Pontiac fever

Alternative names
Legionella pneumonia; Legionnaire’s disease

Legionnaire’s disease is an acute respiratory infection caused by the bacterium Legionella pneumophila, which can cause a broad spectrum of disease from mild cough and fever to a serious pneumonia.

Causes, incidence, and risk factors
The bacteria that cause Legionnaire’s disease have been found in water delivery systems and can survive in the warm, moist, air conditioning systems of large buildings including hospitals. The infection is transmitted through the respiratory system. Person-to-person spread has not been proved.

From the onset of symptoms, the condition typically worsens during the first 4 to 6 days, with improvement starting in another 4 to 5 days. Most infection occurs in middle-aged or older people, although it has been reported in children. Typically, the disease is less severe in children.

Risk factors include cigarette smoking; underlying diseases such as renal failure, cancer, diabetes, or chronic obstructive pulmonary disease; people with suppressed immune systems from chemotherapy, steroid medications, or diseases such as cancer and leukemia; alcoholism; being middle-aged or elderly, and in people on a ventilator for extended periods.


  • Muscle aches and stiffness  
  • Joint pain  
  • Loss of energy  
  • General discomfort, uneasiness, or ill feeling (malaise)  
  • Headache  
  • Fever  
  • Shaking chills  
  • Nonproductive cough  
  • Coughing of blood  
  • Shortness of breath  
  • Chest pain  
  • Diarrhea  
  • Ataxia (lack of coordination)

Signs and tests

  • Listening to the chest with a stethoscope reveals fine crackles.  
  • Sputum direct fluorescent antibody staining shows Legionella.  
  • The causative organism may be cultured from the airway.  
  • Urine tests for the bacteria (urine antigen test) may be positive.  
  • Chest X-ray shows pneumonia.  
  • Arterial blood gas analysis may show low concentrations of oxygen.  
  • CBC shows an increased white blood cell count.  
  • Erythrocyte sedimentation rate is increased.  
  • Low serum sodium.  
  • Liver function tests may show moderate elevation.


The goal of treatment is to eliminate the infection with antibiotics. Treatment is started as soon as Legionnaire’s disease is suspected, without waiting for confirmation by culture results.

The antibiotic commonly used is quinolone (ciprofloxacin, levofloxacin, moxifloxacin, or gatifloxacin) or a macrolide (azithromycin, clarithromycin, or erythromycin).

Supportive treatment includes hospitalization for fluid and electrolyte replacement and oxygen administration by mask or by mechanical ventilation, if the respiratory system becomes severely compromised by the infection.

Expectations (prognosis)
The overall death rate for those with pneumonia is about 15%, and the death rate increases in those with underlying diseases. The mortality for patients who develop Legionnaire’s disease while hospitalized is close to 50%, especially when antibiotics are started late.


Some complications include respiratory failure requiring use of a respirator.

Calling your health care provider
Call your health care provider if breathing difficulties develop.

Active surveillance of infections that were acquired within a hospital can lead to the treatment of contaminated water delivery systems. Detection and treatment of sources outside hospitals usually occurs during or after an epidemic has happened.

Johns Hopkins patient information

Last revised: December 5, 2012
by Potos A. Aagen, M.D.

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