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Legionnaire’s disease (Legionnaire’s disease) Causes, Symptoms and Treatment

What is legionellosis?

Legionellosis is a pulmonary infection caused by Legionella bacteria (the majority of cases are caused by Legionella pneumophila).

The disease is contracted by inhaling contaminated fine water particles, which can invade the lungs and then affect the immune system.

Although legionellosis is very rare, it can be fatal, especially for people whose immune system is weakened. Legionellosis is not contagious from one person to another.

Causes of Legionnaire’s disease

Legionella bacteria live in water and are naturally occurring in the environment. They develop at temperatures of 25 ° C to 42 ° C. They multiply in all types of systems involving water, such as air conditioning cooling towers, water distribution systems, humidifiers or whirlpools. These devices, when poorly maintained, are optimal growth conditions for Legionella bacteria.

Legionnaire’s disease

In 1976, a group of veterans of the Second World War, members of the American Legion met in a hotel in Philadelphia. Nearly 200 of them became ill and about 30 died of pneumonia (a lung infection such as pneumonia).

It was only two years later, in 1978, that the bacterium responsible for this disease was identified. The Legionella pneumophila had spread through the hotel’s air conditioning system and had infected several clients.

Since then, legionellosis has caused various outbreaks related to air conditioning systems and water distribution systems of poorly maintained buildings in different countries.


Untreated legionellosis can lead to life-threatening complications such as:

  • Respiratory insufficiency
  • Septic shock, a sudden decrease in blood flow to vital organs following a generalized bacterial infection.
  • Acute kidney failure.

Mortality is less than 5% in people whose immunity is intact. Generally, mortality is more common in the most fragile individuals, such as the immunocompromised, the elderly or those who have undergone surgery.

Associated disorders

A less severe form of legionellosis is known as Pontiac fever. Caused by the same bacteria, Legionella pneumophila, this disease causes moderate fever, headaches and muscle aches, but does not affect the lungs. This disease is not fatal and generally cures on its own in 3 to 5 days.


Legionella bacteria are found all over the world.

According to the Public Health Agency of Canada, the prevalence of Legionnaires’ disease is about 75 cases reported each year, which equates to 1 to 3 cases per 1 million people .

In France, approximately 1,500 cases of Legionella pneumonia are diagnosed each year, the incidence is estimated at 2 per 100,000 inhabitants per year. ORPHANET data (2007)

Diagnostic of Legionnaire’s disease

Legionellosis is a disease similar to pneumonia . The doctor may have recourse to:

  • A urine test to detect the body’s immune response to the presence of Legionella bacteria.
  • A sputum culture in search of the Legionella bacteria.
  • A blood test to identify the presence of Legionella bacteria in the blood.
  • A radiological examination of the lungs to assess the extent of the infection (in most cases).

The symptoms of legionellosis (Legionnaire’s disease)

Pulmonary legionellosis has an incubation period of 2 to 14 days, that is, the symptoms of the disease appear 2 to 14 days after exposure to Legionella pneumophila.

The first symptoms of legionellosis resemble those of influenza.

  • Fever.
  • Headaches.
  • Chills.
  • Muscular pain.
  • A deterioration of the general state or lethargy.

After a few days, some people may present:

  • A cough with sputum containing blood.
  • Chest pain.
  • A loss of appetite.
  • Shortness of breath
  • Gastrointestinal problems such as diarrhea, nausea or vomiting.
  • Confusion.

People at risk for Legionnaire’s disease

  • The elderly.
  • People suffering from a chronic illness (lung disease, heart disease, kidney failure or diabetes).
  • People whose immune system is weakened (cancer treatment, transplantation, AIDS, taking cortisone chronically).

Risk factors

Legionnaire’s disease does not pose a risk to healthy people.
But some factors can increase the risk of complications:

  • Smoking.
  • Consume a lot of alcohol.
  • Have had recent surgery.

The prevention of legionellosis (Legionnaire’s disease)

Can we prevent?
The system air conditioning and water distribution, as well as swimming pools or heating baths that are found in hospitals, industrial sites, hotels and leisure centers are potential sources of proliferation of bacteria responsible legionellosis.

Regular cleaning and disinfection of these systems is the simplest and most effective way to reduce the risk of bacterial growth and thus reduce the risk of contamination.

At home, cleaning and disinfecting water droplet devices, such as showerheads, whirlpools and humidifiers, reduces the chance of bacteria spreading.


Basic preventive measures
Legionellosis can lead to serious complications, especially in people with weakened immune or respiratory systems.

Avoiding smoking reduces the risk of infection if exposed to the bacteria that causes the disease.

Measures to prevent complications
Legionellosis is treated with antibiotics. The earlier the disease is diagnosed and treated, the lower the risk of complications or mortality related to the disease.

Medical treatment of legionellosis (Legionnaire’s disease)

Antibiotics. The bacterium Legionella pneumophila is naturally resistant to penicillin usually used in the treatment of pneumonia. Erythromycin, azithromycin, clarithromycin, levofloxacin and ciprofloxacin are commonly used to treat legionellosis. The treatments usually last from 7 to 10 days, sometimes several weeks, especially in the case of more vulnerable people.

The earlier the treatments are started, the better the chances of healing and the lower the risk of complications. In general, the improvement of symptoms occurs in 3 to 5 days.

In immunosuppressed patients or in patients with very severe pneumonia from the outset, many may experience fatigue and neurological symptoms 18 months after infection.

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