Legionnaire's disease

Legionnaire's Pneumonia, Legionellosis

What is Legionnaire's disease?

Legionnaire disease is a type of pneumonia caused by Legionella bacteria, most often Legionella pneumophila. Legionella bacteria can cause a range of illnesses that are collectively known as legionellosis. Legionnaire disease, which got its name after an outbreak of the disease occurred among hotel guests attending an American Legion meeting in Philadelphia in 1976, is the best known and most serious of these diseases. In that original outbreak, more than 200 people became ill and 34 people died. The bacteria that caused the illness was identified about six months later.

Another disease caused by the Legionella bacteria is Pontiac fever. Although this is more common than Legionnaire disease, it produces only mild symptoms and often clears of its own accord without treatment. Pontiac fever does not result in pneumonia.

The U.S. Centers for Disease Control and Prevention (CDC) estimates that between 8,000 and 18,000 people are hospitalized with Legionnaire disease in the United States every year. However, because Legionnaire disease is difficult to distinguish from other types of pneumonia, it may be difficult to diagnose and be underreported. The number of people infected each year with Legionnaire disease may be much higher than is currently estimated.

The fatality rate of Legionnaire disease may range from 5 to 30 percent, according to the CDC. It is even higher among hospitalized patients and people with compromised immune systems. According to the National Institutes of Health, the fatality rate can reach almost 50 percent among patients who develop the disease while hospitalized.

Both forms of legionellosis (Legionnaire disease and Pontiac fever) are contracted by coming into contact with water particles that contain Legionella bacteria. Legionella bacteria occur naturally in the environment and can be found in ponds, streams and lakes. The bacteria also thrive in warm water such as that found in hot water tanks, large air-conditioning systems, hot tubs and cooling towers.

Legionellosis can occur when people breathe in contaminated water vapor from a hot tub or air conditioning system. It can also be contracted by drinking contaminated water. Unlike many types of pneumonia, Legionnaire disease cannot be spread from person to person.

Most healthy people exposed to the Legionella bacteria do not develop Legionnaire disease. The likelihood of a person contracting the disease after exposure to the bacteria depends on several factors, including:

  • Contamination level (amount of Legionella bacteria in water)
  • Susceptibility of person exposed to contaminated water
  • Intensity of exposure (e.g., how often or for how long exposed)
The disease is more prevalent in the summer and early fall, although it can occur any time of the year. Legionnaire disease is rare in children. Although outbreaks involving numerous people are sometimes reported in the media, the disease most often occurs sporadically, affecting one or two people at a time. When outbreaks occur, they tend to take place in large buildings that include living quarters, such as hotels or hospitals, where the bacteria may be spread from air conditioning systems or contaminated showerheads.

How is it diagnosed?

Symptoms of Legionnaire's Disease

Individuals complain of high fever, chills, headaches, muscle aches, chest pain, a dry cough, weakness and shortness of breath. As the disease progresses, they may complain of abdominal pain and diarrhea.

Physical exam The physical findings include high fever, slow pulse (bradycardia), severe pneumonia, and altered mental status.

Tests may include chest x-ray and serologic blood testing. The white blood count will be elevated (leukocytosis). A urine specimen and lung fluid sample will be examined for signs of the disease. Because the bacterium does not show up in ordinary sputum cultures, a special sputum culture or stain test will be necessary.

How is Legionnaire's disease treated?

Hospitalization is often required, during which time the individual is given erythromycin, an antibiotic intravenously for several days. Some individuals require supplemental oxygen during this time. After the individual stabilizes, oral antibiotic therapy must be continued for an additional fourteen to twenty-one days.

Additional Information


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What might complicate it?

Complications include lung abscess, respiratory failure, low blood pressure, shock, and kidney failure. In addition, any underlying lung disease or immunological problems may make Legionnaire's disease more resistant to treatment.

Predicted outcome

Most individuals recover from Legionnaire's Disease if it is diagnosed early enough. However, some outbreaks have had mortality rates between fifteen and twenty percent. Roughly one-quarter of individuals who are immunodeficient die.


Some symptoms of Legionnaire's disease are identical to those of pulmonary embolism.

Appropriate specialists

Internist and lung specialist.

Notify your physician if

  • You or a family member has symptoms of Legionnaire's disease.
  • The following occur during or after treatment:
    • Temperature spike to 102°F (38.9° C).
    • Severe chest pain, despite treatment.
    • Increased shortness of breath.
    • Dark or bluish nails, lips or skin.
    • Blood in the sputum.