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Potentially life-threatening emergency
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Potentially life-threatening emergency


Contributors: Neil Mendoza MD, Paritosh Prasad MD
Other Resources UpToDate PubMed


Legionellosis is caused by Legionella pneumophila, a small gram-negative bacillus found in aqueous environments.

Almost all cases of legionellosis are a result of inhalation of aerosolized bacteria residing in warm man-made water bodies, such as water heaters, air conditioning equipment, cooling towers, warm-water baths, warm-water plumbing systems, and recirculating water systems. Contamination of such water systems in hospitals has led to nosocomial outbreaks of disease. Person-to-person transmission does not occur.

Legionellosis is believed to occur throughout the world; 8 000-18 000 cases occur each year in the United States. About a quarter of these cases originate in a hospital environment and are associated with a higher proportion of fatalities. Most cases are sporadic. Less than one-fifth of all cases are associated with an outbreak.

Cigarette smoking, chronic heart and lung disease, male sex, diabetes mellitus, end-stage renal disease, cancer, and advanced age (older than 50 years) are some of the host factors that increase the risk for contracting legionellosis. Patients who are immunosuppressed are at a higher risk for infection with L pneumophila. Occupational exposures increase risk, such as hospital employment, work as a heating and air-conditioning technician, and building maintenance work. Social exposures include hot tub use.

The incubation period is generally between 2-10 days, but cases with incubation periods as long as one month have been reported. A prodromal illness consisting of headaches, myalgias, weakness, diarrhea, and abdominal pain may occur. These symptoms may suggest a viral illness and lead to a misdiagnosis. Cough, shortness of breath, pleuritic chest pain, and fever, often very high (40°C [104°F] or higher), will eventually predominate. Mental confusion may be present. Clinical examination findings are nonspecific and may include focal rales or signs of lung consolidation. The viral-like prodrome that does not progress to pneumonia is also referred to as Pontiac fever and is self-limited. Pontiac fever has been recognized only during outbreaks of legionellosis.

A number of nonspecific findings such as pulse-temperature dissociation, hyponatremia, abnormal liver enzymes, lymphocytopenia, thrombocytopenia, disseminated intravascular coagulation, elevated creatine kinase (CK), and elevated serum lactate dehydrogenase (LDH) have been described with legionellosis. These findings are insufficiently specific and sensitive to be of diagnostic value.

Rare cases of extrapulmonary disease with manifestations such as brain abscess, intra-abdominal abscess, infection of surgical wounds, cellulitis, and myocarditis have been described.


A48.1 – Legionnaires' disease

26726000 – Legionellosis

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Differential Diagnosis & Pitfalls

Agents causing community-acquired pneumonia such as Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis can present with a similar picture. Legionellosis accounts for 2%-5% of cases of community-acquired pneumonias.

Other diagnoses within the differential include:

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Drug Reaction Data

Below is a list of drugs with literature evidence indicating an adverse association with this diagnosis. The list is continually updated through ongoing research and new medication approvals. Click on Citations to sort by number of citations or click on Medication to sort the medications alphabetically.

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Last Reviewed:06/26/2017
Last Updated:12/19/2021
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Potentially life-threatening emergency
Patient Information for Legionellosis
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Contributors: Medical staff writer


Legionellosis is the name for infection caused by the Legionella pneumophila bacterium. Infection can take two forms: the more serious legionnaires' disease, which is a pneumonia, and the less serious Pontiac fever. The L. pneumophila bacterium got its name from an outbreak at a convention of the American Legion in 1976, when many people staying in the same hotel developed an unusual pneumonia. Legionella pneumophila is found naturally in water and is often found in the water systems of large buildings, such as the heating and cooling elements of hotels and hospitals.

Legionellosis is transmitted by inhaling water droplets that are contaminated with L. pneumophila. There is no spread of legionellosis by person-to-person contact. Examples of sources of contaminated water include air conditioning units (usually those servicing very large buildings, not small window units) or steam from a sauna or whirlpool.

Because the infection is somewhat difficult to diagnose, and because common treatments for other respiratory infections will successfully treat legionellosis (even without a diagnosis ever being made), many cases of legionellosis probably go unrecognized. Usually, it is not a serious infection, though in patients who are already ill or who are at risk, mortality may be 30%.

Who’s At Risk

Anyone who is exposed to L. pneumophila is at risk for developing legionellosis. However, healthy children and adults rarely develop a symptomatic infection. People at specific risk include:
  • The elderly
  • The immunocompromised
  • People who are ventilated (on a breathing machine)
  • People with lung diseases such as emphysema or COPD (chronic obstructive pulmonary disease)
  • People who smoke
Healthy people usually recover quickly from legionellosis with appropriate treatment; it may be fatal 30% of the time, however, and those who die usually have some of the risk factors listed above.

Signs & Symptoms

Signs and symptoms of legionellosis usually develop within 14 days of exposure to the bacterium. The symptoms are not very different from many other illnesses, so legionellosis can be difficult to diagnose. The symptoms usually last for less than a week and get better within 5 days.

Symptoms of legionellosis include:
  • Fever
  • Chills
  • Cough, sometimes with blood
  • Feeling generally weak and achy (malaise)
  • Headache

Self-Care Guidelines

Rest and stay well hydrated. Caregivers and household members should note that legionellosis is not contagious from person to person.

When to Seek Medical Care

Always see a doctor when you have fever and cough, particularly if you have any of the risk factors listed above.


To diagnose legionellosis, your doctor will need to perform specific tests on your sputum (the material that you cough up) and your urine to confirm the presence of the bacterium. The doctor may order a chest x-ray to confirm the diagnosis of pneumonia. There is no definitive blood test for legionellosis. Common antibiotics are used to treat legionellosis.
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Potentially life-threatening emergency
A medical illustration showing key findings of Legionellosis : Cough, Diarrhea, Fever, Headache, Nausea, Pleuritic chest pain, Dyspnea, Myalgia
Imaging Studies image of Legionellosis - imageId=2954652. Click to open in gallery.  caption: 'Frontal chest x-ray with opacities in the bilateral lower lobes in this patient with Legionnaires' disease.'
Frontal chest x-ray with opacities in the bilateral lower lobes in this patient with Legionnaires' disease.
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