Author(s): William (Chris) Edens

Infectious Agent

Gram-negative bacteria of the genus Legionella cause Legionnaires’ disease and Pontiac fever. Most cases of Legionnaires’ disease are caused by Legionella pneumophila, but all species of Legionella can cause disease.

Transmission

The most common route of transmission is by inhalation of aerosolized water containing the bacteria, although transmission can sometimes occur through aspiration of water containing the bacteria. A single episode of possible person-to-person transmission of Legionnaires’ disease has been reported.

Legionella is ubiquitous in freshwater sources worldwide, but quantities of Legionella in these environments are insufficient to cause disease. In the built environment, Legionella can amplify in water systems, depending on the conditions. Factors associated with amplification include warm water temperatures of 77°F–108°F [25°C–42°C]); water stagnation; presence of scale, sediment, and biofilm in the pipes and fixtures; and absence of disinfectant.

To cause disease, Legionella spp. must be aerosolized and inhaled by a susceptible host. The most common sources of transmission include potable water (via showerheads and faucets), cooling towers, hot tubs, and decorative fountains.

Epidemiology

Legionella growth and transmission can occur anywhere in the world when the right conditions exist. The capacity to diagnose and report cases of Legionnaires’ disease is better established, however, in industrialized settings. In the United States, the incidence of Legionnaires’ disease is increasing; the number of reported Legionnaires’ disease cases increased nearly 900% between 2000 and 2018.

Legionnaires’ disease cases and outbreaks have been reported worldwide. Large outbreaks associated with cooling towers were reported in Spain in 2001 (449 confirmed cases) and Portugal in 2014 (377 cases). In 2015, a cooling tower in Bronx, New York, was associated with 138 cases of Legionnaires’ disease. Travel-associated outbreaks have also been reported. In 2015, 114 cases (11 confirmed Legionnaires’ disease, 29 suspected Legionnaires’ disease, and 74 Pontiac fever cases) were identified among visitors to a hotel in Chicago, Illinois; and during 2016–2017, 51 confirmed cases of Legionnaires’ disease were associated with travel to Dubai.

Despite the presence of Legionella spp. in many aquatic environments, the risk of developing Legionnaires’ disease is low for most people. Travelers who are >50 years old, are current or former smokers, have chronic lung conditions, or are immunocompromised are at increased risk for infection when exposed to aerosolized water containing Legionella spp. Travel-associated Legionnaires’ disease outbreaks can occur on cruise ships, in hotels, and at resorts. A common feature among these settings is the presence of a large, often complex, water system that can be challenging to maintain properly.

Approximately 10% of all reported cases of Legionnaires’ disease in the United States occur in people who have traveled during the 10 days before symptom onset. Exposures among travelers can occur when a person is in or near a hot tub, showering in a hotel, standing near a decorative fountain, or touring in cities with buildings that have cooling towers. Patients with Legionnaires’ disease often do not recall specific water exposures because exposure frequently occurs during normal activities.

Clinical Presentation

Legionellosis is primarily composed of 2 clinically and epidemiologically distinct syndromes: Legionnaires’ disease and Pontiac fever. Though rare, Legionella spp. have also been associated with disease outside of the lungs (extrapulmonary). Legionnaires’ disease typically presents with severe pneumonia, which usually requires hospitalization and can be fatal in ≈10% of cases. Symptom onset occurs 2–10 days (rarely, ≤19 days) after exposure. In outbreak settings, <5% of people exposed to the source of the outbreak develop Legionnaires’ disease. Nearly all cases of legionellosis in the United States are reported as Legionnaires’ disease.

Pontiac fever is milder than Legionnaires’ disease and presents with fever, headache, or muscle aches, but no signs of pneumonia. Pontiac fever can affect healthy people as well as those with underlying illnesses, and symptoms occur within 72 hours of exposure. Nearly all patients fully recover without antimicrobial drug therapy or hospitalization. Up to 95% of people exposed during outbreaks of Pontiac fever can develop symptoms of disease.

Diagnosis

The preferred diagnostic tests for Legionnaires’ disease are the Legionella urinary antigen test and culture of lower respiratory secretions (sputum, bronchoalveolar lavage) on media that supports growth of Legionella spp. The most common diagnostic test, the urinary antigen test, only detects L. pneumophila serogroup 1; this serogroup accounts for 80%–90% of cases.

Isolation of Legionella by culture is important to detect non–L. pneumophila serogroup 1 infections and is necessary to compare clinical to environmental isolates during an outbreak investigation. Diagnosis by PCR of lower respiratory secretions also is possible, but the number of commercially available tests is limited. Because of differences in the mechanisms of disease, Legionella spp. cannot be isolated in people who have Pontiac fever. Legionnaires’ disease, Pontiac fever, and extrapulmonary legionellosis are nationally notifiable diseases in the United States.

Treatment

For travelers with suspected Legionnaires’ disease, administer specific antimicrobial drug treatment promptly while diagnostic tests are being processed. Preferred antimicrobial agents include fluoroquinolones and macrolides. Patients with severe cases might have prolonged intensive care unit stays. Treating physicians should consult with an infectious disease specialist. Because Pontiac fever is a self-limited illness, antimicrobial drugs have no benefit, and treatment is focused on supportive care.

Prevention

No vaccine for Legionnaires’ disease is available, and antibiotic prophylaxis is not effective. Water management programs for building water systems and devices at risk for Legionella growth and transmission can lower the potential for illnesses and outbreaks. Travelers at increased risk for infection, such as older people or people with immunocompromising conditions (e.g., cancer, diabetes), might choose to avoid high-risk exposures (e.g., hot tubs). If exposure cannot be avoided, travelers should seek medical attention promptly if they develop symptoms of Legionnaires’ disease or Pontiac fever.

CDC website: Legionella