Legionella Outbreaks on the Rise: Could Climate Change Be Responsible?

Lack of plumbing maintenance; aging infrastructure; warm, humid climates; and exposure to the immunocompromised may contribute to an increase in Legionnaires disease.

Are warming global temperatures responsible for the rise of Legionnaires disease (LD) cases? Some epidemiologists say the problem is multifactorial: lack of plumbing maintenance; aging infrastructure; warm, humid climates; and exposure to the immunocompromised.1 Since 2000, the number of LD cases in the United States has increased by a factor of nearly 4.5.1 In 2016, there were 6141 cases of LD reported.1

Ideal Conditions for Legionella Bacteria

Indeed, Legionella has been long overlooked as a cause for hospital admissions for community-acquired pneumonia (CAP), occurring in 2% to 15% of patients.2 Clinicians do not routinely test for it, and 10% of LD cases go undetected.1

Although Legionella occurs throughout the world, ideal conditions include2:

  • Water temperatures of 25° to 42 °C
  • Damp soil
  • Rainfall 6 to 10 days before infection
  • Summer or autumn weather

What might account for the increase in LD cases? The reasons are many; however, increasing rainfall because of climate change may play a role.2 Another theory is that vehicles driving on wet surfaces may further distribute the aerosolized bacteria.

Associate Professor Ryota Sakamoto, MD, PhD, from Kyoto University in Japan, who studies legionellosis in human environments, advises clinicians to use the urinary antigen test of Legionella “at least for the patients with severe CAP. I believe cases with LD exist almost everywhere in this world. My worry is that the number may increase because of global warming, which is estimated to increase precipitation in some regions. The problem is that many clinicians consider LD as a rare disease, but the fact is Legionella is one of the major causes of severe CAP.”

How the Environment Affects Our Health

Warming global temperatures and seas will benefit some microorganisms, which thrive in warm, moist environments.3 In the United States, 900,000 cases and 900 deaths have been associated with water-borne illnesses annually as a result of droughts and flooding.3 And it is not just LD that is increasing. Vibrio infections are on the rise, causing gastrointestinal illnesses globally.3 In addition, nontuberculous mycobacteria lung disease is becoming common in states that have a disproportionate number of weather-related disasters.3 Although Legionella can be found in water and damp soil, the most common route to humans is through aerosolized droplets containing the bacteria, usually via showers and evaporative cooling towers.3

How the Veterans Health Administration Thwarts LD

As the nation’s largest integrated healthcare system, the Veterans Health Administration (VHA) is concerned about preventing and containing LD cases.4 To determine how healthcare-acquired LD was occurring across the country, Shantini D. Gamage, PhD, MPH, associate director of VHA National Infectious Diseases Service, Cincinnati, Ohio, and colleagues reported on the number of cases diagnosed in its facilities for inpatients and outpatients.4

What the researchers found was that LD rates increased both inside and outside the VHA, particularly in the East North Central region.4 Although the overall rate of LD increased during the 2-year study, the rate of healthcare-acquired LD decreased, suggesting that prevention efforts within the VHA were effective. Of the 447 LD cases within the VHA, only 9% could be traced to overnight exposure. The inpatient infection rate was 1.5 to 2.0 per 100,000 enrollees (P =.04), whereas the outpatient infection rate was 2.3 to 3.0 per 100,000 enrollees (P =.04).4

“LD looks like other pneumonia, but in hospitalized patients, it can be quite severe. There is a higher mortality rate,” explained Dr Gamage. “There were more cases east of the Mississippi River, but we do not know why. LD can occur anywhere.”

As for the increase in LD cases, Dr Gamage said, “We do not know why there [has been] an increase in cases. It could be that more clinicians are testing their patients, and it [has] been in the news lately with the New York City cooling tower cases.”

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The Effect of Climate Change Beyond LD

Pulmonologists will see a rise in more than just sea levels, with airborne and waterborne pathogens threatening people with respiratory diseases.5 Warmer temperatures globally will lead to expanding insect populations, whose spread has been documented in places such as Alaska, where allergic insect bites were not previously reported.5 Increasing pollution will lead to more exacerbations for patients with asthma. In a literature review of environmental effects on health, ≥17 studies have linked wildfires to increases in emergency department visits or hospitalizations for exacerbations of respiratory illnesses.5

“Known change factors with current evidence include an increased growth of allergenic plants in response to higher carbon dioxide levels and warmer temperatures,” said Charles S. Barnes, PhD, professor and director of the Allergy, Asthma and Immunology Research Laboratory at Children’s Mercy Hospital and The University of Missouri-Kansas City School of Medicine in Kansas City, Missouri. “This also means an increased production of pollen as well as the appearance of allergenic species (including stinging insects) in new climactic areas and a shift and extension of pollen seasons with warmer springs and later frosts.”

Dr Barnes also cautioned about “increasing damp housing exposure related to sea level rise” and the “expected higher number of ozone alert days and increased pollution in populated areas, along with increased exposure to smoke and particles from wildfires, resulting from heat waves. These are all expected to contribute to a general increase in respiratory disease,” he said.

Summary and Clinical Applicability

Warming temperatures increase the risk for spreading bacteria, and Legionella is no exception. Legionnaires disease favors warm and wet environments, which have been increasing the window of opportunity to spread via aerosolized bacteria, especially in the summer and autumn.

Limitations and Disclosures



1. Rubin R. Why are Legionnaires disease diagnoses becoming more common in the United States? JAMA. 2018;319(17):1753-1754.

2. Sakamoto R. Legionnaire’s disease, weather and climate. Bull World Health Organ. 2015;93(6):435-436.

3. Walker JT. The influence of climate change on waterborne disease and Legionella: a review. Perspect Public Health. 2018;138(5):282-286.

4. Gamage SD, Ambrose M, Kralovic SM, Simbartl LA, Roselle GA. Legionnaires Disease surveillance in US Department of Veterans affairs medical facilities and assessment of health care facility association. JAMA Netw Open. 2018;1(2):e180230.

5. Barnes CS. Impact of climate change on pollen and respiratory disease. Curr Allergy Asthma Rep. 2018;18(11):59.