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Legionnaires' Disease

Legionnaires' disease is an acute bronchopneumonia produced by a gram-negative bacillus. This disease was named for 221 persons (34 of whom died) who became ill during an American Legion convention in Philadelphia in July 1976. Outbreaks, usually in late summer and early fall, may be epidemic or confined to a few cases. The disease may range from a mild illness (with or without pneumonitis) to serious multi­lobed pneumonia with mortality as high as 15%.

Pontiac fever is a less severe, self-limiting form of the illness that subsides within a few days but leaves the patient fatigued for several weeks. This disorder is caused by the same organism as Legionnaires' disease but produces few or no respiratory symptoms, no pneumonia, and no fatalities.

Legionnaires' disease is more common in men man in women and is most likely to affect:

  • elderly people
  • immunocompromised patients (particularly those receiving corticosteroids after transplantation) or those with lymphoma or other disorders associated with impaired humoral immunity
  • patients with chronic underlying disease, such as diabetes, chronic renal failure, or chronic obstructive pulmonary disease
  • alcoholics
  • cigarette smokers (who are three to four times more likely to contract Legionnaires' disease than nonsmokers).


The bacterium responsible for Legionnaires' disease belongs to the genus Legionella. There are approximately 35 Legionella species known to produce the disease. Legionella species are commonly found in any aquatic environment. They can survive for several months in a wet environment and multiply in the presence of algae and organic matter.

Signs and symptoms

When the bacteria enters the lungs, the body's immune system sends out immune cells to destroy them. However, the Legionella bacteria are resistant to the immune cells. They continue to grow and eventually kill the immune cells. Then, even more Legionella bacteria enter the lungs and the infection gets worse. Symptoms begin to appear around 2 to 10 days after infection, and include fever, chills, headache, muscle pain, low appetite, and fatigue. This is followed by a dry cough. Around one to two days later, symptoms progress and can include a fever that goes up and down, chest pain, confusion, delirium, diarrhea, and stomach pain. The cough usually becomes productive, meaning it brings up sputum (saliva and mucus). There may be blood in the sputum and the urine. A small percentage of patients may experience kidney failure. Legionnaires' disease is not contagious, which means it cannot be passed from one person directly to another.

Diagnostic tests

Chest X-ray typically shows patchy, localized infiltration, which progresses to multilobed consolidation (usually involving the lower lobes) and pleural effusion. In fulminant disease, chest X-ray reveals opacification of the entire lung.

Laboratory tests include various blood studies and cultures. Blood test findings may include leukocytosis; increased erythrocyte sedimentation rate; a moderate increase in liver enzyme (alkaline phosphatase, alanine aminotransferase, and aspartate aminotransferase) levels; and decreased parttal pressure of oxygen and, initially, decreased partial pressure of carbon dioxide. Hyponatremia (serum sodium level less than 131 mg/L) is evident on chemistry.

Bronchial washings, blood and pleural fluid cultures, and transtracheal aspirate studies rule out other pulmonary infections. Gram staining reveals numerous neutrophils but no organism. Isolation of the organisms from respiratory secretions or bronchial washings or through thoracentesis is a definitive method of diagnosis.


Antibiotic treatment begins as soon as Legionnaires' disease is suspected and diagnostic material is collected. Treatment need not await test results. Erythromycin and tetracycline are most effective. Azithromycin or other nerve macrolides are preferred for immunocompromised patients. For severely ill patients, a combination of rifampin and a macrolide or quinoline may be used.

Supportive therapy includes administration of antipyretics, fluid replacement. circulatory support with pressor drugs if necessary, and oxygen administration by mask or cannula or by mechanical ventilation with positive end-expiratory pressure.


Since the bacteria thrive in warm stagnant water, regularly disinfecting ductwork, pipes, and other areas that may serve as breeding areas is the best method for preventing outbreaks of Legionnaires' disease. Most outbreaks of Legionnaires' disease can be traced to specific points of exposure, such as hospitals, hotels, and other places where people gather. Sporadic cases are harder to determine and there is insufficient evidence to point to exposure in individual homes.

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