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. 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.

Cases of Legionnaries' disease, 2000–2016

Cases of Legionnaries' disease, 2000–2016

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.


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.

Public health role and response

In the United States and many other countries, legionellosis is a nationally notifiable disease. A health worker who learns of a case of the disease is required to notify officials of this fact, providing relevant information about the infection. Public health workers also interview individuals who have contracted the disease in order to obtain information about possible sources of the disease. When such sources are identified, arrangements can be made for cleaning that source, such as a dirty air conditioning unit. Public health workers can also reduce the risk to individuals of contracting the disease by providing healthcare workers and the general public about the conditions that lead to Legionnaires' disease and the steps that can be taken to reduce the risk of contracting the disease.


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.

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.
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.
A laboratory system for growing bacteria for further study.
DNA probe—
An agent that binds directly to a predefined sequence of nucleic acids.
Refers to conditions in which the immune system is not functioning properly and cannot adequately protect the body from infection.
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.
A disease caused by infection with a Legionella bacterium.
Substance which contains all the nutrients necessary for bacteria to grow in a culture.
The “ingestion” of a piece of matter by a cell.




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.

See also Waterborne diseases .



Uzel, Atac, and E. Esin Hames-Kocabas. Legionella Pneumophila: From Environment to Disease. Hauppage, NY: Nova Science Publications, 2010.


Cristino, S., P. P. Legnani, and E. Leoni. “Plan for the Control of Legionella Infections in Long-term Care Facilities: Role of Environmental Monitoring.” International Journal of Hygiene and Environmental Health 215, 3 (2012): 279-285.

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


Legionnaires' Disease. Medline Plus. http://www.nlm.nih.gov/medlineplus/legionnairesdisease.html (accessed August 18, 2012).

Patient Facts: Learn More about Legionnaires' Disease. http://www.cdc.gov/legionella/patient_facts.htm (accessed August 18, 2012).

Julia Barrett

  This information is not a tool for self-diagnosis or a substitute for professional care.