Legionnaires' Disease

Legionnaires' disease is a type of pneumonia caused by Legionella bacteria. The bacterial species responsible for Legionnaires' disease is L. pneumophila . An acute bronchopneumonia, Legionnaires' disease is produced by a fastidious, gram-negative bacillus. It derives its name and notoriety from the peculiar, highly publicized disease that struck 182 people (29 of whom died) at an American Legion convention in Philadelphia in July 1976.

This disease may occur epidemically or sporadically, usually in late summer or early fall. Its severity ranges from a mild illness, with or without pneumonitis, to multilobar pneumonia, with a mortality as high as 15%. A milder, selflimiting form (Pontiac syndrome) subsides within a few days but leaves the patient fatigued for several weeks; this form mimics Legionnaires' disease but produces few or no respiratory symptoms, no pneumonia, and no fatalities.

Each year, between 8,000 and 18,000 people are hospitalized with Legionnaires' disease in the U.S. However, many infections are not diagnosed or reported, so this number may be higher. More illness is usually found in the summer and early fall, but it can happen any time of year

Causes

The causative agent of Legionnaires' disease, Legionella pneumophila, is an aerobic, gram-negative bacillus that's probably transmitted by an airborne route. In past epidemics, it has spread through cooling towers or evaporation condensers in air-conditioning systems. However, Legionella bacilli also flourish in soil and excavation sites. The disease doesn't spread from person to person.

Legionnaires' disease occurs more often in men than in women and is most likely to affect:

• middle-aged to elderly people
• immunocompromised people (particularly those receiving corticosteroids, for example, after a transplant), or those with lymphoma or other disorders associated with delayed hypersensitivity
• patients with a chronic underlying disease, such as diabetes, chronic renal failure, or chronic obstructive pulmonary disease
• alcoholics
• cigarette smokers (three to four times more likely to develop Legionnaires' disease than nonsmokers).

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).

Diagnosis

The patient history focuses on possible sources of infection and predisposing conditions. In addition, a chest X-ray shows patchy, localized infiltration, which progresses to multilobar consolidation (usually involving the lower lobes), pleural effusion and, in fulminant disease, opacification of the entire lung.

Auscultation reveals fine crackles, progressing to coarse crackles as the disease advances.

Abnormal test findings include leukocytosis, an increased erythrocyte sedimentation rate, an increase in liver enzyme levels (alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase), hyponatremia, decreased partial pressure of arterial oxygen and, initially, decreased partial pressure of arterial carbon dioxide. Bronchial washings and blood, pleural fluid, and sputum tests rule out other infections.

Treatment

Antibiotic treatment begins as soon as Legionnaires' disease is suspected and diagnostic material is collected; it shouldn't await laboratory confirmation. Erythromycin is the drug of choice, but if it's not effective alone, rifampin can be added to the regimen. If erythromycin is contraindicated, rifampin or rifampin with tetracycline may be used.

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

Prevention

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.