Inhalation of a water aerosol containing the bacteria. The bacterium grows in warm freshwater environments. Person-to-person transmission does not occur with either Legionnaires’ disease or Pontiac fever.
Legionellae are ubiquitous worldwide. Most cases of legionellosis are caused by Legionella pneumophila. Disease occurs after exposure to aquatic settings that promote bacterial growth—the aquatic environment is somewhat stagnant, the water is warm (77°F–108°F [25°C–42°C]), and the water must be aerosolized so that the bacteria can be inhaled into the lungs. These 3 conditions are met almost exclusively in developed or industrialized settings. Disease does not occur in association with natural freshwater settings such as waterfalls, lakes, or streams.
Outbreaks of legionellosis have been described in numerous countries throughout the world. The largest outbreak (449 cases) ever reported was traced to a cooling tower on the roof of a city hospital in Murcia, Spain, in 2001. Travel-associated outbreaks are commonly recognized.
Despite the presence of Legionella bacteria in many aquatic environments, the risk of developing legionellosis for most people is low. Travelers who are exposed to aerosolized, warm water containing Legionella are at risk for infection. Travelers who are aged >50 years, are current or former smokers, have chronic lung conditions, or are immunocompromised are at higher risk of developing illness after exposure. Many outbreaks have been associated with exposure to cruise ships, hotels, and resorts. Exposures can occur during recreation in or near a whirlpool spa, while showering in a hotel, or touring in cities with buildings that have cooling towers. Patients often do not recall specific water exposures, as they frequently occur during normal activities.
Legionnaires’ disease typically presents with pneumonia, which usually requires hospitalization and can be fatal in 10%–15% of cases. Symptom onset occurs 2–14 days after exposure. In outbreak settings, <5% of people exposed to the source of the outbreak develop Legionnaires’ disease.
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. Most patients fully recover. Up to 95% of people exposed in outbreak settings can develop symptoms of Pontiac fever.
Isolation of Legionella from respiratory secretions, lung tissue, pleural fluid, or a normally sterile site is an important method for diagnosis of Legionnaires’ disease. Clinical isolates are often necessary to interpret the findings of an investigation through comparison with isolates obtained from environmental sources. Because of differences in mechanism of disease, Legionella cannot be isolated in people who have Pontiac fever.
The most used diagnostic method is the Legionella urinary antigen assay. However, the assay can only detect L. pneumophila serogroup 1, the most common cause of legionellosis. Paired serology showing a 4-fold rise in antibody titer between acute- and convalescent-phase specimens also confirms the diagnosis. A single antibody titer of any level is not diagnostic of legionellosis.
For travelers with suspected Legionnaires’ disease, specific antibiotic treatment is necessary and should be administered promptly while diagnostic tests are being processed. Appropriate antibiotics include fluoroquinolones and macrolides. Treatment may be necessary for up to 3 weeks. In severe cases, patients may have prolonged stays in intensive care units. Consultation with an infectious diseases specialist is advised. Pontiac fever is a self-limited illness that requires supportive care only; antibiotics have no benefit.
There is no vaccine for legionellosis, and antibiotic prophylaxis is not effective. 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 whirlpool spas. If exposure cannot be avoided, travelers should be advised to seek medical attention promptly if they develop symptoms of Legionnaires’ disease or Pontiac fever.
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