Legionnaires' Disease

Legionnaires' Disease

Legionellosis is an infection caused by the bacterium Legionella pneumophila. The disease got its name in 1976 when an outbreak of pneumonia (an inflammation in the lungs) caused by this newly recognized bacterium occurred among persons attending a convention of the American Legion in Philadelphia. The disease has two distinct forms: Legionnaire’s disease, the more severe form of infection which includes pneumonia; and Pontiac fever, a milder illness.

About 5,000 Legionnaires' cases are reported in the United States each year. Some people can be infected with the Legionella bacterium and have mild symptoms or no illness at all. Outbreaks of Legionnaires' disease receive significant media attention. However, this disease usually occurs as a single isolated case not associated with any recognized outbreak. When outbreaks do occur, they are usually recognized in the summer and early fall, but cases may occur year-round. Death occurs in 10% of Legionnaire’s cases.

Symptoms and Testing

People with Legionnaires' disease usually have fever, chills, and a cough. Some people also have muscle aches, headache, tiredness, loss of appetite, and, occasionally, diarrhea. Chest X-rays often show pneumonia. It is difficult to distinguish Legionnaires' disease from other types of pneumonia by symptoms alone; other tests are required for diagnosis. Persons with Pontiac fever experience fever and muscle aches but do not have pneumonia. They generally recover in two to five days without treatment.

The time between the patient's exposure to the bacterium and the beginning of illness for Legionnaire’s disease is two to 10 days. For Pontiac fever, this time is shorter, generally a few hours to two days.

The diagnosis of legionellosis requires special tests not routinely performed on persons with fever or pneumonia. Therefore, a physician must consider the possibility of legionellosis in order to obtain the right test. The most useful tests detect the bacteria in sputum or find Legionella antigens in urine samples.


Macrolides (e.g., azithromycin) and quinolones (e.g., ciprofloxacin, levofloxacin) are the antibiotics of choice for treating people with Legionnaire’s disease. In severe cases, a second drug, rifampin, may be used as well. Progression of infiltrates as seen on chest x-ray is common despite appropriate antibiotic therapy.

Treatment is usually prolonged and recovery may be slow. Pontiac fever requires no specific treatment.

People at Risk

People of any age may get Legionnaire’s disease, but the illness most often affects middle-aged and older persons, particularly those who smoke cigarettes or have chronic lung disease.

Also at increased risk are people whose immune system is suppressed by diseases such as cancer, kidney failure requiring dialysis, diabetes, or AIDS. People who take drugs that suppress the immune system are also at higher risk.

Pontiac fever most commonly occurs in people who are otherwise healthy.


Legionella organisms can be found in many types of water systems. However, the bacteria reproduce to high numbers in warm water (77°-108° F), such as that found in certain plumbing systems and hot water tanks, cooling towers and evaporative condensers of large air-conditioning systems, and whirlpool spas. Cases of legionellosis have been identified throughout the United States and in several foreign countries. It is believed to occur worldwide.

Outbreaks of legionellosis have occurred after breathing mists that come from a water source (e.g., air conditioning cooling towers, whirlpool spas, and showers) contaminated with Legionella bacteria. People may be exposed to these mists in homes, workplaces, hospitals, or public places. Legionellosis is not passed from person-to-person, and there is no evidence of persons becoming infected from auto air conditioners or household window air-conditioning units. Improved design and maintenance of cooling towers and plumbing systems to limit the growth and spread of Legionella organisms are the foundations of legionellosis prevention. During outbreaks, investigators seek to identify the source of disease transmission and recommend appropriate prevention and control measures, such as decontamination of the water source.

To prevent the spread of Legionella, care should be taken to observe the following:

  • When possible, maintain the temperature of water systems at temperatures hostile to the organism; hold cold water below 68º F, hot water above 140º F (60º C); circulate to return at 122º-124º F (50º-51º C).  Temperatures above 131º F (55º C) are bactericidal.  Take measure to prevent scalding.
  • Maintain whirlpool spas as directed by the manufacturer
  • Use sterile water for flushing medical tubes and for water used to provide humidification or irrigation to breathing equipment such as Neti pots or CPAP machines.  DO NOT USE TAP WATER.
  • Use commercially available car windshield washer solution in motor vehicles.  DO NOT USE TAP WATER.
  • Avoid creating an aerosol that could be inhaled with standing warm water, e.g., spraying with garden hoses that have been kept in the sun.
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