How does transmission of Legionella occur?

Answer

Legionella transmission is thought to occur via inhalation of aerosolized mist from water sources, such as the following, that have been contaminated with the bacterium^{[2, 3, 4]}

Cooling systems
Showers
Decorative fountains
Humidifiers
Respiratory therapy equipment
Whirlpool spas
Ice machines
Potting soil (L longbeachae spp)
Compost (L longbeachae spp)
Roadside puddles
Tubs used for water births^[5]

Legionnaires disease may be travel associated from exposure in aircraft or hotel facilities. Person-to-person transmission, however, has not been documented.

The highest incidence of Legionnaires disease occurs during late spring and early fall, when air-conditioning systems are used more frequently.^{[6, 7]} Nosocomial acquisition likely occurs via aspiration, respiratory therapy equipment,^[3] or contaminated water. In addition, transmission has been linked to the use of humidifiers, nebulizers, and items that were rinsed with contaminated tap water.

The following features increase the likelihood of colonization and amplification of legionellae in human-made water environments:

Temperature of 25-42°C
Stagnation
Scale and sediment
Presence of certain free-living aquatic amoebae capable of supporting intracellular growth of legionellae

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van Loenhout JA, van Tiel HH, van den Heuvel J, Vercoulen JH, Bor H, van der Velden K, et al. Serious long-term health consequences of Q-fever and Legionnaires' disease. J Infect. 2014 Jan 25. [Medline].
Franco-Garcia A, Varughese TA, Lee YJ, Papanicolaou G, Rosenblum MK, Hollmann TJ, et al. Diagnosis of Extrapulmonary Legionellosis in Allogeneic Hematopoietic Cell Transplant Recipients by Direct 16S Ribosomal Ribonucleic Acid Sequencing and Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry. Open Forum Infect Dis. 2017 Summer. 4 (3):ofx140. [Medline].
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[Guideline] Arizona Department of Health Services. Guidelines for water immersion and water birth. ADHS. Available at http://www.azdhs.gov/documents/licensing/special/midwives/training/guidelines-for-water-immersion-water-birth.pdf. November 2016; Accessed: June 9, 2017.

Media Gallery

This electron micrograph depicts an amoeba, Hartmannella vermiformis (orange), as it entraps a Legionella pneumophila bacterium (green) with an extended pseudopod. After it is ingested, the bacterium can survive as a symbiont within what then becomes its protozoan host. The amoeba then becomes a so-called "Trojan horse," since, by harboring the pathogenic bacterium, the amoeba can afford it protection. In fact, in times of adverse environmental conditions, the amoeba can metamorphose into a cystic stage, enabling it, and its symbiotic resident, to withstand the environmental stress. Image courtesy of the Centers for Disease Control and Prevention and Dr. Barry S Fields.

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Author

Chinelo N Animalu, MD, MPH Assistant Professor, Division of Infectious Diseases, Department of Internal Medicine, University of Tennessee Health Science Center College of Medicine; Physician, Department of Infectious Disease, University of Tennessee Methodist Physicians (UTMP), Methodist University Hospital

Chinelo N Animalu, MD, MPH is a member of the following medical societies: American College of Physicians, American Medical Association, Infectious Diseases Society of America, Memphis Medical Society, Tennessee Medical Association

Disclosure: Nothing to disclose.

Specialty Editor Board

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Received salary from Medscape for employment. for: Medscape.

Chief Editor

Michael Stuart Bronze, MD David Ross Boyd Professor and Chairman, Department of Medicine, Stewart G Wolf Endowed Chair in Internal Medicine, Department of Medicine, University of Oklahoma Health Science Center; Master of the American College of Physicians; Fellow, Infectious Diseases Society of America; Fellow of the Royal College of Physicians, London

Michael Stuart Bronze, MD is a member of the following medical societies: Alpha Omega Alpha, American College of Physicians, American Medical Association, Association of Professors of Medicine, Infectious Diseases Society of America, Oklahoma State Medical Association, Southern Society for Clinical Investigation

Disclosure: Nothing to disclose.

Additional Contributors

Frank C Smeeks, III, MD Specialty Medical Director of Emergency Medicine, Applied Medico Legal Solutions

Frank C Smeeks, III, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Emergency Medicine, American College of Emergency Physicians, American Association for Physician Leadership, American Medical Association, North Carolina Medical Society

Disclosure: Nothing to disclose.

Burke A Cunha, MD Professor of Medicine, State University of New York School of Medicine at Stony Brook; Chief, Infectious Disease Division, Winthrop-University Hospital

Burke A Cunha, MD is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, Infectious Diseases Society of America

Disclosure: Nothing to disclose.

Acknowledgements

Joseph F John Jr, MD, FACP, FIDSA, FSHEA Clinical Professor of Medicine, Molecular Genetics and Microbiology, Medical University of South Carolina College of Medicine; Associate Chief of Staff for Education, Ralph H Johnson Veterans Affairs Medical Center