Legionnaires' disease causes a serious pneumonia (lung infection). You get the infection by breathing in droplets of water which contain Legionella bacteria. The earlier the illness is treated with antibiotics, the better the likely outcome. The illness is fatal in some cases. Pontiac fever is a milder illness caused by the same bacterium.
Legionnaires' disease (also called legionellosis) is an uncommon infection caused by a bacterium (germ) called legionella. There are two types of illness which can develop after being infected with this bacterium:
In 1976 an outbreak of pneumonia occurred among American legionnaires (ex-servicemen) who attended a conference. 221 people developed pneumonia, and 34 died. A bacterium (germ) that had never been identified before was found to be the cause and was named Legionella pneumophila (pneumophila means lung-loving). Since then, many different types of Legionella bacteria have been identified. Legionella bacteria live in fresh water and mud and are found all over the world. Most cases of illness are caused by L. pneumophila, the type first identified.
The source of the bacteria in an outbreak is usually a man-made water distribution system where the bacteria have multiplied in great numbers. Warm storage tanks where the water stagnates are ideal for legionella bacteria to multiply. The bacteria thrive in water temperatures between 25° and 45°C - about 35°C seems to be the optimum temperature. This means that Legionnaires' disease can be caught from:
You become infected by breathing in water droplets (aerosol) that are contaminated by many legionella bacteria. An outbreak can affect many people in the same hospital, hotel, office complex or other large building, around the same time.
Individual cases occur from time to time. Where only one person is infected, then the source of the bacteria is not usually found. About 2 in 100 cases of pneumonia that occur in previously well people are thought to be due to legionella bacteria.
Anyone can develop Legionnaires' disease. However, you are more likely to develop Legionnaires' disease (and pneumonia caused by other bacteria) if you are already in poor health. Conditions that make you more at risk of Legionnaires' disease, and more at risk of a serious illness if you contract Legionnaires' disease include:
You are also more at risk of contracting Legionnaires' disease if you are a smoker and if you are aged over 50 years. Note: person-to-person spread of Legionnaires' disease does not occur. This means that you cannot catch Legionnaires' disease from an infected person. You cannot catch Legionnaires' disease from drinking contaminated water either. To get the illness you need to inhale water droplets that contain legionella bacteria.
A chest X-ray can confirm that you have pneumonia. However, pneumonia is common and there are many bacteria and other types of germs that can cause pneumonia. (Legionnaires' disease is an uncommon cause of pneumonia.)
Blood tests and sputum tests are then done to try to identify the bacterium or other germ that is causing the pneumonia. These tests are called cultures because the idea is to try to get the legionella bacteria to grow and multiply, so it can be identified under a microscope. More specialised blood tests can look for antibodies against legionella bacteria in the blood. Antibodies are proteins made by the immune system, which fight infection.
In addition, if Legionnaires' disease is suspected, a urine test can confirm most (but not all) cases of legionella infection. The test, called the legionella urinary antigen test, detects a specific protein that is part of the legionella bacterium. These proteins can be detected in the urine, even several months after you have been treated for Legionnaires' disease.
Sometimes a lumbar puncture (LP), sometimes called a spinal tap, is performed to help diagnosis. Here, a sample of cerebrospinal fluid (CSF), or spinal fluid, is removed from the base of the spine, using a needle and some local anaesthetic to numb the area. CSF is the fluid that flows around the brain and spinal cord, to protect and nourish the nerve tissues. The CSF is tested to see if the legionella bacteria have infected the brain (if this has happened, it is known as meningitis). This test is only rarely done, mainly if you are very unwell, to exclude other serious illnesses, particularly those caused by infections. (See separate leaflet called 'Lumbar Puncture' for more information.)
Legionnaires' disease is a serious illness. If you were previously fit and well, you have about a 1 in 10 chance of dying from Legionnaires' disease. If you were previously unwell, (for example, if you already have a lung disorder) then you have a higher risk of dying if you get Legionnaires' disease.
Possible complications of Legionnaires' disease include:
The outlook is best if the illness is diagnosed as early as possible, and you are treated with antibiotics immediately.
After recovery from Legionnaires' disease, you may get symptoms such as tiredness, poor concentration, cough or mild shortness of breath. These may take several months to settle.
Pontiac fever is a milder illness caused by legionella bacteria. It causes a flu-like illness which lasts just a few days. Symptoms include: fever, shivers, headache, muscle aches, tiredness and a dry cough. However, the infection does not affect the lungs and therefore does not cause pneumonia. Usually, no treatment is needed other than paracetamol or ibuprofen to ease fever and muscle aches.
It is not clear why some people get Legionnaires' disease (with pneumonia) and some people get the milder Pontiac fever when infected with legionella bacteria.
It is also thought that some people who are infected with legionella bacteria do not become ill at all (a subclinical infection).
As mentioned, the main cause of outbreaks of Legionnaires' disease is from contaminated water sources. In the UK, and in many other countries, there are regulations on how to maintain water supplies and air conditioning systems used in large buildings. This keeps the number of legionella bacteria very low to minimise the risk of infection. However, it is very difficult to eradicate this bacterium completely. If you are an employer or a landlord, you have a legal duty to ensure all water systems in your premises are properly maintained.
If you have a home humidifier or spa, follow the manufacturer's advice about maintenance and cleaning.
About 3 in 10 cases of Legionnaires' disease are contracted abroad, usually due to poorly maintained water storage and air conditioning systems in hotels. Always mention any recent foreign travel if you are unwell and see your doctor.
At present, there is no vaccine that can protect against legionella bacteria.
An independent organisation, set up by the Government in 2003 to protect the public from threats to health, due to infectious diseases and environmental hazards. Includes information on Legionnaires' disease.