Legionnaires' Disease

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Definition

Legionnaires' disease is a type of pneumonia caused by Legionella bacteria. The bacterial species responsible for Legionnaires' disease is L. pneumophila. Major symptoms include fever, chills, muscle aches, and a cough that is initially nonproductive. Definitive diagnosis relies on specific laboratory tests for the bacteria, bacterial antigens, or antibodies produced by the body's immune system. As with other types of pneumonia, Legionnaires' disease poses the greatest threat to people who are elderly, ill, or immunocompromised.

Description

Legionella bacteria were first identified as a cause of pneumonia in 1976, following an outbreak of pneumonia among people who had attended an American Legion convention in Philadelphia, Pennsylvania. This eponymous outbreak prompted further investigation into Legionella and it was discovered that earlier unexplained pneumonia outbreaks were linked to the bacteria. The earliest cases of Legionnaires' disease were shown to have occurred in 1965, but samples of the bacteria exist from 1947.

Exposure to the Legionella bacteria doesn't necessarily lead to infection. According to some studies, an estimated 5-10% of the American population show serologic evidence of exposure, the majority of whom do not develop symptoms of an infection. Legionella bacteria account for 2-15% of the total number of pneumonia cases requiring hospitalization in the United States.

There are at least 40 types of Legionella bacteria, half of which are capable of producing disease in humans. A disease that arises from infection by Legionella bacteria is referred to as legionellosis. The L. pneumophila bacterium, the root cause of Legionnaires' disease, causes 90% of legionellosis cases. The second most common cause of legionellosis is the L. micdadei bacterium, which produces the Philadelphia pneumonia-causing agent.

Approximately 10,000-40,000 people in the United States develop Legionnaires' disease annually. The people who are the most likely to become ill are over age 50. The risk is greater for people who suffer from health conditions such as malignancy, diabetes, lung disease, or kidney disease. Other risk factors include immunosuppressive therapy and cigarette smoking. Legionnaires' disease has occurred in children, but typically it has been confined to newborns receiving respiratory therapy, children who have had recent operations, and children who are immunosuppressed. People with HIV infection and AIDS do not seem to contract Legionnaires' disease with any greater frequency than the rest of the population, however, if contracted, the disease is likely to be more severe compared to other cases.

Cases of Legionnaires' disease that occur in conjunction with an outbreak, or epidemic, are more likely to be diagnosed quickly. Early diagnosis aids effective and successful treatment. During epidemic outbreaks, fatalities have ranged from 5% for previously healthy individuals to 24% for individuals with underlying illnesses. Sporadic cases (that is, cases unrelated to a wider outbreak) are harder to detect and treatment may be delayed pending an accurate diagnosis. The overall fatality rate for sporadic cases ranges from 10-19%. The outlook is bleaker in severe cases that require respiratory support or dialysis. In such cases, fatality may reach 67%.

— Julia Barrett

An acute, sometimes fatal respiratory disease caused by a bacterium of the genus Legionella, especially L. pneumophila, and characterized by severe pneumonia, headache, and a dry cough.

[So called because it was first recognized when an outbreak occurred during an American Legion Convention in Philadelphia in 1976.]

A type of pneumonia usually caused by infection with the bacterium Legionella pneumophila, but occasionally with a related species (such as L. micdadei or L. dumoffii). The disease was first observed in an epidemic among those attending an American Legion convention in Philadelphia, Pennsylvania, in 1976. The initial symptoms are headache, fever, muscle aches, and a generalized feeling of discomfort. The fever rises rapidly, reaching 102–105°F (32–41°C), and is usually accompanied by cough, shortness of breath, and chest pains. Abdominal pain and diarrhea are often present. The mortality rate can be as high as 15% in untreated or improperly diagnosed cases. Erythromycin, new-generation fluroquinolones, and rifampicin are considered highly effective medications, whereas the penicillins and cephalosporins are ineffective.

While epidemics of Legionnaires' disease (also referred to as legionellosis) can often be traced to a common source (cooling tower, potable water, or hot tub), most cases seem to occur sporadically. It is estimated that Legionella spp. account for approximately 4% of all community- and hospital-acquired pneumonia. Legionnaires' disease is most fequently associated with persons of impaired immune status. Legionella bacteria are commonly found in fresh water and moist soils worldwide and are often spread to humans through inhalation of aerosols containing the bacteria. Legionnaires' disease is not a communicable disease, indicating that human infection is not part of the survival strategy of these bacteria. Therefore, the legionellae are considered opportunistic pathogens of humans. It is technology (air conditioning) and the ability to extend life through medical advances (such as transplantation and treatments for terminal diseases) that have brought these bacteria into proximity with a susceptible population.

For most humans exposed to L. pneumophila, infection is asymptomatic or short-lived. This is attributed to a potent cellular immune response in healthy individuals. Recovery from Legionnaires' disease often affords immunity against future infection. However, no vaccine exists at the present time. See also Medical bacteriology; Pneumonia.

An acute bacterial pneumonia caused by infection with L. pneumophila and characterized by an influenza-like illness followed within a week by high fever, chills, muscle aches, and headache. Contaminated air conditioning cooling towers and stagnant water supplies, including water vaporizers and water sonicators, may be a source of organisms.

For more information on Legionnaire disease, visit Britannica.com.

Legionnaires' Disease, an acute infection manifested principally by pneumonia, takes its name from its first known victims—military veterans who attended the Pennsylvania state convention of the American Legion in Philadelphia, 21–24 July 1976. Within days after the convention, reports reached the legion's headquarters that a number of attendees who stayed at the Bellevue-Stratford Hotel had died of pneumonia. Before the epidemic subsided, 221 cases had been found—182 Legionnaires and 39 others who had been near the hotel—and 34 had died. The cause at first was unknown, although speculation centered on a chemical toxin or sabotage. In December 1976 Joseph McDade at the Centers for Disease Control in Atlanta, Georgia, demonstrated that a previously unknown bacterium, Legionella pneumophila, was the causative agent.

Although Legionella will not grow in the usual diagnostic media of the laboratory, it is hardy and common in nature. It grows in fresh water, preferring a temperature close to that of the human body and an atmosphere laden with carbon dioxide. It is found in potable water systems on the hot water side and in recirculating towers of heat-exchange systems. At the Bellevue-Stratford it was probably spread through the air-conditioning system. There are many strains of Legionella, which cause diseases of varying severity, including 1 to 2 percent of all pneumonia cases in the United States. At the other end of the Legionella spectrum is Pontiac fever, a nonfatal disease in which pneumonia does not occur.

Bibliography

Fraser, David W. "Legionnaires' Disease." In The Cambridge World History of Human Disease. Edited by Kenneth F. Kiple. Cambridge; New York: Cambridge University Press, 1993.

Lattimer, Gary L., and Richard A. Ormsbee. Legionnaires' Disease. New York: Marcel Dekker, 1981.