Written by Ada’s Medical Knowledge Team
What is Legionella infection?
Legionella infection, also known as legionellosis, is a bacterial infection most often affecting the airways and/or lungs. It is frequently caused by a bacterium called Legionella pneumophila, which thrives in warm, wet environments such as hot tubs and air conditioning units.
The infection can be caught by inhaling contaminated water droplets. Transference of the disease between humans generally does not occur, with only one case of possible person-to-person transmission ever reported.
Legionella infection can affect the lungs, causing a potentially severe form of pneumonia known as Legionnaires’ disease. Legionnaires’ disease causes respiratory symptoms, such as coughing, often alongside a combination of other symptoms including:
There is also a similar but milder form of the infection called Pontiac fever, which presents as a mild, flu-like illness. Pontiac fever does not cause pneumonia and usually resolves on its own without treatment.
People of any age can catch Legionella infection, although elderly people and those with a weakened immune system are most commonly affected. Legionella infection is uncommon in children.
Diagnosis is usually made through laboratory tests that check for the presence of Legionella bacteria in the affected person’s urine, blood and/or phlegm. A chest X-ray is typically also recommended when Legionnaires’ disease is suspected to check for pneumonia in the lungs.
Antibiotics are most often prescribed to treat Legionnaires’ disease. Complications are possible, particularly within high-risk groups, such as elderly people and those with certain underlying health conditions, such as chronic lung or kidney disease. In severe cases, Legionnaires’ disease can be fatal. However, with prompt diagnosis and treatment, most people make a full recovery.
Types of Legionella infection
Legionella infection, or legionellosis, refers to two different, but related, conditions:
- Legionnaires’ disease
- Pontiac fever
Both of these conditions are caused by the Legionella bacteria. However, Legionnaires’ disease is the more serious of the two. Legionnaires’ disease is characterized by the presence of pneumonia, whereas Pontiac fever is non-pneumonic.
If you are experiencing possible symptoms of Legionella infection, you can carry out a free symptom assessment with the Ada app.
Symptoms of Legionella infection
The precise symptoms experienced as a result of Legionella infection depend on whether the affected person has Legionnaires’ disease or Pontiac fever.
Symptoms of Legionnaires’ disease
Legionnaires’ disease is a more severe illness, and symptoms tend to appear between 2 to 10 days after becoming infected.
Respiratory symptoms are usually a key feature of Legionnaires’ disease. Respiratory symptoms may include:
- A cough which is dry at first, but begins to produce mucus and phlegm as the condition progresses. This is the most common respiratory symptom of Legionnaires’ disease
- Shortness of breath
- Chest pain
- Bloody phlegm
Respiratory symptoms may not be present at first, but develop as the infection progresses.
Flu-like symptoms are also a common sign of Legionnaires’ disease and can include:
- Muscle aches
- Generally feeling unwell
In addition to the above, further symptoms that have also been associated with Legionnaires’ disease, include:
- Nausea and vomiting
- Abdominal pain
- Joint pain
- Neurological symptoms such as confusion, agitation or a dazed, unresponsive state known as stupor
Good to know: Although Legionnaires’ disease symptoms usually appear around 2 to 10 days after infection by the bacteria, an incubation period of up to 16 days has been recorded in some instances. It is therefore recommended that anyone who may have been exposed to Legionella bacteria monitors themselves for symptoms for around two weeks after exposure. If Legionnaires’ disease is suspected, always contact a doctor.
Symptoms of Pontiac fever
Pontiac fever also causes flu-like symptoms, most commonly fever and muscle aches. However, it is a milder infection than Legionnaires’ disease, and the symptoms experienced are usually less intense.
Most significantly, Pontiac fever does not result in pneumonia. This means that respiratory symptoms such as coughing and shortness of breath are not usually present in people with this condition.
Symptoms of Pontiac fever generally arise around one to three days after first becoming infected. Pontiac fever is a self-limiting condition, which means, the illness usually passes of its own accord without the need for medication. Symptoms generally resolve within a week.
Symptoms of extrapulmonary Legionella infection
Legionella infection most often affects the lungs and airways. However, in very rare cases, the infection can occur outside the respiratory tract. This is known as extrapulmonary Legionella infection and can cause conditions such as:
- Sinusitis, a swelling of the sinuses in the nose
- Cellulitis, a skin infection
- Pancreatitis, a swelling of the pancreas
- Myocarditis, an inflammation of the heart muscle
- Pericarditis, an inflammation of the membrane around the heart
- Infective endocarditis, an inflammation of the heart’s inner lining and valves
- Pyelonephritis, an inflammation of the kidney
- Peritonitis, an inflammation of the abdomen’s inner lining
- Meningitis, an infection of the membranes around the brain and spinal cord
- Osteomyelitis, an infection of the bone
- Septic arthritis
- Soft tissue and brain abscesses
- Surgical site infection
Good to know: Extrapulmonary Legionella infection is rare. However, when it does occur, it can manifest either as a complication of pneumonia or independently of pneumonia.
If you are concerned that you or a loved one may have contracted Legionella infection, always contact a doctor. The free Ada app can also be used to carry out a symptom assessment.
Legionella vs. COVID-19 symptoms
Both an infection with Legionella and COVID-19 can present similar symptoms in the initial stage. These symptoms may include fever, a dry cough, headache, confusion, shortness of breath, nausea, and diarrhea. Both conditions can also cause pneumonia. As the symptoms are so alike, the only way of differentiating between the two is by laboratory testing. Since the cause for both conditions is different, the treatment options are also different, going from antiviral medication for COVID-19 to antibiotics for Legionnaires' disease.
Causes of Legionella infection
The most common cause of Legionella infection in humans is a bacterium called Legionella pneumophila. Legionella pneumophila is the cause of approximately 70 to 90 percent of Legionella infections in adults. Other Legionella species, such as Legionella longbeachae and Legionella micdadei, can also cause the disease, but are much less common.
Transmission of Legionella infection usually occurs when a person breathes in airborne water droplets that are contaminated with Legionella bacteria. The bacteria subsequently enter the person’s respiratory tract, causing Legionnaires’ disease, or, less commonly, Pontiac fever.
However, the likelihood of developing an illness can depend on a number of factors, including:
- The concentration of Legionella bacteria in the water source
- The severity of the particular strain of Legionella bacteria
- The quantity of aerosolized water droplets in the air
- The presence of any risk factors in the affected person, such as advanced age or pre-existing health conditions
Good to know: It is not known precisely what concentration of Legionella bacteria is necessary to cause illness. However, it is believed that high-risk people may develop Legionella infection after relatively limited amounts of exposure.
Legionella pneumophila is often found in bodies of freshwater, such as rivers, lakes and streams. However, as the bacterium thrives in warm, wet environments, it can spread to artificial water systems used by humans. Systems where Legionella pneumophila may be found, include: ref1 ref11
- Air conditioning units that use water-based cooling towers
- Hot tubs and whirlpool spas
- Water birth tubs
- Showers and sink faucets
- Large plumbing systems
- Hot water tanks
- Water fountains
- Respiratory therapy equipment, such as nebulizer machines
- Rainwater puddles on roads
- Soil and compost
Hospitals, large hotels, museums and office blocks often have large and complex water supply systems where Legionella pneumophila can spread quickly. Because the bacterium commonly dwells in specific locations, it is prone to causing outbreaks. The majority of Legionella infection outbreaks, that have occurred in hospitals, have been linked to a contaminated hot water supply.
Good to know: Home and car air conditioning units usually do not use water to cool the air, meaning they are not typical sites for Legionella bacteria. Generally, it is large or industrial-level air conditioning units that use cooling towers containing water and therefore are at higher risk for Legionella growth.
Risk factors for Legionella infection
In the United States, it is estimated that around 8,000 to 18,000 people are admitted to hospital with Legionnaires’ disease every year. Of these, more than 80 percent of cases are spread throughout the year, and the remaining 20 percent are the result of outbreaks during summer and early fall.
In adults, established risk factors for Legionella infection include:
- Advanced age. Middle-aged and older adults are at higher risk of developing Legionnaires’ disease. It is rare in young adults and children
- Gender. Men are more than twice as likely to develop Legionnaires’ disease than women
- Smoking. Tobacco smoke exposure increases a person’s vulnerability to bacterial infections
- Alcohol abuse
- Chronic diseases, such as chronic lung or heart disease
- Weakened immune system, also known as immunodeficiency, which can be due to illness, e.g. HIV infection or drugs that weaken the immune system, e.g. chemotherapy
- Certain underlying health conditions, such as diabetes mellitus, cancers, kidney failure or liver failure
Legionella infection is much less common in children, and the risk factors for children are less well defined than they are for adults. However, some risk factors believed to affect children include:
- Young age. More than a third of reported Legionnaires’ disease cases in children have occurred in infants under the age of one year old
- Weakened immune system
- Respiratory diseases, such as chronic lung disease or asthma
- Water births
Good to know: Risk factors not only increase the chances of a person developing Legionella infection, but some may also increase the severity of the condition. This means that certain high-risk people are also more likely to develop complications as a result of a Legionella infection. If Legionella infection is suspected, contact a doctor immediately.
Diagnosis of Legionella infection
Legionella infection can result in two distinct conditions, Legionnaires’ disease and Pontiac fever. Although these conditions are caused by the same bacteria, Legionnaires’ disease and Pontiac fever are generally not diagnosed in the same way.
Diagnosis of Legionnaires’ disease
A diagnosis of Legionnaires’ disease is usually made based on a combination of a clinical evaluation, a chest X-ray and laboratory analysis of a phlegm, urine and/or blood sample from the affected person.
If Legionnaires’ disease is suspected, a doctor will often begin by asking about a person’s symptoms and personal history. Because Legionella bacteria often dwell in specific locations, the person may be asked whether they have been near high-risk areas, such as hot tubs or cooling towers.
Although a clinical evaluation alone is typically not able to confirm a diagnosis of Legionnaires’ disease, it may help to support a diagnosis. For example, although Legionnaires’ disease is hard to reliably distinguish from other types of pneumonia, it may raise the suspicion that they have Legionnaires’ disease if the affected person is also displaying gastrointestinal symptoms such as vomiting and diarrhea.
If Legionnaires’ disease is suspected after a clinical evaluation, the affected person will usually be moved onto a further stage of the diagnostic process.
A chest X-ray is often recommended for a person with suspected Legionnaires’ disease. This is because Legionnaires’ disease is a form of pneumonia, and a chest X-ray can generally confirm the presence of pneumonia in the lungs, as well as the severity of the infection. However, it is not able to distinguish Legionella pneumonia from other types of pneumonia.
A definitive diagnosis of Legionnaires’ disease can typically be made following a positive laboratory test result, usually on a sample of phlegm, urine or blood. The main tests used to detect Legionella infection include:
Polymerase chain reaction (PCR) is a type of nucleic acid test (NAT). PCR testing on a lower respiratory tract sample, such as phlegm, is often considered the preferable diagnostic method for people with possible Legionnaires’ disease. PCR can detect all types of Legionella bacteria through a sample of a person’s phlegm, urine or blood; however, lower respiratory tract samples are believed to have the highest level of accuracy.
Urinary antigen testing may be suggested if PCR testing is not available or a lower respiratory tract sample cannot be obtained. Urinary antigen testing is fast and sensitive, however, it can only detect Legionella pneumophila. If a negative result is received, but Legionella infection is still suspected, a culture test may be recommended instead.
- Culture testing can be done on nearly any sample type, however, the results typically take three to five days, and it can be less sensitive than other testing methods. If a PCR or urinary antigen test is positive for Legionella infection, a lower respiratory tract sample will often be taken for culture testing to research into the causes of the infection.
As a single test may not be conclusive enough to confirm a diagnosis, a doctor may recommend a combination of laboratory tests to check if a person has Legionnaires’ disease.
Good to know: Testing for Legionella infection is generally recommended for anyone with severe pneumonia that has been acquired outside of a healthcare setting, also known as community-acquired pneumonia (CAP). People with nonsevere CAP may also be recommended for testing if they have specific risk factors or are believed to have been exposed to Legionella bacteria.
People experiencing possible symptoms of Legionnaires’ disease should consult a doctor. You can also use the Ada app to carry out a free symptom assessment.
Diagnosis of Pontiac fever
Pontiac fever, the less serious type of Legionella infection, typically cannot be detected through testing the person’s blood, urine or phlegm. Instead, Pontiac fever is most often diagnosed when the symptoms match the clinical criteria, and other cases of Legionella infection have been confirmed within the same area and timeframe.
Good to know: Pontiac fever is usually not serious. It also typically resolves on its own without the need for treatment. Therefore, swift diagnosis and treatment is not as crucial for Pontiac fever as it is for Legionnaires’ disease.
Treatment of Legionnaires’ disease
Most cases of Legionnaires’ disease can be successfully treated with antibiotics that help to kill the Legionella bacteria in the body. A class of antibiotic drug called fluoroquinolones is often recommended to treat Legionnaires’ disease and can be administered either orally or intravenously. Another class of antibiotic, known as macrolides, may also be suggested.
Due to the potential severity of Legionnaires’ disease, most patients who contract the illness are admitted to hospital to receive intensive antibiotic treatment.
To begin with, antibiotics are usually given intravenously, meaning they are injected directly into the bloodstream. This is because intravenous drugs begin to act faster than those taken orally, and also because people with Legionnaires’ disease may have poor stomach absorption due to their gastrointestinal symptoms.
Good to know: Antibiotic treatment for Legionnaires’ disease may last from a few days up to a number of weeks. The length of treatment can depend on factors such as the severity of the illness, the overall health of the affected person and the specific medication prescribed. With prompt and appropriate treatment, most people with Legionnaires’ disease make a full recovery and begin to see symptomatic improvements within three to five days.
Other treatment methods that may be suggested alongside antibiotic treatment include:
- Medications that help ease pain and high temperatures, such as paracetamol, also known as acetaminophen
- Extra oxygen, because pneumonia can affect the lungs’ ability to breathe properly. Oxygen is usually administered through a face mask or nasal tube
- Extra fluids to help combat dehydration. Urgent fluids are usually administered through an intravenous drip
- Assisted breathing through a ventilator, if the infection is particularly severe
Treatment of Pontiac fever
Pontiac fever does not usually require antibiotic treatment. The condition is generally mild in nature and tends to clear on its own within a week.
However, people with Pontiac fever may wish to manage the flu-like symptoms it causes, such as fever and muscle aches. Measures that may help to ease such symptoms include:
- Drinking lots of water to stay hydrated
- Reducing fever and aches with paracetamol or nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen
- Resting and avoiding strenuous activities
Preventing Legionella infection
The main cause of Legionella infection is contaminated water sources. Therefore, keeping water systems such as cooling towers, hot tubs and water fountains clean and well maintained can help to prevent some cases of infection.
The precise measures taken to ensure water systems are well maintained depend on the system itself, e.g. what the water is used for, how much water is stored in it, what temperature the water is kept at. However, examples of control measures that may help to minimize the growth of Legionella bacteria in water systems include:
- Regular cleaning and disinfecting
- Flushing unused taps on a weekly basis to help reduce stagnation where necessary
- Maintaining chemical levels that help prevent the development of germs where necessary, e.g. chlorine levels in a hot tub
- Implementing a water filter system in high-risk areas, e.g. pediatric hospitals, birthing tubs or when an outbreak is suspected
- Keeping cool water at a temperature below 68 F / 20 C and hot water above 140 F / 60 C where necessaryGood to know: The manufacturer’s instructions should always be followed for guidelines on proper water system maintenance, including home devices such as humidifiers and spas.
Managing Legionella infection outbreaks
Most cases of Legionella infection are caught sporadically, however, occasional outbreaks do occur. If an outbreak is suspected, early identification and decontamination of the source of infection can be hugely beneficial in managing the spread of the infection.
There is no vaccine against Legionella infection.
Good to know: Legionella infection generally does not spread from human to human. Therefore, people with this condition do not require isolation.
Complications of Legionella infection
While Pontiac fever is a relatively mild illness, Legionnaires’ disease can be very severe and often requires hospitalization.
Although most cases of Legionnaires’ disease are successfully treated with antibiotics, complications can occur. In the most serious cases, Legionnaires’ disease can be a life-threatening condition.
Certain groups are at higher risk of developing complications due to Legionnaires’ disease, including:
- The elderly and children under one year old
- People with certain underlying health conditions such as chronic lung disease, diabetes mellitus, cancers or a weakened immune system
- People who received delayed antibiotic treatment
- People who acquired the infection in hospital
As Legionnaires’ disease affects the lungs and causes pneumonia, severe complications of the condition can include:
- Lung failure, also known as respiratory failure
- Kidney failure, also known as renal failure
- Multiple organ failure, also known as multiple organ dysfunction syndrome
- Shock, which occurs when not enough blood or oxygen is reaching a person’s organs and cells
- Sepsis, which occurs when the infection enters the bloodstream
The overall prognosis for people with Legionnaires’ disease is variable. While some people experience rapid improvement upon treatment, others may take much longer to recover. One study found that, out of 190 people with Legionnaires’ disease, many were still experiencing one or more health problems one year after the onset of infection, such as fatigue and reduced general quality of life.
The mortality rate for Legionnaires’ disease can go up to 50 percent in people who contracted the illness in a hospital setting. However, the outlook for Legionnaires’ disease depends on multiple factors including the overall health of the affected person, the promptness and course of treatment, as well as where the infection was acquired.
Other names for Legionella infection
- Legionnaires’ disease
- Pontiac fever
- Legionella pneumonia
- Legionella pneumophila infection
Legionella infection FAQs
Q: What causes Legionella infection?
A: Legionella infection is caused by bacteria called Legionella, of which there are numerous species. Although any species of Legionella bacteria can cause an infection, it is believed that up to 90 percent of adult cases of Legionnaires’ disease are caused by a specific species called Legionella pneumophila.
Q: Do Legionella bacteria live in water?
A: Legionella bacteria are most commonly found in water. Legionella pneumophila is often found in natural freshwater sources, such as rivers and lakes. Legionella becomes a health concern when it spreads to human-made water systems, such as air conditioning cooling towers, hot tubs and water fountains, where it may be breathed in and cause an infection.
Q: What’s the difference between Legionnaires’ disease and Pontiac fever?
A: Both Legionnaires’ disease and Pontiac fever are caused by Legionella bacteria and are a type of Legionella infection. When the bacteria infect the lungs, a potentially severe form of pneumonia called Legionnaires’ disease is caused. This often results in respiratory symptoms, such as coughing and shortness of breath, as well as flu-like symptoms, such as fever and muscle aches. Legionnaires’ disease typically requires hospitalization and immediate antibiotic treatment.
Pontiac fever is a less common and much milder form of Legionella infection. It does not cause pneumonia, and symptoms usually resemble those of a mild flu infection. Pontiac fever is generally self-limiting, meaning it goes away on its own without medical intervention.
Q: Is Legionella infection contagious?
A: Legionella infection is generally not transmitted between humans, likely due to the fact that a high dose of Legionella bacteria is required to cause infection. Only one case of possible person-to-person infection has been reported. Instead, Legionella infection is most often caught when a person inhales airborne water droplets containing the bacteria.
Q: What is the origin and history of Legionnaires’ disease?
A: Legionella bacteria were first discovered after an outbreak of Legionnaires’ disease in 1976. The outbreak occurred at a convention of the American Legion, which is why the bacteria are now known as Legionella. This pneumonic form of the illness subsequently became known as Legionnaires’ disease.
The milder, non-pneumonic form of the infection, called Pontiac fever, was first identified in Pontiac, Michigan during 1968. However, it wasn’t until Legionella bacteria were discovered in 1976, that health professionals were able to confirm the two conditions are both caused by the same bacteria.