Legionnaires Disease on Rise in US--2016 Update

Laura A. Cooley, MD, MPHTM


June 13, 2016

Editorial Collaboration

Medscape &

(Editor's note: This is an update to an article published on Medscape in 2015: What Everyone Needs to Know About Legionnaires Disease.)


Legionnaires disease has been in the news recently, with outbreaks in New York, Illinois, California, and Michigan. Despite these outbreaks garnering media attention, Legionnaires disease continues to be underdiagnosed.

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 is a milder, influenza-like illness that generally resolves on its own. Among those who develop Legionnaires disease, about 10% will die of their illness. This case fatality rate tends to be higher among patients with healthcare-associated Legionnaires disease.

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.

Transmission of Legionella

Legionella are transmitted by aerosolized water containing the bacteria. Less commonly, these bacteria are transmitted by aspiration of drinking water. Legionella are generally not transmitted from person to person; however, a single episode of possible person-to-person transmission of Legionnaires disease has been reported.[1] Most people exposed to the bacteria do not become ill.

Legionella can be found everywhere in natural, freshwater environments, but generally, in those environments the bacteria are present in insufficient numbers to cause disease. In human-made water systems like the plumbing of large buildings (eg, water heaters, storage tanks, and 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) allow for amplification of Legionella. Water systems that are not properly cleaned, maintained, or disinfected are at risk for Legionella amplification.

Most 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 before the onset of illness), and 7% of those with Legionnaires disease stayed overnight in a healthcare facility during their incubation periods. People older than 50 years, smokers, former smokers, and people with underlying medical conditions such as chronic lung disease or weakened immune systems are at increased risk of developing Legionnaires disease.

Pontiac fever has a shorter incubation period (6-48 hours after exposure) and most commonly affects young, healthy adults.[2] 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 for Legionnaires Disease

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; and

  • Exposure to settings where the plumbing has had recent repairs or maintenance work.

Burden of Disease

The number of legionellosis cases reported to the Centers for Disease Control and Prevention (CDC) has been on the rise over the past decade.[3] The reason for this increase is unknown but is likely multifactorial. This rise could represent a true increase in the frequency of disease related to several factors, such as a greater number of persons at risk for legionellosis because of underlying illness or immunocompromising medications, an aging US population, an aging plumbing infrastructure, or changes in the climate. Increased use of diagnostic testing (although still underutilized) because of greater awareness among clinicians and availability of diagnostic tests, as well as more reliable reporting to local and state health departments and CDC, could also be playing a role.

About 5000 cases of Legionnaires disease are now reported to CDC each year.[4] Accurate data reflecting the true incidence of this disease are not available because of underutilization of diagnostic testing.

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