Legionnaires' Disease

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%.

Causes and symptoms

Legionnaires' disease is caused by inhaling Legionella bacteria from the environment. Typically, the bacteria are dispersed in aerosols of contaminated water. These aerosols are produced by devices in which warm water can stagnate, such as air-conditioning cooling towers, humidifiers, shower heads, and faucets. There have also been cases linked to whirlpool spa baths and water misters in grocery store produce departments. Aspiration of contaminated water is also a potential source of infection, particularly in hospital-acquired cases of Legionnaires' disease. There is no evidence of person-to-person transmission of Legionnaires' disease.

Once the bacteria are in the lungs, cellular representatives of the body's immune system (alveolar macrophages) congregate to destroy the invaders. The typical macrophage defense is to phagocytose the invader and demolish it in a process analogous to swallowing and digesting it. However, the Legionella bacteria survive being phagocytosed. Instead of being destroyed within the macrophage, they grow and replicate, eventually killing the macrophage. When the macrophage dies, many new Legionella bacteria are released into the lungs and worsen the infection.

Legionnaires' disease develops 2-10 days after exposure to the bacteria. Early symptoms include lethargy, headaches, fever, chills, muscle aches, and a lack of appetite. Respiratory symptoms such as coughing or congestion are usually absent. As the disease progresses, a dry, hacking cough develops and may become productive after a few days. In about a third of Legionnaires' disease cases, blood is present in the sputum. Half of the people who develop Legionnaires' disease suffer shortness of breath and a third complain of breathing-related chest pain. The fever can become quite high, reaching 104 °F (40 °C) in many cases, and may be accompanied by a decreased heart rate.

Although the pneumonia affects the lungs, Legionnaires' disease is accompanied by symptoms that affect other areas of the body. About half the victims experience diarrhea and a quarter have nausea and vomiting and abdominal pain. In about 10% of cases, acute renal failure and scanty urine production accompany the disease. Changes in mental status, such as disorientation, confusion, and hallucinations, also occur in about a quarter of cases.

In addition to Legionnaires' disease, L. pneumophila legionellosis also includes a milder disease, Pontiac fever. Unlike Legionnaires' disease, Pontiac fever does not involve the lower respiratory tract. The symptoms usually appear within 36 hours of exposure and include fever, headache, muscle aches, and lethargy. Symptoms last only a few days and medical intervention is not necessary.

Diagnosis

The symptoms of Legionnaires' disease are common to many types of pneumonia and diagnosis of sporadic cases can be difficult. The symptoms and chest x rays that confirm a case of pneumonia are not useful in differentiating between Legionnaires' disease and other pneumonias. If a pneumonia case involves multisystem symptoms, such as diarrhea and vomiting, and an initially dry cough, laboratory tests are done to definitively identify L. pneumophila as the cause of the infection.

If Legionnaires' disease is suspected, several tests are available to reveal or indicate the presence of L. pneumophila bacteria in the body. Since the immune system creates antibodies against infectious agents, examining the blood for these indicators is a key test. The level of immunoglobulins, or antibody molecules, in the blood reveals the presence of infection. In microscopic examination of the patient's sputum, a fluorescent stain linked to antibodies against L. pneumophila can uncover the presence of the bacteria. Other means of revealing the bacteria's presence from patient sputum samples include isolation of the organism on culture media or detection of the bacteria by DNA probe. Another test detects L. pneumophila antigens in the urine.

Treatment

Most cases of Legionella pneumonia show improvement within 12-48 hours of starting antibiotic therapy. The antibiotic of choice has been erythromycin, sometimes paired with a second antibiotic, rifampin. Tetracycline, alone or with rifampin, is also used to treat Legionnaires' disease, but has had more mixed success in comparison to erythromycin. Other antibiotics that have been used successfully to combat Legionella include doxycycline, clarithromycin, fluorinated quinolones, and trimethoprim/sulfamethoxazole.

The type of antibiotic prescribed by the doctor depends on several factors including the severity of infection, potential allergies, and interaction with previously prescribed drugs. For example, erythromycin interacts with warfarin, a blood thinner. Several drugs, such as penicillins and cephalosporins, are ineffective against the infection. Although they may be deadly to the bacteria in laboratory tests, their chemical structure prevents them from being absorbed into the areas of the lung where the bacteria are present.

In severe cases with complications, antibiotic therapy may be joined by respiratory support. If renal failure occurs, dialysis is required until renal function is recovered.

Prognosis

Appropriate medical treatment has a major impact on recovery from Legionnaires' disease. Outcome is also linked to the victim's general health and absence of complications. If the patient survives the infection, recovery from Legionnaires' disease is complete. Similar to other types of pneumonia, severe cases of Legionnaires' disease may cause scarring in the lung tissue as a result of the infection. Renal failure, if it occurs, is reversible and renal function returns as the patient's health improves. Occasionally, fatigue and weakness may linger for several months after the infection has been successfully treated.

Prevention

Since the bacteria thrive in warm stagnant water, regularly disinfecting ductwork, pipes, and other areas that may serve as breeding areas is the best method for preventing outbreaks of Legionnaires' disease. Most outbreaks of Legionnaires' disease can be traced to specific points of exposure, such as hospitals, hotels, and other places where people gather. Sporadic cases are harder to determine and there is insufficient evidence to point to exposure in individual homes.

Resources

PERIODICALS

Shuman, H. A., et al. "Intracellular Multiplication of Legionella pneumophila: Human Pathogen of Accidental Tourist?" Current Topics in Microbiology and Immunology 225 (1998): 99.

KEY TERMS

Antibody— A molecule created by the immune system in response to the presence of an antigen. It serves to recognize the invader and help defend the body from infection.

Antigen— A molecule, such as a protein, which is associated with a particular infectious agent. The immune system uses this molecule as the identifying characteristic of the infectious invader.

Culture— A laboratory system for growing bacteria for further study.

DNA probe— An agent that binds directly to a predefined sequence of nucleic acids.

Immunocompromised— Refers to conditions in which the immune system is not functioning properly and cannot adequately protect the body from infection.

Immunoglobulin— The protein molecule that serves as the primary building block of antibodies.

Immunosuppressive therapy— Medical treatment in which the immune system is purposefully thwarted. Such treatment is necessary, for example, to prevent organ rejection in transplant cases.

Legionellosis— A disease caused by infection with a Legionella bacterium.

Media— Substance which contains all the nutrients necessary for bacteria to grow in a culture.

Phagocytosis— The "ingestion" of a piece of matter by a cell.

Legionnaires' disease

Legionnaires' disease is a type of pneumonia caused by Legionellabacteria . The bacterial species re sponsible 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.

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 out-break) 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%.

Causes and symptoms

Legionnaires' disease is caused by inhaling Legionella bacteria from the environment. Typically, the bacteria are dispersed in aerosols of contaminated water . These aerosols are produced by devices in which warm water can stagnate, such as air-conditioning cooling towers, humidifiers, shower heads, and faucets. There have also been cases linked to whirlpool spa baths and water misters in grocery store produce departments. Aspiration of contaminated water is also a potential source of infection, particularly in hospital-acquired cases of Legionnaires' disease. There is no evidence of person-to-person transmission of Legionnaires' disease.

Once the bacteria are in the lungs, cellular representatives of the body's immune system (alveolar macrophages) congregate to destroy the invaders. The typical macrophage defense is to phagocytose the invader and demolish it in a process analogous to swallowing and digesting it. However, the Legionella bacteria survive being phagocytosed. Instead of being destroyed within the macrophage, they grow and replicate, eventually killing the macrophage. When the macrophage dies, many new Legionella bacteria are released into the lungs and worsen the infection.

Legionnaires' disease develops two-10 days after exposure to the bacteria. Early symptoms include lethargy, headaches, fever, chills, muscle aches, and a lack of appetite. Respiratory symptoms such as coughing or congestion are usually absent. As the disease progresses, a dry, hacking cough develops and may become productive after a few days. In about a third of Legionnaires' disease cases, blood is present in the sputum. Half of the people who develop Legionnaires' disease suffer shortness of breath and a third complain of breathing-related chest pain . The fever can become quite high, reaching 104°F (40°C) in many cases, and may be accompanied by a decreased heart rate.

Although the pneumonia affects the lungs, Legionnaires' disease is accompanied by symptoms that affect other areas of the body. About half the victims experience diarrhea and a quarter have nausea and vomiting and abdominal pain. In about 10% of cases, acute renal failure and scanty urine production accompany the disease. Changes in mental status, such as disorientation, confusion, and hallucinations, also occur in about a quarter of cases.

In addition to Legionnaires' disease, L. pneumophila legionellosis also includes a milder disease, Pontiac fever. Unlike Legionnaires' disease, Pontiac fever does not involve the lower respiratory tract. The symptoms usually appear within 36 hours of exposure and include fever, headache, muscle aches, and lethargy. Symptoms last only a few days and medical intervention is not necessary.

Diagnosis

The symptoms of Legionnaires' disease are common to many types of pneumonia and diagnosis of sporadic cases can be difficult. The symptoms and chest x rays that confirm a case of pneumonia are not useful in differentiating between Legionnaires' disease and other pneumonias. If a pneumonia case involves multisystem symptoms, such as diarrhea and vomiting, and an initially dry cough, laboratory tests are done to definitively identify L. pneumophila as the cause of the infection.

If Legionnaires' disease is suspected, several tests are available to reveal or indicate the presence of L. pneumophila bacteria in the body. Since the immune system creates antibodies against infectious agents, examining the blood for these indicators is a key test. The level of immunoglobulins, or antibody molecules, in the blood reveals the presence of infection. In microscopic examination of the patient's sputum, a fluorescent stain linked to antibodies against L. pneumophila can uncover the presence of the bacteria. Other means of revealing the bacteria's presence from patient sputum samples include isolation of the organism on culture media or detection of the bacteria by DNA probe. Another test detects L. pneumophila antigens in the urine.

Treatment

Most cases of Legionella pneumonia show improvement within 12-48 hours of starting antibiotic therapy. The antibiotic of choice has been erythromycin, sometimes paired with a second antibiotic, rifampin. Tetracycline, alone or with rifampin, is also used to treat Legionnaires' disease, but has had more mixed success in comparison to erythromycin. Other antibiotics that have been used successfully to combat Legionella include doxycycline, clarithromycin, fluorinated quinolones, and trimethoprim/sulfamethoxazole.

The type of antibiotic prescribed by the doctor depends on several factors including the severity of infection, potential allergies, and interaction with previously prescribed drugs. For example, erythromycin interacts with warfarin, a blood thinner. Several drugs, such as penicillins and cephalosporins, are ineffective against the infection. Although they may be deadly to the bacteria in laboratory tests, their chemical structure prevents them from being absorbed into the areas of the lung where the bacteria are present.

In severe cases with complications, antibiotic therapy may be joined by respiratory support. If renal failure occurs, dialysis is required until renal function is recovered.

Prognosis

Appropriate medical treatment has a major impact on recovery from Legionnaires' disease. Outcome is also linked to the victim's general health and absence of complications. If the patient survives the infection, recovery from Legionnaires' disease is complete. Similar to other types of pneumonia, severe cases of Legionnaires' disease may cause scarring in the lung tissue as a result of the infection. Renal failure, if it occurs, is reversible and renal function returns as the patient's health improves. Occasionally, fatigue and weakness may linger for several months after the infection has been successfully treated.

Prevention

Since the bacteria thrive in warm stagnant water, regularly disinfecting ductwork, pipes, and other areas that may serve as breeding areas is the best method for preventing outbreaks of Legionnaires' disease. Most outbreaks of Legionnaires' disease can be traced to specific points of exposure, such as hospitals, hotels, and other places where people gather. Sporadic cases are harder to determine and there is insufficient evidence to point to exposure in individual homes.

Resources

books

Edelstein, Paul H., and Richard D. Meyer. "Legionella Pneumonias." In Respiratory Infections: Diagnosis and Management. 3rd ed. James E. Pennington, ed. New York: Raven Press, Ltd., 1994.

Johnson, Caroline C., and Sydney M. Finegold. "Pyogenic Bacterial Pneumonia, Lung Abscess, and Empyema." In Volume 1: Textbook of Respiratory Medicine. 2nd ed., John F. Murray and Jay A. Nadel, eds. Philadelphia: W.B. Saunders Company, 1994.

Koch, A.L. Bacterial Growth and Form. Dordrecht: Kluwer Academic Publishers, 2001.

periodicals

Rossier, O., and N.P. Cianciotto. "Type II Protein Secretion is a Subset of the PilD-dependent Processes that Promote Infection by Legionella pneumophila." Infection and Immunity 68 (2001): 2092-2098.

Shuman, H.A., M. Purcell, G. Segal, L. Hales, and L.A. Wiater. "Intracellular Multiplication of Legionella pneumophila: Human Pathogen of Accidental Tourist?" Current Topics in Microbiology and Immunology 225 (1998): 99.

Stout, Janet E., and Victor L. Yu. "Legionellosis," The New England Journal of Medicine 337 (September 4, 1997): 682.

Julia Barrett

KEY TERMS

Antibody

—A molecule created by the immune system in response to the presence of an antigen (a foreign substance or particle). It marks foreign microorganisms in the body for destruction by other immune cells.

Antigen

—A molecule, usually a protein, that the body identifies as foreign and toward which it directs an immune response.

Culture

—A means of growing bacteria and viruses in a flask or on a plate. The culture medium usually is agar, a form of gelatin, that may be enriched with broth or blood.

DNA probe

—An agent that binds directly to a predefined sequence of nucleic acids.

Immunocompromised

—A condition in which the immune system is not functioning properly and cannot adequately protect the body from infection.

Immunoglobulin

—The protein molecule that serves as the primary building block of antibodies.

Immunosuppressive therapy

—Medical treatment in which the immune system is purposefully thwarted. Such treatment is necessary, for example, to prevent organ rejection in transplant cases.

Legionellosis

—A disease caused by infection with a Legionella bacterium.

Media

—Substance which contains all the nutrients necessary for bacteria to grow in a culture.

Phagocytosis

—The "ingestion" of a piece of matter by a cell.

Legionnaires' disease

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.

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 (the bacterium's name was derived from this group's name). This 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 does not 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 some type of 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%.

Legionnaires' disease is caused by inhaling Legionella bacteria from the environment. Typically, the bacteria are dispersed in aerosols of contaminated water. These aerosols are produced by devices in which warm water can stagnate, such as air-conditioning cooling towers, humidifiers, shower heads, and faucets. There have also been cases linked to whirlpool spa baths and water misters in grocery store produce departments. Aspiration of contaminated water is also a potential source of infection, particularly in hospital-acquired cases of Legionnaires' disease. There is no evidence of person-to-person transmission of Legionnaires' disease.

Once the bacteria are in the lungs, cellular representatives of the body's immune system (alveolar macrophages) congregate to destroy the invaders. The typical macrophage defense is to phagocytose the invader and demolish it in a process analogous to swallowing and digesting it. However, the Legionella bacteria survive being phagocytosed. Instead of being destroyed within the macrophage, they grow and replicate, eventually killing the macrophage. When the macrophage dies, many new Legionella bacteria are released into the lungs and worsen the infection.

Legionnaires' disease develops 2–10 days after exposure to the bacteria. Early symptoms include lethargy, headaches, fever, chills, muscle aches, and a lack of appetite. Respiratory symptoms such as coughing or congestion are usually absent. As the disease progresses, a dry, hacking cough develops and may become productive after a few days. In about a third of Legionnaires' disease cases, blood is present in the sputum. Half of the people who develop Legionnaires' disease suffer shortness of breath and a third complain of breathing-related chest pain. The fever can become quite high, reaching 104°F (40°C) in many cases, and may be accompanied by a decreased heart rate.

Although the pneumonia affects the lungs, Legionnaires' disease is accompanied by symptoms that affect other areas of the body. About half the victims experience diarrhea and a quarter have nausea and vomiting and abdominal pain. In about 10% of cases, acute renal failure and scanty urine production accompany the disease. Changes in mental status, such as disorientation, confusion, and hallucinations, also occur in about a quarter of cases.

In addition to Legionnaires' disease, L. pneumophila legionellosis also includes a milder disease, Pontiac fever. Unlike Legionnaires' disease, Pontiac fever does not involve the lower respiratory tract. The symptoms usually appear within 36 hours of exposure and include fever, headache, muscle aches, and lethargy. Symptoms last only a few days and medical intervention is usually not necessary.

The symptoms of Legionnaires' disease are common to many types of pneumonia and diagnosis of sporadic cases can be difficult. The symptoms and chest x rays that confirm a case of pneumonia are not useful in differentiating between Legionnaires' disease and other pneumonias. If a pneumonia case involves multisystem symptoms, such as diarrhea and vomiting, and an initially dry cough, laboratory tests are done to definitively identify L. pneumophila as the cause of the infection.

If Legionnaires' disease is suspected, several tests are available to reveal or indicate the presence of L. pneumophila bacteria in the body. Since the immune system creates antibodies against infectious agents, examining the blood for these indicators is a key test. The level of immunoglobulins , or antibody molecules, in the blood reveals the presence of infection. In microscopic examination of the patient's sputum, a fluorescent stain linked to antibodies against L. pneumophila can uncover the presence of the bacteria. Other means of revealing the bacteria's presence from patient sputum samples include isolation of the organism on culture media or detection of the bacteria by DNA probe. Another test detects L. pneumophila antigens in the urine.

The type of antibiotic prescribed by the doctor depends on several factors including the severity of infection, potential allergies , and interaction with previously prescribed drugs. For example, erythromycin interacts with warfarin, a blood thinner. Several drugs, such as penicillins and cephalosporins, are normally ineffective against the infection. Although they may be deadly to the bacteria in laboratory tests, their chemical structure prevents them from being absorbed into the areas of the lung where the bacteria are present. In severe cases with complications, antibiotic therapy may be joined by respiratory support. If renal failure occurs, dialysis is required until renal function is recovered.

Appropriate medical treatment has a major impact on recovery from Legionnaires' disease. Outcome is also linked to the victim's general health and absence of complications. If the patient survives the infection, recovery from Legionnaires' disease is usually complete. Similar to other types of pneumonia, severe cases of Legionnaires' disease may cause scarring in the lung tissue as a result of the infection. Renal failure, if it occurs, is reversible and renal function returns as the patient's health improves. Occasionally, fatigue and weakness may linger for several months after the infection has been successfully treated.

Because the bacteria thrive in warm stagnant water, regularly disinfecting ductwork, pipes, and other areas that may serve as breeding areas is the best method for preventing outbreaks of Legionnaires' disease. Most outbreaks of Legionnaires' disease can be traced to specific points of exposure, such as hospitals, hotels, and other places where people gather. Sporadic cases are harder to determine and there is insufficient evidence to point to exposure in individual homes.

See also Pneumonia, bacterial and viral

Legionnaires' disease

Legionnaires' disease is an acute respiratory infection caused by a common bacteria that results in a serious case of pneumonia. It first became a well-known disease in 1976 when a serious outbreak occurred among a large number of people attending an American Legion convention. Researchers eventually discovered that the bacteria can be easily found in nature wherever there is warm and moist stagnant water, and that it is transmitted by breathing it in.

A mysterious outbreak

During July 21 to 24, 1976, over 4,000 members of the American Legion met in Philadelphia, Pennsylvania, to attend their fifty-eighth annual convention and to celebrate the nation's two hundredth birthday. When the meetings were over, the attendees and their families returned home, but not all was right. On July 27, only three days after the convention, one of the legionnaires who had been in Philadelphia died from a pneumonia-like illness. On July 30, a physician in Bloomsburg, Pennsylvania, realized that the three patients he was treating for a similar condition had all attended the convention in Philadelphia. That same day, a nurse in the nearby Chambersburg Hospital noted a similar condition in three patients who had gone to the same convention. However, it was not until August 2 that state officials were able to put together the illness with its victims' whereabouts and to realize that there was some undeniable connection between this serious febrile (pronounced FEH-brile) or feverish illness and the legionnaires' convention. By that date, eighteen legionnaires had already died. Federal officials at the Center for Disease Control (CDC) were notified and became immediately involved in what was now a mysterious and spreading outbreak.

Words to Know

Antibody: A protein produced by certain cells of the body as an immune (disease-fighting) response to a specific foreign antigen, or any substance that the body considers foreign, such as a bacterial cell.

Bacteria: Single-celled microorganisms that live in soil, water, plants, and animals that play a key role in the decay of organic matter and the cycling of nutrients. Some are agents of disease.

Pneumonia: Any of several diseases caused by bacteria or viruses in which the lungs become inflamed.

"Philly Killer"

By this time the media realized that it had a major story on its hands, and from then on health officials had to work under the close watch of radio, television, and newspaper reporters. Still, no one, including the CDC, was able to pinpoint the immediate cause of this disease. Since it was now directly connected to the Philadelphia legionnaires' convention, the media referred to it as the "Philly Killer," "Legion Malady," "Legion Fever," "Legion Disease," and finally the name that took hold, "Legionnaires' disease." As this name became used regularly, some members of the American Legion thought it was bad for the organization since it might suggest that they were somehow responsible for the disease. Other members thought the opposite and considered it a sort of tribute or honor to those who had already died. Despite their opinions, the name for the disease stuck, and it is still called that today by non-scientists.

Probably the main reason for the name sticking was that researchers could not identify the organism causing this disease and therefore had nothing to name it. Technically, the CDC simply called it "Respiratory Infection-Pennsylvania." For several months as they studied the disease, this name persisted until an April 1977 CDC report made reference to "Legionnaires' disease." From then on, even after the bacterium that caused the disease was identified and named, the press and even some in the medical community would refer to it by its popular name.

Discovery by CDC

It was not until nearly six months after the first outbreak that the cause of this disease was identified positively by the CDC. On January 18, 1977, the CDC announced that their investigators had isolated the cause of Legionnaires' disease. Using a piece of lung tissue taken from one of its dead victims, researchers finally were able to demonstrate that a bacterium they would name Legionella pneumophila (pronounced leejuh-NELL-uh new-mo-FEE-lee-uh) was the culprit. By then however, a total of 221 people had contracted the disease and 34 of them had died. Isolating the actual bacterium enabled the CDC not only to learn how it had spread and how to fight it, but it showed researchers that this was a complicated organism responsible for many past unexplained outbreaks.

Course of disease

The Legionella bacterium was an unusual and complicated germ because it was found to cause two diseases, one very serious known as Legionnaires' disease and another milder form called Pontiac Fever. Although Pontiac Fever is caught by 95 percent of the people exposed to it, most of them simply experience flulike symptoms that pass in two to five days. Legionnaires' disease is much harder to catch, with only two to five percent of those exposed actually contracting it. But once contracted, usually by at-risk individuals who are more susceptible, it will not go away without medication and it kills between five and fifteen percent of the people it infects.

Source of infection

CDC researchers named the species of bacteria Legionella pneumophila because the second word means "lung-loving" in Latin. This bacteria is actually very common in the natural world and only causes trouble when it gets into people's respiratory systems. It finds our lungs to be an especially comfortable place because they have conditions the bacteria prefer—they are warm and moist. Legionella are found to exist naturally in stagnant water, and in the Philadelphia case, the CDC traced the outbreak source to the hotel's air conditioning system whose condenser was vented very close to its air intake system. This meant that the large air conditioning system, which had not been cleaned for some time, had the common Legionella germ growing in it, which people then inhaled after the organism had gotten into the air intake pipes.

Attacks the susceptible

The fact that this is the only way that people can contract the disease was discovered by the CDC. Unlike many diseases, you cannot "catch" this disease from another person. The Legionella germ must penetrate deep into the lungs. Further, the cilia (pronounced SIL-lee-uh) in most people's lungs usually capture and expel the bacteria. However, for those who are somehow at risk—like smokers, alcoholics, older people with chronic lung problems, or someone with a weak immune system—these short hairlike projections called cilia do not work the way they should. The Legionella can then get in and infect a person. Another unusual

thing about this disease is that the infecting bacteria invade the body's white blood cells and multiply inside them. These are the very cells that the body uses to fight such invaders. Normally, these attack white cells called phagocytes (pronounced FA-go-sites) surround and engulf or swallow up a bacterial invader. Although the phagocytes do manage to engulf the Legionella, they are unable to digest it and soon the attacker becomes the attacked. In fact, the Legionella becomes a parasite and actually begins multiplying inside the phagocyte, who now becomes its host. After doubling its numbers every two hours, the Legionella eventually overloads its host, which bursts and spreads even more invading cells throughout the body. The CDC discovered that the antibiotic erythromycin (pronounced eh-RI-throw-MY-sin) is effective. However, it works not by killing the bacteria but rather by stopping it from multiplying in the cells, therefore giving the body a good chance to combat it on its own.

A new "old" disease

Once the real cause of the disease was known and well understood, researchers realized that this was not some new bacterium that had suddenly emerged but one that had been around all the time. It was simply one that science had never identified. With hindsight, they found that an estimated 8,000 to 18,000 people get some form of Legionnaires' disease every year in the United States. Further, they found that the disease occurs worldwide. For example, it is so common in Australia that roughly one-third of the population has antibodies for it in their blood (meaning that they have been exposed to it at some point in their lives and their bodies have developed a way to combat it).

Since Legionella has been found in cooling towers and evaporative condensers of large air conditioning systems, as well as in spas and showers, all of which have temperature conditions that allow it to thrive, it is important to keep these systems clean and well-maintained. Legionella is easily killed by heating water to high temperatures. It dies off quickly if it dries out and it is also killed by simple exposure to the ultraviolet radiation of the Sun. There is no evidence that people can be infected by air conditioners in their cars or by window units in their homes.

Legionnaires' disease is a major bacterial disease that had existed without being detected until 1976. What made it suddenly known to science was the fact that so many people in the same place got sick all at once, attracting a lot of media attention and suggesting that something had infected them. What they had in common was the fact that they all had spent some time in the same convention hall. Remarkably, the CDC eventually found that these people were the victims of a fairly common, natural bacteria that has been invading humans and other hosts for centuries but of which no one had any knowledge.

[See also Disease ]

LEGIONNAIRES' DISEASE

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.

Elizabeth W.Etheridge/c. w.

See alsoCenters for Disease Control and Prevention ; Epidemics and Public Health .