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# Legionnaires disease and Pontiac fever

Legionnaires disease is a severe pneumonia caught by breathing water droplets with Legionella bacteria. Pontiac fever is a mild form without pneumonia.

Updated 2026-11-01

## Structures involved

Tap one to see what it is and open it in 3D.

- [Right lower lobe](https://durgical.com/en/anatomy/inferior-lobe-of-right-lung/)
- [Left lower lobe](https://durgical.com/en/anatomy/inferior-lobe-of-left-lung/)
- [Right upper lobe](https://durgical.com/en/anatomy/superior-lobe-of-right-lung/)
- [Pleura](https://durgical.com/en/anatomy/pleura/)
- [Trachea](https://durgical.com/en/anatomy/trachea/)

## Microorganisms that cause it

- [Legionella pneumophila](https://durgical.com/en/microbiology/legionella-pneumophila/)

## Why it happens

- [Patterns of acute inflammation: serous, fibrinous, purulent, abscess, ulcer](https://durgical.com/en/general-pathology/patterns-of-acute-inflammation-serous-fibrinous-purulent-abscess-ulcer/)
- [The alveolus: type I and II pneumocytes, surfactant, the blood–air barrier](https://durgical.com/en/histology/the-alveolus-type-i-and-ii-pneumocytes-surfactant-the-blood-air-barrier/)

## What it is

Infection by Legionella bacteria, mainly L. pneumophila, acquired by inhaling contaminated water aerosols. It causes two illnesses: Legionnaires disease, a pneumonia that can be severe, and Pontiac fever, a short self-limited flu-like illness.

## Who gets it

Found worldwide, with sporadic cases and outbreaks linked to cooling towers, hotels, hospitals, spas and plumbing systems. Most cases are in people over 50 and in men. It causes a small but important share of community-acquired pneumonia and of severe hospital-acquired pneumonia. It is probably underdiagnosed in Latin America, including Colombia, where specific tests are not routinely requested. There is no person-to-person spread.

## Causes

- Legionella pneumophila, especially serogroup 1; other species such as L. longbeachae (potting soil)
- Bacteria that multiply in warm water (about 25–45 °C) and inside amoebae and biofilms
- Inhalation of aerosols from cooling towers, showers, hot tubs, humidifiers and decorative fountains, or aspiration of contaminated water
- Incubation of 2–10 days for Legionnaires disease and hours to 2 days for Pontiac fever

## Risk factors

- Age over 50 years and male sex
- Current or past smoking and heavy alcohol use
- Chronic lung, heart or kidney disease and diabetes
- Immunosuppression: transplant, cancer, corticosteroids or anti-TNF agents
- Recent travel with hotel stays, recent hospitalisation or work with water systems

## Pathophysiology

Inhaled bacteria are taken up by alveolar macrophages, where they block phagosome-lysosome fusion and multiply inside the cell. Cell death releases more bacteria and causes a neutrophil-rich alveolar pneumonia that can spread to several lobes. Toxins and the systemic inflammatory response explain the high fever and frequent extrapulmonary features (diarrhoea, confusion, hyponatraemia, kidney and liver involvement). Pontiac fever is thought to be a reaction to inhaled bacteria or their products without lung invasion.

## Classification

- Legionnaires disease: pneumonia, community-acquired, travel-associated or healthcare-associated
- Pontiac fever: non-pneumonic, self-limited febrile illness
- Extrapulmonary legionellosis (rare): endocarditis, wound or soft-tissue infection

## Symptoms and signs

- High fever, chills, malaise and myalgia
- Cough, at first dry, with dyspnoea and pleuritic chest pain
- Diarrhoea, nausea and abdominal pain
- Headache and confusion or other changes in mental status
- Relative bradycardia (pulse lower than expected for the fever)
- Pontiac fever: fever, headache, myalgia and fatigue without pneumonia, resolving in about a week

## Red flags

- Respiratory failure, low oxygen saturation or rapidly spreading infiltrates
- Septic shock or confusion
- Severe hyponatraemia
- Rising creatinine or very high creatine kinase (kidney injury, rhabdomyolysis)
- Pneumonia not responding to beta-lactam antibiotics alone

## Diagnosis

**Clinical**
Pneumonia cannot be told apart from other causes on clinical grounds, but the clue set is high fever, diarrhoea, confusion and hyponatraemia, severe illness, failure of beta-lactams, and exposure to water aerosols or an outbreak.

**Tests**
Urinary antigen test, rapid but detecting only L. pneumophila serogroup 1; culture on special media or PCR of sputum or bronchoalveolar lavage, which detects all species and allows outbreak tracing. Ideally both are taken together. Routine labs often show hyponatraemia, raised [liver](https://durgical.com/en/anatomy/liver/) enzymes, high CRP, haematuria or proteinuria and raised creatine kinase.

**Imaging**
[Chest](https://durgical.com/en/anatomy/region/chest/) X-ray shows patchy or lobar consolidation that may progress to several lobes and may lag behind clinical improvement; CT for complications or unclear findings.

## Differential diagnosis

- Pneumococcal and other bacterial community-acquired pneumonia
- Mycoplasma or Chlamydophila pneumonia
- Psittacosis and Q fever
- Influenza or COVID-19 pneumonia
- Leptospirosis with lung involvement

## Goal of treatment

Start Legionella-active antibiotics early, support breathing and circulation, and find and control the environmental water source to prevent more cases.

## Treatment

**Medical**
Antibiotics that act inside cells: macrolides (azithromycin) or respiratory fluoroquinolones (levofloxacin, moxifloxacin); doxycycline is an alternative. Empirical regimens for severe community-acquired pneumonia include Legionella coverage. Beta-lactams alone are ineffective. Pontiac fever needs no antibiotics.

**Acute**
Oxygen, ventilatory support and intensive care for respiratory failure or shock; correction of sodium and fluids.

**Follow-up**
Oxygen saturation, electrolytes, [kidney](https://durgical.com/en/anatomy/kidney/) function and creatine kinase; notification to public health so the water source can be investigated.

## Complications

- Respiratory failure and ARDS
- Septic shock
- Acute kidney injury and rhabdomyolysis
- Hyponatraemia
- Lung abscess or empyema (uncommon)
- Death, especially in older and immunosuppressed people or with delayed treatment

## Prognosis

Most treated patients recover, though fatigue can last weeks to months. Case fatality is about 10% overall, higher in healthcare-associated cases and in untreated immunosuppressed people; delayed effective antibiotics is a key predictor of death. Pontiac fever resolves without sequelae.

## Prevention

- Water safety plans for buildings: cooling-tower maintenance, biocides and hot water kept above about 50 °C
- Regular cleaning and disinfection of spas, humidifiers and fountains; flushing of unused taps
- Sterile water for respiratory devices and extra precautions in hospitals; no vaccine exists

## Test yourself

Think of the answer, then tap the question to see it.

## How is Legionella transmitted to humans?

By inhaling contaminated water aerosols (or aspirating contaminated water); there is no person-to-person spread.

## Which electrolyte disturbance is classically associated with Legionnaires disease?

Hyponatraemia.

## What is the limitation of the Legionella urinary antigen test?

It detects only L. pneumophila serogroup 1, so a negative test does not exclude other serogroups or species.

## Which antibiotic classes treat Legionnaires disease?

Macrolides (especially azithromycin) and respiratory fluoroquinolones; beta-lactams alone do not work.

## How does Pontiac fever differ from Legionnaires disease?

It is a short, self-limited flu-like illness without pneumonia that needs no antibiotics.

[Add to my cards (5)](https://durgical.app/estudiar/?add=/en/conditions/legionnaires-disease-and-pontiac-fever/)

## Related conditions

- [Community-acquired pneumonia](https://durgical.com/en/conditions/community-acquired-pneumonia/)
- [Hospital-acquired and ventilator-associated pneumonia](https://durgical.com/en/conditions/hospital-acquired-and-ventilator-associated-pneumonia/)
- [Hyponatraemia](https://durgical.com/en/conditions/hyponatraemia/)
- [Psittacosis](https://durgical.com/en/conditions/psittacosis/)
- [Q fever](https://durgical.com/en/conditions/q-fever/)
- [Influenza (flu)](https://durgical.com/en/conditions/influenza/)
- [COVID-19](https://durgical.com/en/conditions/covid-19/)
- [Rhabdomyolysis](https://durgical.com/en/conditions/rhabdomyolysis/)
- [Acute kidney injury](https://durgical.com/en/conditions/acute-kidney-injury/)
- [Acute respiratory distress syndrome (ARDS)](https://durgical.com/en/conditions/acute-respiratory-distress-syndrome/)
- [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Legionellosis](https://www.who.int/news-room/fact-sheets/detail/legionellosis) (World Health Organization, 2022); [Clinical Guidance for Legionella Infections | Legionella | CDC](https://www.cdc.gov/legionella/hcp/clinical-guidance/index.html) (Centers for Disease Control and Prevention, 2024); [Clinical Features of Legionnaires' Disease and Pontiac Fever | Legionella | CDC](https://www.cdc.gov/legionella/hcp/clinical-signs/index.html) (Centers for Disease Control and Prevention, 2024).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/conditions/legionnaires-disease-and-pontiac-fever/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/conditions/legionnaires-disease-and-pontiac-fever/