’ disease. The key to prevention is making sure that water systems, especially in large buildings, are well maintained by those who own or control them. In the UK and many other countries there are regulations covering such maintenance.

    Seek medical attention immediately if you think you might have Legionnaires’ disease, and tell your doctor about any foreign travel or potential exposure. Although there’s no vaccine, there are some steps you can take to reduce your risk of catching the disease.

    • If you smoke, try to stop – you’re more likely to get Legionnaires’ disease if you’re a smoker.
    • If you have a home humidifier or spa/hot tub, follow the manufacturer’s instructions about maintenance and cleaning carefully.
    • If you have a weakened immune system, consider avoiding hot tubs or spas if you can’t tell how well they’re maintained. This may include those in hotels abroad or on cruise ships. See our section on causes of Legionnaires’ disease above for more information about who is most at risk.

  • If you’re an employer, you need to follow health and safety guidelines to reduce the risk of Legionnaires’ disease to your employees. These guidelines are set by the Health and Safety Executive (HSE).

    More information

    As an employer you must carry out the following to reduce the risk of Legionnaires’ disease to your employees.

    • Identify and assess any sources of risk. This includes checking whether present conditions could help bacteria to grow – for example, checking that the water temperature is correct.
    • Prepare a plan to prevent or control an outbreak of Legionnaires’ disease.
    • Systems should be regularly monitored and a person should be put in charge of the plan.
    • Keep records of what has been done to reduce the risk.
    • If appropriate, tell your local authority that you have a cooling tower in the workplace.

    These guidelines also apply to people in control of buildings, including landlords.

    Visit the Health and Safety Executive website for much more information about your duties and advice on how to carry them out.

  • Yes, you can catch Legionnaires’ disease from a spa bath if it’s not properly maintained or cleaned regularly.

    More information

    Legionella bacteria are a particular problem in spa baths because the water is kept at a temperature that they can grow in and multiply. The water is usually vigorously stirred up which sends tiny droplets into the air. These droplets may contain the bacteria which you then breathe in.

    Spa baths should be kept clean and monitored regularly to reduce the risk of an outbreak of Legionnaires’ disease. If you have a spa bath at home it’s important to follow the manufacturer’s instructions about cleaning and maintenance. If there’s one where you work there should be clear instructions to follow too.

    You might not be sure whether an unfamiliar spa bath or hot tub has been properly maintained, for example if you’re on holiday. If so, you may choose to avoid using it. You may also wish to be especially careful if you’re at higher risk of getting Legionnaires’ disease. See our section on causes above for more information.

    You can find more information about spa baths and reducing the risks of legionella infection from the Health and Safety Executive website.

  • Pontiac fever and Legionnaires’ disease are both caused by legionella bacteria. Pontiac fever got its name from an outbreak in Pontiac, Michigan in 1968.

    Pontiac fever is a mild flu-like illness. Symptoms can include headaches, fever and muscle aches, but unlike Legionnaires’ disease, you won’t have pneumonia. It often affects previously healthy people and young people.

    Pontiac fever will usually clear up on its own without any treatment. Antibiotics are usually only recommended if you have severe symptoms. Because Pontiac fever has similar symptoms to many other viral illnesses, you can probably have it without knowing that’s what you’ve got.

    Doctors don’t know why some people who inhale legionella bacteria get Legionnaires’ disease and some get Pontiac fever. It’s likely that others get no illness at all.

    If you have flu-like symptoms that are increasing, or you’re concerned about them, contact your GP.

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Related information

    • Legionella infection. BMJ Best practice., last updated 4 April 2016
    • Legionnaires’ disease. Medscape., updated 30 March 2016
    • Legionella and Legionnaires’ disease. PatientPlus., last checked 11 February 2014
    • Sepsis (septicaemia). PatientPlus., last checked 15 April 2016
    • Map of Medicine. Community-acquired pneumonia. International View. London: Map of Medicine; 2016 (Issue 2)
    • Legionnaires’ disease in England and Wales – 2015. Public Health England., published October 2016
    • Pontiac fever. Public Health England., published 22 November 2007
    • Legionella (Legionnaires’ disease and Pontiac fever). CDC, Centers for Disease Control and Prevention., accessed 26 January 2017
    • Legionella and Legionnaires’ disease. Health and Safety Executive., accessed 26 January 2017
    • Control of legionella and other infectious agents in spa-pool systems. Health and Safety Executive, 2017.

  • Reviewed by Dr Kristina Routh, Freelance Health Editor, January 2017
    Expert reviewer, Professor Robert Read, Professor of Infectious Diseases
    Next review due January 2020

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