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Pathogenicity, Immunology and Epidemiology: Legionnaires’ Disease

Outbreak and Bacteria

In September of 2017, several people visiting Anaheim, California, including the Disneyland park located there, fell ill. Ten people were hospitalized, and one person died due to complications from co-morbidities. Overall, twelve cases of bacteria-associated illness were diagnosed. Officials identified it as Legionnaire’s disease, a serious lung infection (Barboza, 2017). The Legionella bacterium causes the disease. It is commonly found in freshwater environments, but can also be observed in artificial human systems such as cooling towers, fountains, water tanks, and plumbing systems. It is a gram-negative, aerobic bacterium. It stains poorly. It has a short rod form with the length up to 5 μm. When cultured, it has an elongated filamentous bacillus form that can be 10-25 μm in length (“Legionella pneumophila,” n.d.).

Outbreaks – Globally

Outbreaks Globally

Map of risk distribution of Legionella outbreaks globally, with Western Europe and Latin America being highly dangerous areas (Yang, LeJeune, Alsdorf, Lu, Shum, & Liang, 2012).

Small-scale outbreaks occurred around the world in the last four years. An outbreak happened at Dubai, UAE in 2016 as it was noticed of persistent cases of Legionella in Europeans returning from the area. Approximately 22 cases were reported. A series of outbreaks occurred in Western Europe in the 2017-2018 year, stretching from England to several outbreaks in Spain and Lisbon, Portugal. Lisbon had a large outbreak with over 50 cases and five deaths. Sydney, Australia, and New Zealand have experienced several small-scale (ranging 3-5 cases) outbreaks in the last two years as well. The only reported case in South and Latin America occurred in a hotel in Peru. Most of the reported cases were investigated, and it was found that isolated water sources such as swimming pools, water tanks, or cooling towers were the source of the disease (HCInfo, n.d.).

Outbreaks – United States

Outbreaks United States

The United States has experienced several relatively large outbreaks recently with many smaller-scale ones as well. The most massive outbreak occurred in 2015 in New York City when more than 120 people contracted the disease in South Bronx due to cooling towers. Since then, various neighborhoods of NYC have experienced smaller outbreaks every single year despite increased regulation of water in cooling towers. Other states, including Florida, Texas, Washington, Ohio, Tennessee, California have experienced small-scale (3-5 cases) outbreaks in a variety of locations such as hotels, medical centers, assisted living facilities, entertainment parks, gyms and prisons (HCInfo, n.d.). This shows that the Legionella bacterium can be present virtually in any water source or system, even if it is under careful regulation. However, most often it is found in stagnant water sources with massive public use such as hotel pools and spas.

Transmission to Humans

The Legionella bacterium grows and spreads throughout water systems. This includes building water systems. It is spread to humans through tiny water droplets which are inhaled out of the air. It is rare that people get infected via drinking the contaminated water. That occurs only in cases of accidental chocking which allows water droplets to enter the lungs. The disease cannot, in any manner, be spread from human to human contact (Centers for Disease Control and Prevention, 2018). The water contaminated with the bacterium can be aerosolized through common water sources including showers, fountains, faucets, and cooling towers. The prevalent source of Legionella, especially during summer, is cooling towers because they are used in air conditioning systems and changes to water chemistry at higher temperatures (Prussin, Schwake, & Marr, 2017).

Symptoms and Clinical Presentation

The infection with the Legionella bacterium presents itself in two common forms of legionellosis: Legionnaire’s disease and a less dangerous Pontiac fever. Legionellosis symptoms are often similar to pneumonia and include coughing, high fever, diarrhea, chest pain, nausea, vomiting, chest pain (with difficulty breathing), and potential neurological problems. Fatalities can range from 10 to 50% of the infected population (Prussin et al., 2017). At risk, populations include being over the age of 50, smoking habits, weakened immune system, and presence of chronic lung disease. Legionnaire’s Disease presents itself clinically as pneumonia after a 2-10-day incubation period. Pontiac fever has much milder effects and often goes unreported. Flu-like symptoms of fatigue and chills are standard, but there is usually not impact on the lungs (Centers for Disease Control and Prevention, 2018).

Symptoms and Clinical Presentation

Legionnaire’s disease is a pulmonary infection and can be classified as atypical pneumonia. It presents respiratory and inflammatory symptoms as well as evident features of hyponatremia due to abnormal ADH secretion. In radiological X-ray images, Legionnaire’s disease presents itself as multifocal and bilateral with lower lung zone predominance. Pleural effusions are noticeable as well. Since the disease often affects older adults, complications are common, mostly due to co-morbidities and can result in death. The earlier the disease is diagnosed and treated, the more optimistic is the prognosis. However, common complications include:

  1. Respiratory failure – lung function shut down as pneumonia overwhelms the organ and requires external oxygen.
  2. Acute kidney injury – infection decreases kidney function. With renal failure, toxic products accumulate in the bloodstream
  3. Severe sepsis – overwhelming infection of the organism causing shock and multisystem organ failure (Henderson, 2017).


A chest X-ray is often the first and simplest test to identify Legionnaire’s disease. However, it may be inaccurate since a wide range of bacteria can cause regular pneumonia. If there is suspicion for Legionnaire’s disease, blood and phlegm tests are performed in the laboratory. Blood cultures are examined and used in antibody tests to determine the presence of the Legionella bacterium. The legionella urinary antigen test (urine sample) can also outline the presence of the Legionella by tracing a specific protein unique to the bacterium. In extreme cases, a lumbar puncture (spinal tap) may be performed by collecting samples of the cerebrospinal fluid. It can be analyzed for the bacteria as well since its presence in the neural tissue can cause legionella meningitis (Henderson, 2017).

Disease Treatment

Signs and symptoms of legionnaires disease and pontiac fever
(Brown, 2016)

Legionnaires’ disease results in hospitalization. Patients are treated with antibiotics given intravenously for quicker effect. Erythromycin is the most common antibiotic used with treatment lasting one to three weeks. Oxygen is usually provided for the patient to aid in troubled breathing due to pneumonia-causing decreased lung function and low oxygen levels. IV fluids are used to keep patients hydrated. In extreme cases, assisted ventilation may be provided as supportive treatment since severe pneumonia can cause lung function to shut down (Henderson, 2017).

Preventive Measures

There are no existing vaccines for Legionnaires’ disease. All preventive measures are aimed at testing and maintaining the quality of water systems, particularly in buildings. Any large-scale industrial, infrastructure or plumbing systems using water elements are susceptible to the bacteria. The CDC has developed a water management program with appropriate recommendations and a toolkit which building owners can use to prevent Legionella growth (Centers for Disease Control and Prevention, 2018). Therefore, the strategy of Legionella management is two-fold. Since it is an environmental-based disease, all attempts should be made to control water sources through adequate maintenance and safe cleaning agents. Meanwhile, health care providers should dedicate the time and resources to test pneumonia patients that may be at risk for Legionnaires’ disease since early diagnosis can be potentially life-saving.

Interesting Facts

The Bellevue-Stratford Hotel

The Bellevue-Stratford Hotel, site of the American Legion Convention in 1976 and one of the first recorded Legionella outbreaks (Philadelphia Architects and Buildings, n.d.).

The Legionella bacterium was named at the first recorded outbreak at the American Legion Convention in Philadelphia in 1976 where numerous people got pneumonia.

  • Legionella is common in natural water sources where it poses little danger. It becomes most potent when human-made water sources are contaminated.
  • More than 54 species of Legionella exist with the Pneumophila type being most prevalent in humans.
  • Although only about 6,100 cases of Legionnaires’ disease are reported in the United States annually, the incidence rate is considered much higher due to the high probability of misdiagnosis for the condition (Centers for Disease Control and Prevention, 2018).


Barboza, T. (2017). Disneyland shuts down 2 cooling towers after Legionnaires’ disease sickens park visitors. Los Angeles Times.

Brown, C. H. (2016). Legionnaires’ disease and Pontiac fever

Centers for Disease Control and Prevention. (2017). Legionnaires’ disease. Web.

Centers for Disease Control and Prevention. (2018). Legionella (Legionnaires’ Disease and Pontiac Fever). Web.

Henderson, R. (2017). Legionnaires’ disease

HCInfo. (n.d.). Legionnaires’ disease outbreaks 2014-2016

Kerkar, P. (n.d.). What is Legionnaires Disease: Signs, symptoms, treatment, pathophysiology

KIRO7. (2016). Legionella bacteria found in UWMC water supply; 2 patients die

Legionella pneumophila. (n.d.).

Legionella pneumophila. (2016).

MRSA Today. (n.d.). Legionella

Philadelphia Architects and Buildings. (n.d.). The Bellevue-Stratford Hotel

Prussin, A. J., Schwake, D. O., & Marr, L. C. (2017). Ten questions concerning the aerosolization and transmission of Legionella in the built environment. Building and Environment, 123, 684-695. doi:10.1016/j.buildenv.2017.06.024

Water Technology. (2014). Treating Legionella outbreaks in water systems

Yang, K., LeJeune, J., Alsdorf, D., Lus, B., Shum, C.K., Liang, S. (2012). Global distribution of outbreaks of water-associated infectious diseases. Plos Neglected Tropical Disease, 6(2). doi:10.1371/journal.pntd.0001483

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StudyKraken. (2022, September 9). Pathogenicity, Immunology and Epidemiology: Legionnaires’ Disease. Retrieved from


StudyKraken. (2022, September 9). Pathogenicity, Immunology and Epidemiology: Legionnaires’ Disease.

Work Cited

"Pathogenicity, Immunology and Epidemiology: Legionnaires’ Disease." StudyKraken, 9 Sept. 2022,

1. StudyKraken. "Pathogenicity, Immunology and Epidemiology: Legionnaires’ Disease." September 9, 2022.


StudyKraken. "Pathogenicity, Immunology and Epidemiology: Legionnaires’ Disease." September 9, 2022.


StudyKraken. 2022. "Pathogenicity, Immunology and Epidemiology: Legionnaires’ Disease." September 9, 2022.


StudyKraken. (2022) 'Pathogenicity, Immunology and Epidemiology: Legionnaires’ Disease'. 9 September.

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