Legionnaire’s Disease (Legionellosis)

What is Legionnaire’s Disease?

Legionnaires’ disease (LD), also known as legionellosis, is a severe form of lung infection producing pneumonia-like symptoms. Legionnaires’ disease is a water-borne disease caused by Legionella pneumophila bacteria. Legionnaires’ disease is contracted by inhaling the Legionella bacteria. Smokers, older adults, and people with weak immunity are more vulnerable to developing Legionnaires’ disease.

In its mild form, Legionnaires’ disease is a benign illness known as Pontiac fever. As Pontiac fever it is characterized by mild fever and muscle pain but no pneumonia. Pontiac fever is self-limited (meaning that it resolves on its own) and does not require medical attention in most cases. However, untreated Legionnaires’ disease can be life-threatening.

How Common is Legionellosis?

Legionnaires’ disease occurs worldwide. It is one of the most common infectious diseases acquired in the community, though sporadic cases are also reported. In the US, up to 18,000 cases of Legionnaires’ disease are reported per year. It often causes severe pneumonia in patients. Males and elderly persons carry a greater risk of contracting Legionella infection.

What Happens in Legionellosis?

The Legionella bacterium is a tiny, waterborne bacterium that cannot move (non-motile).  Legionella infection is acquired by inhaling mist from water sources contaminated with the bacterium. Contaminated showers, whirlpools, cooling towers, ice machines, roadside puddles, potting soil, or compost can produce Legionella containing mist. Direct inhalation of mist from such sources is a very frequent method of transmission. The bacteria replicate inside the lungs.

Upon entering the body, the presence of the bacteria prompts T cells (a type of immune cells) to produce certain chemicals called lymphokines, which stimulate macrophages (another type of immune cells). Activated macrophages then kill the invading bacterium and stop its growth. However, the bacteria can also infect these macrophages and yet another type of immune cells known as monocytes. People with weakened immune systems (like elderly patients or HIV positive people) are unable to clear Legionella bacterium in this way. This increases their susceptibility to Legionnaires’ disease.

Signs and Symptoms

After incubation period

The incubation period is the time from when the bacteria enter the body to the time that symptoms first appear. This means that during the incubation period there are no symptoms. Incubation periods vary with different infections. In Legionnaires’ disease the incubation period is between 2 to 10 days.

Early symptoms

The first symptoms of Legionella bacteria after the incubation period includes :

  • Headache
  • Fever (104 F or higher)
  • Muscle pain
  • Chills

Later symptoms

Within 48 to 72 hours of the appearance of the first symptoms, the following symptoms develop:

  • Cough (with phlegm and/or blood)
  • Breathlessness
  • Chest pain
  • Loss of appetite
  • Tiredness
  • Gastrointestinal symptoms, like diarrhea, abdominal pain, nausea, and vomiting
  • Mental changes (like confusion)

Legionnaires’ disease mostly affects the lungs, but may also cause infections in wounds and other organs of the body (such as the heart, pancreas, and kidneys). In the milder form of Legionnaires’ disease (Pontiac fever), infection does not reach the lungs.

Causes of Legionnaires’ Disease

Majority of the cases of Legionnaires’ disease are caused by the L. pneumophila but there are many other species of the Legionella bacteria can also be responsible. Legionella bacteria can survive and thrive in all kinds of indoor water systems (such as air conditioners, hot tubs, plumbing systems, and mist sprayers in grocery stores). The water droplets need to be airborne for it to inhaled and then infect the lung tissue.

Picture of Legionella pneumophila (scanning electron micrograph) from Wikimedia Commons

The infection spreads through mist containing Legionella bacteria. People can easily become infected by inhaling the microscopic water droplets from faucets, showers, or whirlpools. Water dispersing through ventilation systems in large buildings may also be contaminated with Legionella bacteria. Cooling towers in air conditioners are an example. Physical therapy equipments, swimming pools, decorative fountains, and water systems in hospitals, hotels, and nursing homes, can all have Legionella contamination.

Apart from water droplets, Legionella bacteria can sometimes be transmitted by:

  • Working in the garden or using infected potting soil.
  • Coughing or choking while drinking. The liquids containing Legionella bacteria may accidentally enter the lungs.

Who is at Risk?

The following factors increase the risk of developing Legionnaires’ disease:

  • Low immunity: A weak immune system, caused by certain diseases (like HIV/AIDS) or medications (like corticosteroids and immunosuppressants taken after organ transplants to suppress immune system), increases the risks of catching Legionnaires’ disease.
  • Smoking: Smoking damages the lungs. Damaged lungs are susceptible to all types of lung infections including Legionnaires’ disease.
  • Older age: People above 65 years of age are at a higher risk of developing Legionnaires’ disease.
  • Illness: Chronic lung disease (like emphysema), kidney disease, diabetes, or cancer can increase the susceptibility to Legionnaires’ disease.
  • Lifestyle: Jobs in industrialized settings increase exposure to Legionella bacteria. Examples include maintenance of cooling towers in air conditioning systems, working with soil, maintenance of ICUs and operation theaters in hospitals and nursing homes.

Diagnosis of Legionellosis

Legionnaires’ disease causes symptoms that resemble those of other types of pneumonia. Therefore further diagnostic investigations are recommended.

  • Legionella infection is commonly confirmed by determining the presence of Legionella antigens in urine samples.
  • Blood tests can also confirm the presence of Legionella bacteria.
  • Sputum culture or lung fluid cultures can detect Legionella infection. These tests are preferred over blood tests.
  • X-ray scan of the chest may be helpful in determining the amount of infection in the lungs.
  • In case of neurological symptoms, a computed tomography (CT) scan of the brain is taken.
  • The fluid extracted through a spinal tap (lumbar puncture) can also reveal the presence of the bacterium in the brain.

Complications of Legionellosis

If untreated, Legionnaires’ disease can lead to life-threatening complications like:

Respiratory failure

In case of heavy infection, lungs cannot function properly. Infected lungs cannot provide the body with sufficient oxygen or remove sufficient carbon dioxide from the blood. This leads to respiratory failure.

Septic shock

If there is a severe, sudden drop in blood pressure due to overwhelming Legionella infection, vital organs like the kidneys and brain get less blood. The heart pumps more blood to compensate the situation. This extra workload weakens the heart and results in further reduction in blood flow.

Acute kidney failure

The kidneys may suddenly lose their ability to filter waste material from the blood. This increases the accumulation of fluids and wastes to dangerous levels.


In people with weak immune systems due to some diseases or medications, Legionnaires’ disease may be fatal. Such cases need effective and prompt treatment.

Treatment and Drugs

The milder form of Legionnaires’ disease, i.e., Pontiac fever, generally goes away on its own and does not require medical treatment. Full blown Legionnaires ‘disease, however, requires immediate treatment.


  • Antibiotics (tigecycline, doxycycline, and azithromycin) are the main mode of treatment.
  • Antibiotics eradicate the bacteria and prevent serious complications including death.
  • Rifampin is recommended for severe infections.
  • Antibiotics are administered intravenously or orally.

How to Prevent Legionellosis

Outbreaks of Legionnaire’s disease can be prevented by thorough cleaning and disinfection of water bodies like pools, water distribution systems, and spas.

Smoking significantly increases the susceptibility of individuals to develop Legionnaires’ disease upon exposure to Legionella bacteria. Therefore, quitting smoking lowers the risk of infection and prevents unnecessary complications.





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