Legionellosis

Infections caused by species of the genus Legionella are collectively termed legionellosis, or Legionnaires’ disease. Legionellosis is an acute bronchopneumonia produced by the gram-negative bacillus L. pneumophila. Outbreaks usually occur in late summer and early fall and may be either epidemic or confined to a few cases. The infection may range from a mild illness (with or without pneumonitis) to serious multilobed pneumonia with mortality as high as 15%. Legionellosis is most likely to affect middle-aged or elderly people as well as immunocompromised patients and those with a chronic underlying disease, such as diabetes or chronic renal failure.

Pontiac fever is a less severe, self-limiting form of legionellosis that subsides within 2 to 5 days but leaves the patient fatigued for several weeks. This disorder is caused by the same microorganism that causes legionellosis but produces few or no respiratory symptoms, no pneumonia, and no fatalities.

Causes

Legionellosis infections are caused by L. pneumophila, an aerobic, gram-negative bacillus that’s probably transmitted by air. The primary reservoir for the microorganism seems to be water distribution systems, such as humidifiers, whirlpool spas, decorative water fountains, respiratory therapy devices, and shower heads. The infection isn’t spread from person to person. The Legionella microorganisms enter the lungs after aspiration or inhalation and then migrate to the pili. Although alveolar macrophages phagocytize the Legionella, the microorganisms aren’t killed and proliferate intracellularly. The cells rupture, releasing the Legionella microorganisms, and the cycle starts again.

Lesions develop a nodular appearance and the alveoli become filled with fibrin, neutrophils, and alveolar macrophages. Conditions impairing mucociliary clearance (such as smoking, lung disease, or alcoholism) predispose the patient to infection.

Complications

Patients in whom pneumonia develops may experience hypoxia and acute respiratory failure. Other complications include hypotension, delirium, seizures, heart failure, arrhythmias, renal failure, and shock, which is usually fatal.

Assessment Findings

The patient’s history may include proximity to a suspected source of infection. Onset of illness may be gradual or sudden. After a 2- to 10-day incubation period (or a 1- to 2-day incubation period in Pontiac fever), the patient may report nonspecific prodromal symptoms, including diarrhea, anorexia, malaise, diffuse myalgia and generalized weakness, headache, and recurrent chills. He or she may also complain of a cough (initially nonproductive but eventually productive), dyspnea and chest pain, nausea, vomiting, and abdominal pain. Sputum is grayish or rust-colored, nonpurulent and, occasionally, streaked with blood. Some patients demonstrate tachypnea, bradycardia (about 50% of patients), and neurologic signs, especially an altered level of consciousness. Chest percussion may disclose dullness over areas of secretions and consolidation or pleural effusions. Auscultation may reveal fine crackles that develop into coarse crackles as the disease progresses.

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