Legionnaires' Disease

Alternate Names: Legionella, Legionellosis

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  • The Facts

    Legionnaires' disease, or legionella pneumonia , is a serious lung infection caused by bacteria of the species Legionella.

    This organism was first discovered in 1976, when 221 delegates at an American Legion conference in Philadelphia fell mysteriously ill. Of the 221 delegates, 34 died. After the possibility of food poisoning had been ruled out, it became apparent that they had inhaled a bacterium that was later named after these first known victims.

    This bacterium had caused a similar outbreak 2 years earlier in 1974 at the same hotel, when 11 delegates became ill, although this outbreak went unnoticed. Other studies that looked at samples from 81 patients who became ill with an unknown respiratory infection at a Washington, DC, psychiatric hospital in 1965 confirmed that Legionella was the culprit. It appears that Legionella had been causing disease for many years before it was discovered, and it only became a serious threat with the arrival of modern, mechanically ventilated buildings.

    Most cases of Legionnaires' disease are isolated and sporadic. Although it occurs most commonly in the late summer and early fall, it can occur at any time of the year.


    Legionella bacteria live in water or wet soil, and they must be inhaled to cause infection. In nature, there is nothing that will spread tiny drops of Legionella-infected water in the air. However, the modern cooling towers or evaporative condensers in large buildings make it easier for these waterborne bacteria to get in the air.

    Such a tower works by spraying water through circulating air. The smaller the drops, the more efficient the cooling, so various devices are used to break large drops into smaller and smaller ones. These towers may be contaminated with limescale and algae, both of which encourage the growth of Legionella.

    In older, badly designed or poorly maintained buildings, evaporated water or tiny droplets can get into air conditioning through breaches in the ducts, especially if these are too close to the cooling tower.

    Contaminated droplets and vapour also exit directly from the top of the cooler, potentially contaminating other towers in nearby buildings. Most cases of Legionnaires' disease appear in large institutional buildings, especially in hotels and hospitals or on cruise ships. It is transmitted via air-conditioning, heating, and cooling systems.

    Legionella can easily survive in hot water tanks. It has been found coming out of shower heads that create spray fine enough to reach the lungs. Electric humidifiers and hospital assisted-breathing equipment have also been responsible for cases of Legionnaires' disease. Occasionally, it's been transmitted outdoors, usually where heavy digging disturbs wet soil.

    The bacteria may also be transmitted by aspiration. Aspiration occurs when liquids or food is accidentally inhaled into the lungs instead of being swallowed and going into the stomach.

    While anyone can get Legionnaires' disease, some people are more susceptible than others. Middle-aged people and seniors are more at risk. The following conditions increase the risk of Legionnaires' disease:

    • cancer
    • chronic kidney disease
    • chronic obstructive pulmonary disease (COPD)
    • corticosteroid use (e.g., prednisone, dexamethasone)
    • diabetes
    • organ transplant
    • smoking
    • anything else that weakens the immune system, such as a viral infection

    Legionnaires' disease doesn't spread from one person to another, so it's not contagious. Evidence also suggests that people can't get it from drinking contaminated water unless it is aspirated and breathed in.

    Symptoms and Complications

    Legionella can cause a mild flu-like infection without pneumonia, called Pontiac fever. Although Pontiac fever is less common than Legionnaires' disease, one of the largest outbreaks of Pontiac fever occurred in Canada. The symptoms are fever, aches and pains, and a headache. A cough, if present, is mild. People usually recover on their own in about one week.

    Usually, Legionella causes pneumonia. Pneumonia is a condition in which pockets in one or both lungs start to fill up with liquid or pus. Pneumonia due to Legionnaires' disease is fatal for about 10% of people infected, but this number may be higher in seniors, people with weakened immune systems, and people who become infected in a hospital.

    The first symptoms of Legionnaires' disease are general unwellness, fatigue, headache, muscle aches, diarrhea, nausea, and abdominal pain. These symptoms typically appear 2 to 10 days after infection. In full-blown Legionnaires' disease, dry cough, high fever, chills, joint pain, and chest pain quickly develop, followed by pneumonia that starts in one lung but often spreads to the other. Some people may also experience confusion, memory changes, and hallucinations.

    Complications due to Legionnaires' disease can include respiratory failure (when the lungs can't provide enough oxygen), kidney failure, and septic shock (when blood pressure drops and not enough oxygen gets to body tissues).

    Making the Diagnosis

    Legionnaires' disease can't be distinguished from other severe forms of pneumonia by symptoms alone. A chest x-ray is used to confirm pneumonia, but other tests are required to determine if it is caused by Legionella. These may involve a urine test and a lab test that looks at sputum or fluid from the lungs.

    Treatment and Prevention

    Antibiotics are needed to treat Legionnaires' disease. Treatment should be continued for 14 days, but in people with a weakened immune system, treatment should be continued for 3 weeks to prevent relapses. The chances of survival depend on the strength of the lungs, the immune system, and the spread of the bacteria within the lungs.

    Nothing can guarantee protection from Legionnaires' disease, but the following can help:

    • avoid viral respiratory infections such as the common cold and influenza
    • disinfect humidifiers, shower heads, hot tubs; and whirlpools on a regular basis
    • don't smoke
    • get an annual flu shot
    • have water-cooling equipment disinfected (not just cleaned) regularly
    • wash your hands after contact with people who have respiratory infections
