Legionnaire's Disease

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University of Oklahoma Study Abroad Microbiology in Arezzo, Italy[1]

Scanning electron microscope image of Legionella pneumophila. From: MicrobLog [2]

Etiology/Bacteriology

Taxonomy

Description

Legionnaires disease is a type of pneumonia caused by the pathogenic proteobacterium Legionella pneumophila. L. pneumophila are motile, aerobic, non-spore forming, Gram-negative rods that reside in freshwater complexes with high algae and amoeba count. [12]

Pathogenesis

Transmission

Transmission of L. pneumophilais spread through contaminated mist from water sources such as hot tubs
From: legionellaprevention.org [3]

Legionella pneumophila survive and thrive in warm, damp environments, typically those that have a high amoeba count. This includes full bodies of freshwater, soil, and various areas of fresh dirt (Australian Health Page). L. pneumophila are often spread through airborne water droplets, known as a aerosols. Optimal growth of the bacteria is 28-40°C, but the bacteria does not survive past 60°C (emedicine) Because of the high temperature in which the bacteria are able to survive, man-made public water systems such as showers, hot tubs, and fountains are the perfect place for L. pneumophila to thrive. (Australian Health Page). The transmission of the bacteria can come from a mist or vapor emitted from a fresh water system that has not been properly disinfected (CDC). The bacteria, however, cannot be spread through person-to-person contact, unlike other forms of pneumonia. (CDC) Many people that are exposed to the L. pneumophila do not develop any symptoms; therefore, those that surround an infected person in a work or school place are not at risk of developing the disease (CDC).

Infectious Dose, Incubation, and Colonization

Epidemiology

While Legionnaires disease can be found worldwide, the most common places for infection are the United States, Canada, Australia, parts of Asia, and Europe (science direct). There are approximately 8,000-18,000 cases of Legionnaires disease reported in the United States alone each year, and there are many cases that remain unreported. (emedicine) Although the disease can be contracted sporadically throughout the year, the peak season for the pneumonia is June-October. The humidity and warm, wet climate found in during these months tend to be optimal living conditions for L. pneumophila, causing the number of bacteria to increase (science direct). Legionellosis is known to be the cause of 5-15% of community-acquired pneumonia (e medicine). The disease usually occurs in adults over the age of 50, and the ratio of men to women affected is 4:3 (science direct).

The largest known outbreak of Legionella acquired pneumonia was in July 1976 in Philadelphia. Approximately 182 people were infected, 147 required hospitalization, and of those, 29 people died. The source of the outbreak has been linked to the cooling towers at the Bellevue Stratford Hotel. The bacteria is believed to have been colonized in the cooling towers and distributed amongst the guests at the hotel through the air conditioner units. (Pennsylvania book).

Virulence Factors

Clinical Features

Chest x-ray of Legionnaires disease patient
From: newyorkmedicaljournal.org [4]

Many of the symptoms of Legionnaires Disease are characteristic of pneumonia. Symptoms typically begin with a shortness of breath followed by a cough. A high fever, headaches, muscle aches, and malaise are other symptoms that occur with pneumonia caused by Legionella pneumophila. [2] Signs of symptoms can appear 2-14 days after being exposed to the pathogen. Many people that come into contact with Legionella pneumophila do not show symptoms. Legionnaires disease can be found in anyone; however, it is usually found in patients over 50, heavy smokers, and immunocompromised patients. [1]

Morbidity/Mortality

Due to the very low infection rate, the morbidity rate of Legionnaires disease is low as well. About 1-5% of the patients that contract the disease require hospitalization, and those patients that are generally treated and cured through antibiotic treatment (medicine) . Many factors such as the severity of the disease, the age of the patient, and how quickly the patient begins antibiotic treatment all contribute to the survival rate of the patient. [13] The mortality rate of Legionnaires disease is approximately 5-30% [13]. An increase in knowledge and awareness about how Legionellosis is spread has allowed the mortality rate to drop drastically worldwide in recent years. [13]

Diagnosis

On the surface, Legionnaires disease is difficult to differentiate from other types of pneumonia, there are multiple simple tests that can be run in order to determine if the symptoms are caused by Legionella pneumophila. One of the most affective and efficient tests is a urine antigen test. [12] Urine antigen tests take approximately one hour to detect the antigen produced by L. pneumophila and is 98-100% specific [13]. There are, however, a number of serotypes that cannot be detected in a urine sample. Other options in diagnosing Legionnaires disease include culturing blood or patient's sputum or detection of the pathogen in respiratory secretion through a fluorescent antibody test [12].

Treatment

There are approximately 8,000-10,000 Legionnaires disease patients that are hospitalized each year in the United States alone [12]. Nevertheless, Legionellosis can be easily treated with the proper dosage of antibiotics. For milder cases, broad-spectrum antibiotics are sometimes accepted as a choice of treatment [4]. For cases in which the patient experiences much more severe symptoms, a combination of Rifampicin and Erythromycin can be used. However, various types fluoroquinolone drugs are the most common and most effective way to treat Legionnaires disease [4]. The use of fluoroquinonlone removes the need for Rifampicin therapy, and is therefore more efficient because only one antibiotic is required [4]. Many other strands of pneumonia are often treated with Beta-lactam antibiotics; Legionnaires disease, on the other hand, does not respond to Beta-lactam drugs [4].

Prevention

Risk Avoidance

Immunization

Host Immune Response

References

Rathore, M. and A. Alvarez. 2014. eMedicine: Legionella infection. http://www.emedicine.com/ped/topic1288.htm (Description)

Sheff, Barbara. Microbe of the month: Legionella pneumophila. Nursing2004. 34:74. http://web.a.ebscohost.com.ezproxy.lib.ou.edu/ehost/pdfviewer/pdfviewer?sid=4b6f670a-bf77-4c0a-a19a-768789fa1a0f%40sessionmgr4002&vid=2&hid=4207 (Symptoms)

Centers for Disease Control and Prevention (CDC). Legionella (Legionnaires' Disease and Pontiac Fever). Available at http://www.cdc.gov/legionella/about/index.html (Symptoms)

Centers for Disease Control and Prevention (CDC). Top 10 Things Every Clinician Needs to Know About Legionellosis. Available at http://www.cdc.gov/legionella/clinicians.html#footnotes (Morbidity)

Bartram, Jamie, et al. 2007. Legionella and the prevention of legionellosis. World Health Organization Library Catalogue. http://www.who.int/water_sanitation_health/emerging/legionella.pdf (Morbidity)

Created by MaKenzi Burke, student of Tyrrell Conway at the University of Oklahoma.