Legionella

Legionella is a genus of Gram-negative, aerobic bacteria comprising more than 60 species, with Legionella pneumophila as the species most commonly associated with human disease. Many Legionella species are environmental organisms found in natural freshwater and in human-made water systems.

Legionella bacteria inhabit freshwater environments and colonize engineered water systems such as cooling towers, hot water systems, showers, decorative fountains, and spa pools. Humans acquire infection primarily by inhaling aerosolized water droplets containing the bacteria; person-to-person transmission is rare and not a recognized route for Legionnaires' disease.

Legionnaires' disease is a severe form of pneumonia characterized by fever, cough, shortness of breath, and sometimes gastrointestinal symptoms. Pontiac fever is a milder, self-limited illness with flu-like symptoms and no pneumonia. Onset after exposure is typically several days.

Pathogens enter alveolar macrophages and replicate intracellularly, aided by the Dot/Icm type IV secretion system. Legionella pneumophila serogroup 1 accounts for the majority of laboratory-confirmed cases worldwide; however, other species also cause disease.

Diagnosis relies on urinary antigen testing (primarily for L. pneumophila serogroup 1), culture on buffered charcoal yeast extract agar with iron and cysteine, and PCR. Urinary antigen testing may miss non-serogroup 1 infections. Treatment consists of antibiotics such as macrolides (azithromycin or clarithromycin) or fluoroquinolones (levofloxacin, moxifloxacin); duration generally 7–14 days for Legionnaires' disease, longer in severe cases. Pontiac fever does not require antibiotics.

Prevention focuses on controlling Legionella growth in water systems. Measures include maintaining appropriate disinfectant levels and temperatures, preventing stagnation, and conducting system maintenance and remediation in facilities such as hospitals, hotels, and large buildings. Outbreaks are typically linked to cooling towers, hot water systems, spa and decorative fountains, or potting soils in some contexts.