Legionnaires’ Disease

Published: March 23, 2016
Last reviewed: March 12, 2018

What is Legionnaires’ disease?

Legionnaires’ disease is a form of atypical pneumonia caused by the bacterium Legionella pneumophila. The disease has got its name after the outbreak at the convention of the American Legion in a hotel in Philadelphia in the United States in 1976, during which more than 182 attendants got infected, and later 34 died [11]. The bacteria were presumably spread by the hotel’s air conditioning system.

Legionnaires’ disease can be mild or life-threatening and often needs to be treated in the hospital.

Legionellosis is a general term for the infection with Legionella, which includes Legionnaires’ disease, which affects the lungs, and a milder disease called Pontiac fever, which does not affect the lungs.

The Cause — the Bacterium Legionella pneumophila

The bacterium Legionella pneumophila lives in the artificial water systems, such as cooling towers and plumbing in big buildings. When you inhale the mist of contaminated water, the bacteria can invade the white blood cells and macrophages in your lung’s air sacs (alveoli) and cause pneumonia [4].

Symptoms and Signs

Early symptoms (in the first 2 days) include a mild headache, fatigue, muscle pain and low-grade fever.

Later symptoms, which can persist for several weeks, can include high fever (102-105 °F or 39-40.5 °C), chills, dry cough or coughing up mucus, blood in sputum, shortness of breath, chest pain during deep breathing (pleuritic pain), confusion, depression, abdominal pain, loss of appetite, vomiting and diarrhea [5,7].

By using the stethoscope (auscultation), a doctor may hear localized crackles in your lungs and decreased heart rate (bradycardia) [5].

Pontiac Fever

Pontiac fever is an infection by Legionella with flu-like symptoms, such as a dry cough, fever, chills, muscle and joint pain but without pneumonia — with a normal X-ray. The incubation period is 24-48 hours; 90% of individuals exposed to contaminated water may get infected. Symptoms usually resolve within 5 days without any specific treatment [2,11].

Is Legionnaires’ disease contagious?

Legionnaires’ disease does not likely spread from person to person [1]. You can contract Legionella by inhaling aerosolized water (mist, vapor) from [4,8,10,11,20]:

  • Cooling towers and HVAC systems (Heating, Ventilating and Air Conditioning) in big buildings; the bacteria can spread indoors through air condition or outdoors by air
  • Indoor or outdoor decorative fountains
  • Whirlpools, hot tubs, spas, jacuzzis and thermal springs
  • Humidifiers
  • Showers in hospitals, nursing homes, hotels, prisons, cruise ships, swimming pools and other settings with complex water systems [18] (lack of agreement about this) [21]
  • Oxygen masks, nebulizers, mechanical ventilators and dental unit waterlines
  • Ice machines
  • Plant sprays
  • Domestic plumbing during maintenance
  • Compost and potting soil

Legionella can grow in stagnant water (water tanks, boilers) with the temperatures 77-122 °F (25-50 °C) [4,11]; the optimal temperatures for growth are 90-105 °F (32-40 °C). The creation of biofilm (slim) on the inside of the pipes and sediment on the bottom of water tanks also promote bacterial growth.

Water high in iron, zinc and manganese and low in copper promotes Legionella growth [18].

Legionella from cooling towers may travel several kilometers through the air, so it may be considered both waterborne and airborne infection [11].

Other forms of the Legionella transmission, such as aspiration of contaminated drinking water or infection through a skin wound, seem to be rare [1,11].

There seems to be minimal or no risk of contracting Legionella from domestic central heating radiators, steam irons, coffee makers, dishwashers, air conditioning without humidifiers in private houses and cars, properly installed solar boilers, garden ponds, chlorinated swimming pools, saunas or flower exhibitions [10].

Natural water sources, such as rain, fog, rivers, waterfalls, lakes and sea, also do not seem to be likely sources of the infection [12].

Legionnaires’ disease can occur year round but mainly in the late spring, summer and early autumn [4,11,18]. Incubation period–the time from the exposure to bacteria to first symptoms–is 2-10 days or, rarely, up to 19 days [2]. According to Centers for Disease Control and Prevention (CDC) in the United States, only about 5% of individuals exposed to the source of an outbreak develop pneumonia [2].

Risk Factors

Risk factors for Legionnaires’ disease include age over 50 or very young age (newborns), heavy smoking, recent travel, alcoholism, diabetes mellitus, chronic lung, liver or kidney disease, impaired immunity due to HIV/AIDS, use of corticosteroids, immunosuppressants (after organ transplantation) or chemotherapy, iron overload and systemic cancer, such as leukemia [4,8].

Legionnaires’ disease occurs mainly in adults and only rarely in children, in which usually has a milder course; few infants got infected during “water birth” delivery [13]. Pregnant women can get infected, but pregnancy by itself is not considered a risk factor [11].

Recent Legionella Outbreaks

HCinfo.com has a list of more than 40 Legionnaires’ disease outbreaks that occurred in various parts of the world in 2014 and 2015.

  • Flint, Michigan, United States. There were 87 cases of the disease and 10 deaths from June 2014 to October 2015; the outbreak was possibly related to a change of source of municipal water, poorly maintained piping and increased lead concentration in water [17].
  • Bronx, New York, United States. At least 12 deaths from 3 outbreaks were reported in 2015; the suspected sources included cooling towers and air conditioning systems on the roofs of some large buildings [14,15,16].


A combination of an X-ray that shows white patches in the lungs (pneumonia) and either the urinary antigen test or a sputum culture positive for Legionella is required for the diagnosis of Legionnaires’ disease [3].

  • X-ray usually shows white shadows at the bottom of the lungs from the 3rd day after symptoms onset [9,11].
  • Urinary antigens are detectable within days 1-3 after the onset of symptoms and can be positive for months after the disease healing [9].
  • The results of the culture of sputum or samples obtained by bronchoscopy may be available only after 3-5 days [9].

The specificity of both the culture and urinary antigen test is 100%, which means that positive tests confirm an infection with Legionella, but the sensitivity of the culture is only 20-80% and of antigen test 70-100%, so these tests may be false negative and do not reliably exclude an infection [3].

Differential Diagnosis

Conditions that may have similar symptoms than Legionnaires’ disease:


Mild Legionnaires’ disease is treated with oral and severe disease with intravenous antibiotics, such as azithromycin, ciprofloxacin, clarithromycin, doxycycline, levofloxacin, rifampin, tigecycline, and trimethoprin and sulfamethoxazole [6]. Severe infection may require hospitalization for several weeks.

Recovery Time and Prognosis

Antibiotic treatment of a mild disease usually results in an improvement within 3-5 days [6].

Severe disease, even if treated with antibiotics, may last for several weeks; in some individuals, fatigue and poor memory and white shadows on X-ray may persist for more than a year [4,9,11,19].

An early onset of treatment significantly increases the chance of good outcome [9].

The untreated severe disease has a high mortality rate. Mortality is 5-30% [2] and among those treated in hospitals up to 50% [4,5].


Complications of Legionnaires’ disease can include [4,5,11]:

  • Pleural effusion
  • Hyponatremia due to syndrome of inappropriate ADH secretion (SIADH) [5]
  • Muscle breakdown (rhabdomyolysis)
  • Kidney failure
  • Fulminant respiratory failure
  • Inflammation of the paranasal sinuses (sinusitis), pancreas (acute pancreatitis), heart muscle (myocarditis), brain (encephalitis), heart lining (endocarditis) in individuals with prosthetic valves
  • Septic or hypovolemic shock
  • Coma
  • Death


Prevention of Legionnaires’ disease in big buildings can include disinfection of water by heating to 70-80 °C or copper-silver ionization [6]. Ultraviolet (UV) radiation, hyperchlorination and ozonation are no longer recommended [22].

If you are at increased risk for Legionnaires’ disease, you can try the following to prevent it [11]:

  • Do not smoke or drink excessive amounts of alcohol.
  • Avoid places with artificial sources of mist (cooling towers, decorative fountains, hot tubs, showers in hospitals, hotels and cruise ships).
  • Maintain home water heaters (geysers): keep hot water at 140 °F (60 °C) and cold water below 68 °F (20 °C). The warm water at faucet should be 122 °F (50 °C) [12]. You can kill bacteria by periodically flushing the entire plumbing with water heated to 158 °F (70 °C) and allowing it to flow through all faucets for 20 minutes [20].
  • Drain hot water tanks periodically to remove scale and sediment [20].
  • Periodically chlorinate the home water system to produce 10 ppm free residual chlorine and flush all taps until you detect a distinct chlorine smell at all faucets.
  • Other methods of water disinfection include ozonation and ultraviolet (UV) radiation; commercial inline UV systems are available [20].
  • Ask someone to regularly clean the shower head (do not do it by yourself, because you may get infected). You can install a shower head with water filter with a 0.22-micron pore size.

Currently, there is no vaccine against Legionella.

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