Gram-negative bacteria of the genus Legionella. Most cases of Legionnaires’ disease are caused by Legionella pneumophila, but all species of Legionella can cause disease.
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 (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 then 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.
Legionella growth and transmission can occur anywhere in the world when the right conditions exist. However, capacity for diagnosing and reporting of cases of Legionnaires’ disease are most well established in industrialized settings. In the United States, Legionnaires’ disease is on the rise; the number of reported Legionnaires’ disease cases has increased 350% from 2000 through 2016. Legionnaires’ disease cases and outbreaks have been reported worldwide. Large outbreaks associated with cooling towers have been reported in Spain (449 confirmed cases, 2001) and Portugal (377 cases, 2014). In 2015, a cooling tower in Bronx, New York, was associated with 138 cases of Legionnaires’ disease. Travel-associated outbreaks are commonly recognized. In 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 for most people is low. Travelers who are aged >50 years, 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 in settings such as cruise ships, hotels, and resorts. Approximately 10%–15% 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, as they frequently occur during normal activities.
Legionellosis is composed of 2 clinically and epidemiologically distinct syndromes: Legionnaires’ disease and Pontiac fever. Legionnaires’ disease typically presents with severe pneumonia, which usually requires hospitalization and can be fatal in approximately 10% of cases. Symptom onset occurs 2–10 days (rarely, up to 19 days) after exposure. In outbreak settings, <5% of people exposed to the source of the outbreak develop Legionnaires’ disease. Legionnaires’ disease accounts for nearly all cases of legionellosis reported in the United States.
Pontiac fever is milder than Legionnaires’ disease and presents as an influenzalike illness, with fever, headache, and 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 antibiotic therapy. Up to 95% of people exposed during outbreaks of Pontiac fever can develop symptoms of this disease.
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 commonly used 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 species and serogroups and is necessary to compare clinical to environmental isolates during an outbreak investigation. Diagnosis by PCR of respiratory secretions is a newer, evolving technique. Because of differences in mechanism of disease, Legionella spp. cannot be isolated in people who have Pontiac fever. Legionnaires’ disease and Pontiac fever are nationally notifiable diseases in the United States.
For travelers with suspected Legionnaires’ disease, specific antibiotic treatment is necessary and should be administered promptly while diagnostic tests are being processed. Preferred agents include fluoroquinolones and macrolides. In severe cases, patients may have prolonged stays in intensive care units. Consultation with an infectious disease specialist is advised. Pontiac fever is a self-limited illness that requires supportive care only; antibiotics have no benefit.
There is no vaccine for Legionnaires’ disease, 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 the elderly or those with immunocompromising conditions such as cancer or diabetes, may choose to avoid high-risk areas such as hot tubs. If exposure cannot be avoided, travelers should seek medical attention promptly if they develop symptoms of Legionnaires’ disease or Pontiac fever.
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