What is Legionnaires' disease? Also known as Legionellosis, it is a form of pneumonia.
Legionellosis is an infection caused by the bacterium Legionella pneumophila. The disease has two distinct forms: Legionnaires' disease, the more severe form of infection which includes pneumonia, and Pontiac fever, a milder illness. Legionnaires' disease acquired its name in 1976 when an outbreak of pneumonia occurred during a convention of the Pennsylvania Department of the American Legion in Philadelphia at the Bellevue Stratford Hotel .
This was the first known outbreak of the disease. In that outbreak, approximately 221 people contracted this previously unknown type of bacterial pneumonia, and 34 people died. The source of the bacterium was found to be contaminated water used to cool the air in the hotel's air conditioning system.
Later, the bacterium causing the illness was named Legionella.
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Legionnaires' disease is most often contracted by inhaling mist from water sources such as whirlpool baths, showers, and cooling towers that are contaminated with Legionella bacteria. There is no evidence for person-to-person spread of the disease.
Symptoms of Legionnaires' disease include fever, chills, and a cough that may or may not produce sputum. Other symptoms include abdominal pain, diarrhea, and confusion. This list of symptoms, however, does not readily distinguish Legionnaires' disease from other types of pneumonia. Legionnaires' disease is confirmed by laboratory tests that detect the presence of the bacterium, Legionella pnuemophila, or the presence of other bacteria in the family Legionellaceae. It is the most often treated with the antibiotic drug Erythromycin.
Although Legionnaires' disease has a mortality rate of 5 to 15 percent, many people may be infected with the bacterium that causes the disease, yet not develop any symptoms. It is likely that many cases of Legionnaires' disease go undiagnosed.
Legionnaires' disease can be viewed as an example of how our physical environment affects our health. Relative humidity, temperature, and other environmental factors can alter the incidence and the fatality rates of infectious diseases, including Legionnaires' disease. For example, cooling towers and evaporative condensers of large air conditioning systems have been associated with outbreaks of the disease, and the highest incidence of Legionnaires' disease occurs in the warmest months of the year, the time when air conditioning systems are used the most.
An estimated 10,000 to 15,000 persons get Legionnaires' disease in the United States each year. An additional unknown number are infected with the Legionella bacterium and have mild symptoms or no illness at all.
Outbreaks of Legionnaires' disease have received the most media attention; however, most often the disease occurs as single, isolated cases not associated with any recognized outbreak. Outbreaks are usually recognized in the summer and early fall, but cases may occur year-round. About 5% to 15% of known cases of Legionnaires' disease have been fatal.
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Patients with Legionnaires' disease usually have fever, chills, and a cough, which may be dry or may produce sputum. Some patients also have muscle aches, headache, tiredness, loss of appetite, and, occasionally, diarrhea. Laboratory tests may show decreased function of the kidneys. 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 and do not have pneumonia. They generally recover in 2 to 5 days without treatment.
The time between exposure and onset of illness for Legionnaires' disease is 2 to 10 days; for Pontiac fever, it is shorter, generally a few hours to 2 days.
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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 appropriate tests.
Several types of tests are available. The most useful are detecting the bacteria in sputum, finding Legionella antigens in a urine sample, and comparing antibody levels to Legionella in two blood samples obtained 3 to 6 weeks apart.
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People of any age may get Legionnaires' diasease, 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 persons whose immune system is suppressed by diseases such as cancer, kidney failure requiring dialysis, diabetes, or AIDS. Those that take drugs that suppress the immune system are also at higher risk.
Pontiac fever most commonly occurs in persons who are otherwise healthy.
Erythromycin is the antibiotic currently recommended for treating persons with Legionnaires' disease. In severe cases, a second drug, rifampin, may be used in addition. Other drugs are available for patients unable to tolerate erythromycin.
Pontiac fever requires no specific treatment.
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Outbreaks of legionellosis have occurred after persons have inhaled aerosols that come from a water source (e.g., air conditioning cooling towers, whirlpool spas, showers) contaminated with Legionella bacteria. Persons may be exposed to these aerosols in homes, workplaces, hospitals, or public places. Infection cannot be acquired from another person with legionellosis, and there is no evidence of persons becoming infected from auto air conditioners or household window air-conditioning units.
Legionella organisms can be found in many types of water systems. However, the bacteria reproduce to high numbers in warm, stagnant water (95-115oF), 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. The disease likely occurs worldwide.
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Improved design and maintenance of cooling towers and plumbing systems to limit the growth and aerosolization of Legionella organisms are the foundations of legionellosis prevention.
During outbreaks, CDC and health department investigators seek to identify the source of disease transmission and recommend appropriate prevention and control measures, such as decontamination of the water source. Current research will likely identify additional prevention strategies.
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