Legionnaires' Disease

Legionellosis is a pneumonia-causing infection by Legionella; pathogens come from contaminated water and enter via the respiratory tract. A milder febrile form with headache/myalgia is called Pontiac fever.

Common Symptoms

Shock; respiratory failure; myalgia; myalgia; anorexia; confusion; fatigue; cyanosis; nausea

Causes and Pathogenesis

Legionella are aerobic pleomorphic Gram-negative rods ubiquitous in nature; sources are humans, water, and air-conditioning systems, airborne. (1) Etiology: Legionellaceae is a single family with one genus Legionella (42 species, 64 serogroups; ~19 from patients). China has Lp1, Lp3, Lp5, Lp6, Lp9. Main pathogens: L. pneumophila (15 serogroups, ~90% are sg 1) and L. micdadei. Legionella are 0.3-0.9 × 2-4 µm, motile with polar flagella, aerobic; do not grow on ordinary media; BCYE agar is optimal (colonies at 2-4 days, 1-2 mm); F-G agar shows pinpoint colonies with yellow UV fluorescence; weakly catalase-positive, nitrate/urease-negative; cell wall has 14-17 carbon branched fatty acids. Enzymes/toxins include protease, phosphatase, DNase, β-lactamase, cytotoxin. They live in freshwater/biofilms, surviving 2-4 months in distilled water, 3 months in river water, ~1 year in tap water; free-living amoebae support intracellular growth and alter virulence. (2) Pathogenesis: L. pneumophila impairs alveolar macrophages by inhibiting phagosome-lysosome fusion and modulating phagosomal pH; it captures Mφ via depolarization changes, escapes immunity, uses host nutrients, then lyses Mφ causing acute alveolar epithelial/endothelial damage with edema/fibrinous exudate; it also induces apoptosis. Direct damage is via hemolysins/cytotoxins/enzymes. Cell-mediated immunity is key; cytokines inhibit intracellular growth; then antibody/complement/PMNs clear bacteria. Lesions are usually lobar consolidation; fatal cases involve both lungs. Microscopy shows fibrinopurulent pneumonia with neutrophils/macrophages/fibrin/RBCs; some have diffuse alveolar damage with hyaline membranes. Pontiac fever (12-36 h incubation) may be toxin-mediated or an immune reaction to waterborne microbes.

Treatment

Macrolides or fluoroquinolones, supportive therapy

Preventive Measures

Mortality ~15%, higher with age/comorbidities; deaths are mostly respiratory failure, then shock/acute renal failure; early diagnosis lowers mortality. No effective vaccine. (1) Chlorinate/boil drinking water. (2) Shut down and disinfect AC systems; maintain water systems/humidifiers/sprayers. (3) Vaccine in development. Manage cooling towers (clean/disinfect, biocides); maintain hot tub chlorine; hot water ≥60°C, cold ≤20°C; chlorination (1 ppm continuous, 50 ppm intermittent) corrodes pipes and rebound occurs when reduced.

Department to Visit

It is recommended to visit the Respiratory department; an appointment can be booked on this platform before the visit.

Warm reminder: The above information is for health science reference only and cannot replace professional medical diagnosis. If you have related symptoms, please seek medical attention at a regular hospital promptly.