What Everyone Needs to Know About Legionnaires Disease

Preeta K. Kutty, MD, MPH


October 26, 2015

Editorial Collaboration

Medscape &

Editor's Note: For an updated article from CDC on this topic, see Legionnaires Disease on Rise in US-2016 Update.

Legionnaires Outbreaks

Legionnaires disease has been in the news lately, with outbreaks in New York City, New York; Quincy, Illinois; and San Quentin, California. Despite these outbreaks garnering media attention, Legionnaires disease continues to be underdiagnosed and underreported. Here is what you need to know.


Legionellosis is a respiratory infection caused by Legionella bacteria; infection can manifest as either Legionnaires disease or Pontiac fever. Legionnaires disease is a common form of severe pneumonia requiring hospitalization, whereas Pontiac fever generally resolves on its own. Among those who develop Legionnaires disease, 5%-30% will die of their illness.

There are at least 60 different species of Legionella, and most are considered capable of causing disease. However, most disease is caused by Legionella pneumophila, particularly serogroup 1.


Legionella are transmitted by aerosolized water containing the bacteria. Less commonly, these bacteria can be transmitted by aspiration of drinking water. Legionella are not transmitted from person to person, and most people exposed to the bacteria do not become ill.

Legionella can be found everywhere in natural, freshwater environments but generally are present in insufficient numbers to cause disease. In man-made water systems like the plumbing of large buildings (eg, hot water heaters, storage tanks, pipes), cooling towers, decorative fountains, or hot tubs, Legionella can amplify and be transmitted to susceptible hosts through aerosolization. Certain conditions (eg, temperature, the amount of nutrients, pH) allow for amplification of Legionella. Water systems that are not properly cleaned, maintained, or disinfected are at risk for Legionella amplification.

The majority of legionellosis outbreaks are associated with hotels, resorts, cruise ships, hospitals, and long-term care facilities. More than 20% of all persons with Legionnaires disease have traveled during their incubation periods (2-14 days after exposure). And 7% of those with Legionnaires disease stayed overnight in a healthcare facility during their incubation periods. Most people who develop Legionnaires disease have a medical condition that makes them more susceptible to developing the infection or are smokers.

Pontiac fever has a shorter incubation period (6-48 hours after exposure) and most commonly affects young, healthy adults.[1] The pathogenesis of Pontiac fever is poorly understood, and why exposure to Legionella may result in these two clinically and epidemiologically distinct syndromes is not known.

Risk Factors

Risk factors for developing Legionnaires disease include:

  • Renal or hepatic failure;

  • Diabetes;

  • Chronic lung disease;

  • Systemic malignancy;

  • Smoking (current or historical);

  • Immune system disorders; and

  • Age ≥50 years.

Risk factors for exposure to Legionella include:

  • Recent travel with an overnight stay outside of the home, including a stay in a healthcare facility;

  • Exposure to hot tubs or other recreational water; and

  • Exposure to domestic plumbing that has had recent repairs or maintenance work.

Burden of Disease

The number of legionellosis cases reported to Centers for Disease Control and Prevention (CDC) has been on the rise over the past decade.[2] This rise may reflect a true increase in the frequency of disease due to a number of factors (eg, older US population, more at-risk individuals, aging plumbing infrastructure, climate). It may also be a result of increased use of diagnostic testing or more reliable reporting to local and state health departments and to CDC.

Between 2008 and 2012, a total of 3000-4000 cases of Legionnaires disease were reported to CDC each year.[3] Yet, research studies with thorough diagnostic testing estimate that 8000-18,000 hospitalized cases of the disease may occur in the United States each year. Accurate data reflecting the true incidence of this disease are not available because of underutilization of diagnostic testing and underreporting.

More illness is usually found in the summer and early fall, but legionellosis can happen any time of year. Legionellosis is reported more commonly in the mid-Atlantic and nearby states than in other parts of the country.

Diagnosis and Testing

Clinical features of Legionnaires disease include cough, fever, and radiographic pneumonia. For Pontiac fever, clinical features include flu-like illness (ie, fever, chills, malaise) without pneumonia.

Indications that warrant testing for Legionnaires disease include (Infectious Diseases Society of America/American Thoracic Society consensus guidelines on the management of community-acquired pneumonia in adults can be found here):

  • Patients who have failed outpatient antibiotic therapy for community-acquired pneumonia;

  • Patients with severe pneumonia, in particular those requiring intensive care;

  • Immunocompromised patients with pneumonia;

  • Patients with pneumonia in the setting of a legionellosis outbreak;

  • Patients with a travel history (patients who have traveled away from their home within 2 weeks before the onset of illness); and

  • Patients suspected of having healthcare-associated pneumonia.

The preferred diagnostic tests for Legionnaires disease are culture of respiratory secretions on selective media and the Legionella urinary antigen assay. Isolation of Legionella from respiratory secretions or lung tissue is confirmatory and an important method for diagnosis, despite the convenience and specificity of urinary antigen testing. If your patient has pneumonia, and the urinary antigen test is positive for Legionella, then your patient is considered to have Legionnaires disease. This test is designed to detect the most common cause of legionellosis (L pneumophila serogroup 1). However, all species and serogroups of Legionella are potentially pathogenic, so a patient with a negative urinary antigen result may have legionellosis caused by some other member of the Legionella genus. In addition, if urinary antigen testing is negative, but Legionnaires disease is still suspected, then a respiratory culture is required.

Finally, molecular techniques can be used to compare clinical isolates to environmental isolates and confirm the source of an outbreak. Thus, best practice for detection of Legionella and for public health surveillance is to also obtain respiratory specimens for culture at the time urinary antigen testing is ordered, preferably before the administration of antibiotics.

Most cases of Pontiac fever are diagnosed in association with an outbreak, on the basis of clinical signs and symptoms, often along with cases of Legionnaires disease. The urinary antigen test as well as serology can be used to confirm the diagnosis; however, owing to the low sensitivity of these tests in the setting of Pontiac fever, they cannot be used to rule it out. Serologic confirmation requires a fourfold change between acute and convalescent sera collected 3-6 weeks apart.[4]


If your patient has Legionnaires disease, see the most recent guidelines for treatment of community-acquired pneumonia. If your patient has Pontiac fever, antibiotic therapy should not be prescribed. It is a self-limited illness that does not benefit from antibiotic treatment. Recovery usually occurs within 1 week.


Legionellosis is a nationally notifiable disease in the United States that is monitored through two surveillance systems at the national level. With improved diagnosis and reporting, public health experts can better understand the true burden of legionellosis.

Timely identification and reporting of legionellosis cases is important because this allows public health officials to quickly identify and stop potential clusters and outbreaks. Outbreaks among travelers can be difficult to detect because of the low attack rate, long incubation period, and the dispersal of people from the source of the outbreak, so collecting and reporting information about overnight travel in the 14 days prior to onset is important. Healthcare facility exposures can be difficult to ascertain if the patient has not been in the same facility for the entire incubation period or was discharged prior to onset and readmitted. Outpatient, employee, and visitor exposures should be reported because they can help determine the scope and source of an outbreak. Timely reporting of healthcare-associated cases ensures that steps can be taken to protect these highly susceptible populations.


The key to preventing legionellosis is maintenance of the water systems in which Legionella may grow, including drinking water systems, hot tubs, decorative fountains, and cooling towers. If Legionella bacteria are found, facilities should be prepared to eliminate them, especially if they serve people at higher risk for legionellosis. CDC encourages all building owners and especially healthcare facilities to develop comprehensive water safety management plans. Persons at increased risk for infection may choose to avoid high-risk exposures, such as being in or near a hot tub.