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To detect and diagnose Legionella bacteria as the cause of pneumonia or flu-like symptoms; to investigate an outbreak caused by Legionella
When you have a cough, body aches, shortness of breath, headache, and fever with chills, and may have abdominal pain, diarrhea, nausea, and/or mental changes that a healthcare provider suspects may be due to an infection caused by Legionella
A urine sample, a sputum sample, blood sample, and/or respiratory secretions or other body fluids collected by a healthcare practitioner
Legionella is a type of bacteria that can cause a serious form of pneumonia called Legionnaires disease. Legionella testing detects the presence of the bacteria or parts of it (antigens) to help diagnose the cause of a person's pneumonia.
A legionella infection affects the lower respiratory tract, often requires hospitalization, and has a significant mortality rate. In addition to pneumonia, Legionella bacteria can also cause Pontiac fever, a milder and self-limiting illness with flu-like symptoms (without pneumonia). Together, Legionnaires disease and Pontiac fever are referred to as legionellosis.
Legionella bacteria are found naturally throughout the environment. They prefer warm, stagnant water and can grow in the plumbing systems of large buildings such as hotels, hospitals, and cruise ships. The bacteria may contaminate whirlpool spas, drinking and bathing water, hot water tanks, air conditioning cooling towers, ice machines, humidifiers, and public fountains.
Infections occur when an individual inhales airborne water droplets such as steam or mist that are contaminated with Legionella bacteria. The bacteria are not passed from one person to another, but many people can become infected by the same contaminated water source, potentially resulting in a legionellosis outbreak. According to the Centers for Disease Control and Prevention (CDC), legionellosis outbreaks occur any time two or more people become ill in the same place at about the same time.
Legionella infection or Pontiac fever is confirmed when a person showing typical symptoms of the disease has a positive confirmatory laboratory test (usually a positive culture or urine test).
State and local health departments have jurisdiction over legionellosis outbreaks in their state; however, legionellosis is a nationally notifiable disease and state health departments should report all laboratory-confirmed cases to the CDC.
Pontiac fever typically develops within 24-48 hours while Legionnaires disease develops within a few days to a couple of weeks. According to the CDC, about 8,000 to 18,000 people are hospitalized with Legionnaires disease each year. The actual number of people affected by legionellosis is likely much higher as most cases of Pontiac fever and many cases of Legionnaires disease are thought to go undiagnosed.
Although anyone may be affected, adults over 50 are at the highest risk for Legionnaires disease, as are those who smoke, have lung diseases, or have compromised immune systems (such as those with HIV/AIDS, cancer, or who are taking immunosuppressant medications). People with chronic underlying diseases such as diabetes, kidney disease, or liver disease are also more vulnerable, and men are affected more often than women. Most cases of Legionnaires disease are sporadic, but they may also be seen in outbreaks, which typically occur during the summer and early fall. Pontiac fever is frequently linked with outbreaks.
There are many different types of Legionella bacteria, but only a few are common. In the United States, as many as 80% of legionella infections in adults are caused by Legionella pneumophila, serogroup 1 (a subtype of the species). Other serogroups of L. pneumophila, including 4 and 6, cause a number of cases. Other species of Legionella, such as Legionella micdadei, Legionella bozemanii, Legionella dumoffii, and Legionella longbeachae, may cause infections in children and/or are more prevalent in other parts of the world.
How is the sample collected for testing?
The sample collected depends on the test to be performed.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
Testing is performed to detect the presence of Legionella bacteria to identify the cause of a person's symptoms. Tests are primarily used to diagnose Legionnaires disease and distinguish it from other causes of pneumonia and/or other conditions that have similar symptoms. Since Legionnaires disease can often require hospitalization and has a significant mortality rate, it is important for a healthcare practitioner to identify it and initiate appropriate treatment with antibiotics.
Typically, one or more of the following tests may be ordered. According to the Centers for Disease Control and Prevention (CDC), a urine test for the Legionella antigen and Legionella culture of a respiratory sample are the preferred tests to diagnose Legionnaires disease and are used to confirm cases showing typical clinical features of Legionellosis.
Legionella testing may be ordered when a person has pneumonia or symptoms associated with pneumonia, especially when the person also has gastrointestinal symptoms such as diarrhea and/or nausea and mental changes such as confusion.
The Centers for Disease Control and Prevention (CDC) recommends testing when a person:
Signs and symptoms of Legionnaires disease typically include:
and may include:
Additional symptoms may be experienced if Legionella bacteria also infect other organs such as the heart, brain, liver, or kidney.
Results of testing must be carefully interpreted and evaluated in conjunction with a complete medical history, physical exam, and other tests such as a chest X-ray for pneumonia.
Negative legionella test results, of any type, do not rule out a legionella infection. A person may be infected by a Legionella species other than the one(s) tested or the bacteria may be present in the sample in numbers too low to be detected or may not have grown in culture. Repeat testing may be required.
If a Legionella pneumophila urine antigen test is positive, then it is likely that a person has a Legionella pneumophila serogroup 1 infection. If the person has symptoms consistent with pneumonia, then it is likely that the person has Legionnaires disease.
If a legionella culture is positive, then a person is confirmed as having a legionella infection and whatever species is present in the culture is identified as the cause.
If a Legionella species by PCR genetic test is positive, then it is likely that the person has a legionella infection. However, there are a small number of false positives associated with this test.
Blood tests for Legionella pneumophila antibody are no longer routinely performed but may be available from some reference laboratories. The test detects antibodies, the body's immune response to a legionella infection, in the blood. It is not useful for immediate clinical decision-making but can be ordered to confirm a recent infection. Usually two blood samples taken weeks apart are tested, and an increase in the level of antibodies against the bacteria indicates an active infection. A single positive antibody test cannot be used to diagnose a recent infection as up to 15% of the general population may have legionella antibodies in their blood, indicating exposure to the bacteria at some point in their life. Some people, such as those who have compromised immune systems, can have a legionella infection without having a normal antibody response.
Legionella pneumophila by direct fluorescent antibody (DFA) is a test that is no longer routinely recommended. It is not as sensitive as other testing, is labor-intensive, and requires a relatively large respiratory sample.
Other tests may be performed to help evaluate a person's overall health status and to distinguish between conditions that cause similar symptoms. These may include:
Legionnaires disease can affect other blood test results. These tests are not used to help diagnose a legionella infection but may be additional clues for the healthcare provider and information about the person's health status. Often seen are:
Yes, and it frequently is tested to monitor complex water systems, to detect the source of legionellosis outbreaks, and to monitor the effectiveness of water treatment when Legionella bacteria are detected.
The opportunities for them to occur can be minimized, but the risk cannot be totally eliminated. Legionella are very common bacteria in the environment. In building water systems, they can resist low levels of chlorine treatment and can persist in sediment found in pipes. Home and public hot tubs should be properly disinfected and pH of the water regularly checked and maintained at appropriate levels. Individuals at increased risk for infection may choose to avoid high-risk exposures, such as being in or near a hot tub.
In Legionella outbreak situations, hyperchlorination of hospital water (using high levels of chlorine) or raising the temperature of the hot water to over 70⁰C, or a combination of both methods has been proven effective for suppressing the growth of Legionella.
Pontiac fever, the type of legionella infection that causes flu-like systems, is generally self-limiting and resolves in most cases without treatment, so testing is not considered routinely necessary. Testing might be warranted in cases where Legionnaires disease symptoms are associated with travel, hospitalization, a stay in an assisted-living facility, or other risk factors (e.g., smoking, older than age 50, chronic lung disease, etc).
The disease and bacteria were named after an incident in which a significant number of people who went to an American Legion convention in Philadelphia in 1976 developed pneumonia. A new type of bacteria was identified as the cause and was subsequently named Legionella. This outbreak caused illness in 182 people and resulted in 29 deaths.
Sources Used in Current Review
2018 Review completed by Yanal Murad, PhD, D(ABMM).
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