Legionnaires’ disease, more commonly called legionellosis, is a lung infection caused by the Legionella pneumophila bacterium. Fortunately, in the majority of cases, inhalation of this bacterium does not cause major symptoms, although, in fragile people or with particular risk factors, a form of severe pneumonia can occur.
The Legionella genus was named in 1976, after an epidemic spread among attendees at the American Legion rally at the Bellevue Stratford Hotel in Philadelphia. On that occasion, 221 people contracted this previously unknown form of pneumonia, and of these 34 died. The source of bacterial contamination was later identified in the hotel’s air conditioning system. «Legionella has hot water as an element of risk – explains the virologist Fabrizio Pregliasco, health director of the Irccs Galeazzi-Sant’Ambrogio and professor of general hygiene at the State University of Milan – The disease is thus generally contracted through the respiratory route by inhalation or aspiration of aerosol containing legionella.
A graphic to explain what legionella is
Among the environments with the greatest risk of contagion are showers and air conditioning systems that use water from aqueducts to humidify the air or misting systems.” Over 60 species of legionellae are known and are divided into 71 serotypes. These are present in both natural and artificial aquatic environments and therefore spring waters, lakes, thermal waters. These waters, through various collection basins and pipes, reach the city pipes, thus creating risk situations in swimming pools, fountains, but also showers and air conditioners. «In the majority of cases the inhalation of legionella does not lead to particular symptoms – continues the professor – other than giving rise to a clinical form, called Pontiac fever, which is characterized by general malaise, headache and fever». Pontiac fever occurs after an incubation of 1 to 2 days and resolves within 2 to 5 days. Legionellosis, on the other hand, has an average incubation period of 2 or 10 days and causes pneumonia; in more severe cases this can lead to the impairment of other organs and can cause death (the fatality rate is 10% in the general population). This disease can affect the elderly, people with lung diseases or immunosuppressed people. The progression of this pneumonia, in addition to high fever, very severe pain and respiratory failure, can also give extra-pulmonary, gastrointestinal, neurological, cardiac manifestations with signs of decompensation, and in the most serious cases sepsis and thrombosis.
According to the latest available data, in Italy, a total of 3111 cases of legionellosis were notified to the Istituto Superiore di Sanità in 2022, with an increase of 14% compared to the previous year. The number of reported cases therefore returned, after two years, to pre-pandemic values. Of the 3111 cases of legionellosis, 3039 were classified as confirmed cases and 72 as probable cases.
«As regards public places, such as hospitals, RSAs and hotels – explains the virologist – today there is legislation in force on the matter, precisely to prevent these episodes, however, the situation is different for private homes where regular checks are not foreseen . Good maintenance and cleaning of systems, including air conditioning, is so fundamental in terms of prevention. Whether in public or private places, the advice is to let the hot water run before entering the shower, so as to avoid inhaling potentially harmful vapours”.
