Legionellosis is a bacterial infection with two distinct clinical forms: 1) Legionnaires’ disease (LD), the more severe form characterized by pneumonia, and 2) Pontiac fever, an acute-onset, self-limited flu-like illness without pneumonia. Legionella bacteria are common inhabitants of aquatic systems that thrive in warm environments. Ninety percent of cases of LD are caused by Legionella pneumophila serogroup 1, although at least 46 Legionella species and 70 serogroups have been identified. Transmission occurs through inhalation of aerosols containing the bacteria or by aspiration of contaminated water. Person-to-person transmission does not occur. The case fatality rate for LD ranges from 5% to 30%, but can be higher in outbreaks occurring in a hospital setting. Most cases can be treated successfully with antibiotics and healthy people usually recover from the infection. People of any age may get LD, but the disease most often affects middle-aged and older persons, particularly those who are heavy smokers, have chronic lung disease, or whose immune systems are suppressed by illness or medication. Symptoms may include malaise, fever, headaches, muscle aches, chills, nonproductive cough, and pneumonia. Multi-system involvement includes diarrhea, changes in mental status, hyponatremia, and abnormal kidney and liver function tests. Diagnostic tests are by culture of respiratory secretions and/or tissue, urine antigen testing, direct fluorescent antibody (DFA) stain of lung tissue or sputum, serologic testing by IFA. LD incidence is usually found in the summer and early fall, but can happen any time of the year. Each year between 8,000 and 18,000 people are hospitalized with Legionnaires disease in the U.S. However, many infections are not diagnosed or reported, so this number may be higher. According to the CDC, cases of Legionnaire’s disease in the U.S. have tripled in the past decade.
In January, 2011, the CDC has added Legionella to the Active Bacterial Core Surveillance project, a population-based effort for testing clinical specimens for a range of bacterial pathogens. This is in collaboration with state health departments to identify the reasons for the rising number of Legionella cases.