What is legionellosis?

Legionellosis is caused by Legionella pneumophila (and other Legionella species) — bacteria that live in freshwater environments and thrive in warm, stagnant water in man-made systems. It presents as two syndromes: Legionnaires' disease (a severe pneumonia with 5-15% mortality in hospitalised patients) and Pontiac fever (a mild, self-limiting flu-like illness). Critically, legionellosis is not transmitted person-to-person — it is always acquired by inhaling water droplets from contaminated sources.

Where does Legionella grow?

Legionella multiplies in warm (25-45°C), stagnant water in man-made systems: cooling towers for building air conditioning (the most common source of large outbreaks); hot tub spas; hospital and hotel hot water systems; decorative fountains; car wash equipment; and humidifiers. Legionella grows inside amoebae and protozoa in water — this intracellular niche protects it from disinfection. Prevention: maintaining hot water systems at ≥60°C (Legionella is killed at 60°C); regular cooling tower cleaning and disinfection; water safety plans.

Risk factors for Legionnaires' disease: age >65; smoking (the most powerful risk factor — multiplies risk approximately 5-fold); immunosuppression (transplant, cancer, HIV, biologic therapy); chronic lung disease; diabetes; renal failure; and male sex. Travel (hotel/cruise ship stay) accounts for approximately 30% of cases — Legionella-contaminated hotel water systems and hot tubs are common sources. Healthcare-associated legionellosis is particularly serious given the immunocompromised patient population.

How is Legionnaires' disease diagnosed?

Urinary antigen test (UAT): first-line — detects L. pneumophila serogroup 1 antigen (80% of cases). Results in 15 minutes; positive within 24 hours of onset; remains positive for weeks. Lower respiratory culture (sputum/BAL): the reference standard — required for non-serogroup-1 disease detection and outbreak investigation (environmental matching). Serology: paired sera (4-fold titre rise) — retrospective diagnosis only (2-3 weeks for antibody response). Pneumococcal UAT should always be done simultaneously.

How is Legionnaires' disease treated?

First-line: levofloxacin (500mg or 750mg daily, 5-10 days) — preferred due to superior intracellular penetration and pharmacodynamic profile. Alternative: azithromycin (500mg daily, 5-10 days). Severe/immunocompromised: consider combination (fluoroquinolone + rifampicin — though rifampicin monotherapy is not recommended). Importantly, beta-lactams (amoxicillin, cephalosporins) are not effective for Legionnaires' disease. Most patients respond to treatment within 3-5 days.

Is Legionnaires' disease increasing?

Yes — Legionnaires' disease incidence has been increasing in many countries over the past 20 years. Contributing factors: ageing population (more immunocompromised patients); increased air conditioning and cooling tower use; better testing (more UAT use); and climate change (warmer temperatures favour Legionella proliferation in water systems, longer exposure seasons). In Europe, ECDC reports approximately 20% increases in notified cases per year.

How does GMJ News cover legionellosis?

GMJ News covers WHO legionellosis data, the Philadelphia 1976 outbreak, cooling tower and hospital water management, urinary antigen testing, levofloxacin treatment, climate change amplification, and hospital infection control for water systems. Legionellosis is directly relevant to PHIG's hospital accreditation programme (water safety management is a core accreditation standard) and the Georgian Medical Journal.

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Legionellosis

GMJ News knowledge hub · last reviewed September 2026 · Georgian Medical Journal

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Legionellosis — caused by Legionella pneumophila and related species — presents as either Legionnaires’ disease (severe pneumonia, 5-15% case fatality in hospitalised patients) or Pontiac fever (self-limiting flu-like illness). Legionella grows in warm, poorly maintained water systems — cooling towers, hot tubs, hospital water systems, decorative fountains — transmitted exclusively by inhalation of contaminated aerosols; it does not spread person-to-person (WHO). An estimated 10,000-15,000 people are hospitalised with Legionnaires’ disease in the US alone each year; the global burden is massively underestimated. Climate change — warming temperatures favouring Legionella proliferation — is increasing the risk.

Key messages

Building water systems — the hidden reservoir

Legionella grows in warm, poorly maintained water systems — cooling towers, hot tubs, hospital water, decorative fountains. Unlike most respiratory infections, legionellosis is entirely acquired from the environment — not from other people.

5-15% case fatality — vastly underdiagnosed

Legionnaires' disease has a case fatality rate of 5-15% in hospitalised patients (higher in immunocompromised). Estimated 10,000-15,000 US cases/year but the global burden is vastly underestimated due to lack of testing.

Urinary antigen test — the key diagnosis

The urinary antigen test (UAT) for L. pneumophila serogroup 1 — which causes approximately 80% of Legionnaires' disease — is rapid, inexpensive and highly specific. It is positive within 24 hours of illness onset and remains positive for weeks.

Fluoroquinolones or azithromycin

Levofloxacin (respiratory fluoroquinolone) is the preferred treatment for Legionnaires' disease — superior intracellular penetration. Azithromycin is an effective alternative. Doxycycline and rifampicin are alternatives.

Climate change amplifies risk

Warming temperatures increase Legionella proliferation in water systems and cooling towers. Higher summer temperatures and more frequent rainfall (creating favourable aerosol dispersal) are linked to increasing Legionnaires' disease incidence in Europe and North America.

Prevention through water management

WHO water safety plans, regular maintenance and cleaning of cooling towers, hot water systems maintained at ≥60°C (Legionella killed at 60°C), and chlorination of potable water systems prevent legionellosis. Hospital water management plans are essential.

Key statistics

5-15%

case fatality rate in hospitalised Legionnaires' disease

WHO

10-15K

estimated hospitalised Legionnaires' cases/year in USA

CDC/WHO

~80%

of Legionnaires' disease from L. pneumophila serogroup 1

WHO

>60°C

water temperature that kills Legionella — prevention standard

WHO

person-to-person transmission documented

WHO

1976

year of first identified Legionnaires' outbreak (Philadelphia)

WHO/CDC/History

Legionnaires' disease — risk factors and exposure settings (% of cases)

Source: WHO/ECDC. Cooling towers and travel-related exposure account for most investigated outbreaks.

Glossary of key terms

Legionella pneumophila
WHOA Gram-negative, facultative intracellular bacterium — the causative agent of approximately 84% of Legionnaires' disease (serogroup 1 — 70% of LP cases). Resides and multiplies inside amoebae and protozoa in water; this intracellular parasitism within macrophages explains its pathogenicity in humans.

Pontiac fever vs Legionnaires' disease
WHOTwo clinical forms of legionellosis: Pontiac fever — a self-limiting flu-like illness (fever, myalgia, headache) without pneumonia; incubation 24-48 hours; attack rate near 100% in exposed group; no deaths; cause unclear (possibly endotoxin rather than live bacteria). Legionnaires' disease — pneumonia with up to 15% case fatality; incubation 2-10 days; lower attack rate; requires antibiotic treatment.

Urinary antigen test (UAT)
WHO/ESCMIDAn immunochromatographic assay detecting L. pneumophila serogroup 1 antigen in urine — the standard first-line diagnostic test. Advantages: rapid (15 minutes), inexpensive, positive within 24 hours of symptom onset, remains positive for weeks. Limitation: only detects serogroup 1 (approximately 70-80% of Legionnaires' disease). Culture from respiratory specimens (sputum/BAL): the reference method needed for outbreak investigation.

Cooling tower Legionella
WHO/RegulationCooling towers — used in air conditioning and industrial cooling — create warm, aerosolised water that provides ideal Legionella growth conditions. They can disperse Legionella-containing aerosols kilometres downwind. Regular cleaning, disinfection and water treatment with biocides, combined with water temperature management, are the key controls.

Intracellular Legionella pathogenesis
ResearchLegionella survives and multiplies inside alveolar macrophages — using a specialised secretion system (Dot/Icm) to evade phagosome maturation and lysosomal fusion. This intracellular survival explains: why beta-lactam antibiotics (which cannot penetrate cells well) are less effective; why fluoroquinolones and macrolides (excellent intracellular penetration) are preferred; and why immunocompromised patients are at highest risk.

1976 Philadelphia outbreak
WHO/CDC/HistoryThe defining outbreak: 221 cases, 34 deaths among attendees of the American Legion convention in Philadelphia in July 1976. The causative organism was isolated from cooling tower water in January 1977 — named Legionella pneumophila. Retrospective testing found the same organism caused multiple earlier unexplained pneumonia outbreaks dating back to 1947.

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