Legionellosis and legionnaires’ disease

Legionellaceae are Gram-negative bacilli, of which Legionella pneumophila is the principal cause of human infections. Their natural habitats are freshwater streams, lakes, thermal springs, moist soil, and mud, but the principal source for large outbreaks of legionellosis is cooling systems used for air conditioning and other cooling equipment. Legionella spp. are principally transmitted to humans through contaminated water aerosols. Middle-aged men, smokers, regular alcohol drinkers, and those with comorbidity are most at risk. Aside from supportive care, the first-choice antibiotics are macrolides (mainly azithromycin) and/or fluoroquinolones (especially levofloxacin). Case fatality is 5–15% in previously well adults, but much higher in those who are immunocompromised or develop respiratory failure. Prognosis is improved by early administration of effective anti-legionella antibiotic therapy. Prevention is by the correct design, maintenance, and monitoring of water systems.