ABSTRACT

Legionella pneumophila plays a crucial role as an important causative agent in severe community-acquired pneumonia. There are no distinctive, unique radiologic feature that differentiate Legionnaires’ disease from other types of bacterial pneumonia. Unilateral, unilobar involvement  is the most frequent radiographic finding on presentation. When effective therapy is administered at the onset of the disease, progression of shadowing is observed in about 30% of cases. Extensive infiltrates are associated with a high yield of isolation Legionella from respiratory samples. Furthermore, recent studies demonstrate that the extent of shadowing correlates with the severity of clinical manifestations and the outcome. Pleural effusion is reported in one third of  patients. Cavitation is usually only found in the immunocompromised population. Radiographic clearing is frequently slow in legionellosis. Complete resolution may take as long as four months. Radiologists must be aware of the increasing number of extrapulmonary localizations reported. In the immunocompetent patient, non-Legionella pneumophila radiographic features tend to overlap those observed in L. pneumophila pneumonia. In the immunosuppressed population pseudonodular infiltrates that eventually cavitate have been sometimes described.