What is legionellosis?

o legionellosis any legionnaires’ disease It is an atypical and severe form of pneumonia produced by several bacterial species of the legionella genus, especially for Legionella pneumophila.

The term “legionellosis” also includes pontiacum fever, a disease caused by the same bacteria, but milder and not focused on the lungs.

The name is due to one of the first outbreaks identified as Legionella affected attendees at an American Legion convention that was held in 1976 in Philadelphia. 182 people contracted the disease, 29 of them died.


Around 90% of legionellosis cases are caused by the spice Legionella pneumophila. In other cases, species such as L. longbeachae, L. feeleii, L. micdadei or any L. anis.

Legionella is found naturally in streams and freshwater bodies and can contaminate hot water tanks and pipes, cooling towers and ventilation systems, especially in large air conditioning systems.

Usually transmitted by air by breathing aerosols of contaminated water, dust or soil. However, most exposed people do not become infected, and person-to-person transmission is very low.

By inhaling the contaminated particles, the bacteria reach the lungs. Here it is attacked initially by macrophages, a type of phagocyte present in mucous membranes.

The macrophage phagocytoses the legionella and tries to kill the bacteria, but the bacteria can survive inside and multiply until the macrophage is killed. When the macrophage dies, the bacteria are released and can infect new macrophages.

The virulent strains of Legionella prevent the action of macrophages by blocking the fusion of phagosomes and lysosomes, which are cell organelles that contain enzymes to kill bacteria.

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Risks of infection include older age, smoking, a suppressed immune system, or the presence of chronic lung disease.

In cases of severe pneumonia or pneumonia accompanied by an upcoming trip, specific tests to detect legionellosis are recommended, usually through urine and sputum antigen tests.

The treatment of legionellosis consists of the administration of antibiotics such as fluoroquinolone, azithromycin or doxycycline, and hospitalization of the patient is usually required. Mortality is around 10%.

Although the exact number of global cases is not known, it is estimated that 2-10% of all cases of community-acquired pneumonia are due to Legionella.

There is currently no vaccine for Legionella, so much attention must be paid to prevention.


Legionella bacteria grow in water in symbiosis with various types of amoebas. It can also infect amoebas and survive inside them as a parasite. The ideal growth temperature for Legionella is 35ºC, although it grows well between 25 and 45ºC. It dies above 60°C.

Reservoirs can be created where there is adequate humidity and temperature for their growth, such as hot water tanks and pipes, cooling towers and condensers, air conditioning systems such as those installed in large buildings including hotels, communities and hospitals, so it is an infection. relatively common hospital.

Reservoirs can also appear in air humidifiers, foggers, spas, showers, ornamental fountains, compost, etc.


The most characteristic symptoms of legionellosis are high fever, cough, shortness of breath, muscle pain and headache. They take between 2 and 10 days to appear after exposure to the bacteria. In some rare cases, it may take longer.

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A cough produces sputum in half of the cases, and the sputum can be useful for taking a culture, looking for the bacteria responsible, and diagnosing the disease. In one third of patients the sputum is bloody.

Vomiting, diarrhea, loss of appetite and general weakness may also appear. Gastrointestinal symptoms appear in approximately half of patients, as well as ataxia and motor incoordination to varying degrees.

Kidney and liver function and electrolyte levels are affected, often with hyponatremia (low levels of sodium in the blood), although it usually does not produce any apparent symptoms and is only detected with laboratory tests.

On chest radiographs, pneumonia is seen in both lungs and is very difficult to distinguish from other pneumonias. Therefore, additional tests are needed to make a differential diagnosis.

In pontiac fever the symptoms are similar, but milder. Pneumonia does not develop, but fever and muscle aches may occur. Pontiac fever develops more quickly than Legionnaires’ disease, within a few hours to two days after exposure to the bacteria. In this case, recovery occurs on its own without treatment in 2-5 days.


Legionella pneumophila under the electron microscope Severe legionella pneumonia at various stages of evolution in a 42-year-old patient

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