Legionnaires’ disease / Legionellosis / Legion Fever

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Legionnaires’ disease, legionellosis or “legion fever” is a severe form of pneumonia caused by a type of bacteria. The infection is likely to spread when people inhale tiny water droplets containing the bacteria. It is an infectious and potentially life- threatening disease.

Legionnaires’ disease

More than 90% of Legionnaires’ disease cases are caused by gram negative aerobic bacteria of the genus Legionella. The most common causative agent is Legionella pneumophilia, a commonly occurring aquatic organism that thrives in an optimum temperature between 25 and 45 degree centigrade. L. pneumophila has been established as a specific pathogen that infects the respiratory tract. Some people who have this infection acquire no illness or may demonstrate very few symptoms.

The disease takes two distinct forms:

a) Legionnaires disease or "legion fever" where the patient suffers from high fever and pneumonia.

b) Pontiac fever where the patient suffers from mild respiratory illness that resembles acute influenza. No pneumonia is present. This form of fever resolves by itself.

In 1976, Legionnaires disease acquired its name when there was an outbreak of pneumonia among those participating in a convention of the American Legion at Philadelphia. Later, the causative agent was identified to be a bacterium which was thereupon named ‘Legionella’.

Although outbreaks of Legionnaires' disease may occur during summer or autumn, it usually occurs as single, isolated cases. The disease usually affects those who are middle-aged or older. Some people who have this infection may never show any signs of the illness and may demonstrate very few symptoms.

An individual does not get Legionnaires’ disease through person–to-person contact but by inhaling the bacteria. Timely treatment with antibiotics helps in controlling the infection, despite the fact that some people continue to have problems even after treatment. Those who are affected are mostly older individuals, smokers or those with weak immune system. The disease, if untreated, can prove to be fatal.


Legionnaires' disease usually manifests itself within 2- 14 days after being exposed to the Legionella bacteria. It frequently begins with the following signs and symptoms -

Headache and muscle pain
Fever with a temperature of 104°F or higher
Symptoms on the second or third day may include:
Difficulty in breathing
Chest pain
Cough which may produce mucus or even blood
Loss of appetite
Gastrointestinal symptoms such as nausea, vomiting and diarrhea
Confusion / loss of co ordination (ataxia)

Legionnaires' disease primarily impacts the lungs, although occasionally it can cause infections in other body parts, including the heart. Relative bradycardia or slow heart rate may also be observed. Also, the patients’ renal functions, electrolytes and liver function tests may not exhibit normal values. Chest x-rays reveal consolidation in the lungs. But other tests are required to confirm the diagnosis.

Pontiac fever presents itself as muscle aches without pneumonia. The fever usually resolves within 2 to 5 days without treatment.

The complications from the disease include respiratory failure, septic shock or acute kidney failure.

Causes and Risk Factors

The bacterium Legionella pneumophila which is responsible for the majority of the cases of Legionnaires' disease survives in soil and water, but the outdoor bacteria hardly cause infections.

The Legionella bacteria thrive in various water systems such as hot tubs, air conditioners and mist sprayers used in grocery store produce departments. Although Legionnaires’ disease can be contracted in domestic set-ups through plumbing systems, the disease is mostly acquired through the complex systems of large buildings, where the bacteria are allowed to grow and spread more easily.

Legionella infection is likely to spread when people inhale tiny water droplets containing the bacteria. This could be a spray of water from a shower or faucet or water that runs through the ventilation system in buildings. Some of the common sources include -

Hot tubs
Whirlpools on cruise ships
Cooling towers in air conditioning
Swimming pools
Decorative fountains
Physical therapy equipments
Water systems in hospitals, hotels and hospitals

There is no consensus on how many bacteria are required to cause an infection but there are reports that indicate the involvement of very few drops of contaminated water in causing an infection. It must also be kept in mind that Legionella bacteria are capable of travelling in air as far as four miles.

Legionella bacteria can also be transmitted through aspiration when you choke on any liquid and it accidentally enters your lungs. Some people have been afflicted with Legionnaires' disease while tending to their garden and touching contaminated soil. Construction sites are another possible area of contamination.

You are at a risk of getting the disease if you are a smoker, are elderly, immune compromised, have lung disease such as emphysema or if your work is to maintain cooling towers in air conditioning systems.

Diagnosis and Treatment

Legionnaires' disease is similar to other types of pneumonia. A urine test may help to detect Legionella bacteria by analyzing the presence of Legionella antigens. Some of the other tests to detect the disease include blood tests, chest X-ray, study of sputum samples, analyzing spinal tap and CT scan of brain.

Legionnaires' disease is treated with antibiotics. The sooner the therapy begins, the chances are higher for the patient to avoid complications or even fatality. Pontiac fever does not require treatment but resolves by itself. Meticulous cleaning of water bodies and boosting one’s immune system are the best methods of preventing Legionnaires’ disease.

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