Lauri A. Hicks, Nicole T. Alexander

Infectious Agent

Legionellosis is caused by gram-negative bacteria of the genus Legionella.

Mode of Transmission

Transmission occurs by inhalation of a water aerosol containing the bacteria. The bacterium grows in warm freshwater environments.

Occurrence

• Legionellae are ubiquitous worldwide. Most cases of legionellosis are caused by L. pneumophila.
• Disease occurs following 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 three conditions are met almost exclusively in developed or industrialized settings.
• Disease does not occur in association with natural settings such as waterfalls, lakes, or streams.
• Outbreaks of legionellosis have been described in numerous countries throughout the world.
  • In Australia and the United States, rare cases of legionellosis caused by L. longbeacheae have been associated with exposure to potting soil.
  • 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.
  • Outbreaks have been reported on cruise ships.
  • CDC conducts enhanced surveillance for travel-associated legionellosis cases in an effort to identify clusters and recommend measures to prevent ongoing transmission.
• Person-to-person transmission does not occur with either Legionnaires’ disease or Pontiac fever.

Risk for Travelers

• U.S. legionellosis patients frequently report overnight stays at hotels and travel outside their state of residence.
• Travelers who are exposed to aerosolized, warm water are at risk for infection.
• Despite the presence of Legionella bacteria in many aquatic environments, the risk of developing legionellosis for most individuals is low.
• Elderly and immunocompromised travelers, such as those being treated for cancer, are at higher risk.
• Exposures can occur during recreation in or near a whirlpool spa (e.g., on a cruise ship), while showering in a hotel, or touring in cities with buildings that have cooling towers.

Clinical Presentation

• 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, fewer than 5% of persons exposed to the source of the outbreak develop Legionnaires’ disease.
• Pontiac fever differs from Legionnaires’ disease in that Pontiac fever presents as an influenza-like illness, with fever, headache, and myalgias, but no signs of pneumonia. Pontiac fever can affect healthy individuals as well as those with underlying illnesses, and symptoms occur within 72 hours of exposure. Full recovery is the rule. Up to 95% of people exposed in outbreak settings can develop symptoms of Pontiac fever.

Diagnosis

• 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 environmental investigation. Culture cannot be used to confirm Pontiac fever.
• The most common 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 four-fold rise in antibody titer between acute- and convalescent-phase specimens confirms the diagnosis. A single antibody titer of any level is not diagnostic of legionellosis.
• Additional information can be found at CDC’s Legionellosis Resource Site (www.cdc.gov/legionella/index.htm).

Treatment

• For travelers with 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.

Preventive Measures for Travelers

• 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 (e.g., cancer, 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.

References

1. Fields BS, Benson RF, Besser RE. Legionella and Legionnaires’ disease: 25 years of investigation. Clin Microbiol Rev. 2002;15:506–26.
2. CDC. Legionnaires’ disease associated with potting soil—California, Oregon, and Washington, May–June 2000. MMWR Morb Mortal Wkly Rep. 2000;49:777–8.
3. Steele TW, Lanser J, Sangster N. Isolation of Legionella longbeachae serogroup 1 from potting mixes. Appl Environ Microbiol. 1990;56:49–53.
4. CDC. Surveillance for travel-associated Legionnaires’ disease—United States, 2005–2006. MMWR Morb Mortal Wkly Rep. 2007;56:1261–3.
5. Jernigan DB, Hofmann J, Cetron MS, et al. Outbreak of Legionnaires’ disease among cruise ship passengers exposed to a contaminated whirlpool spa. Lancet. 1996;347:494–9.
6. CDC. Cruise-ship-associated Legionnaires disease, November 2003–May 2004. MMWR Morb Mortal Wkly Rep. 2005;54:1153–5.
7. Burnsed, LJ, Hicks LA, Smithee LM, et al.; the Legionellosis Outbreak Investigation Team. A large, travel-associated outbreak of legionellosis among hotel guests: utility of the urine antigen assay in confirming Pontiac fever. Clin Infec Dis. 2007;44:222–8.
8. Garcia-Fulgueiras A, Navarro C, Fenoll D, et al. Legionnaires’ disease outbreak in Murcia, Spain. Emerg Infect Dis. 2003;9:915–21.
9. Mandell LA, Wunderink RG, Anzueto A, et al. Infectious Diseases Society of America/American Thoracic Society consensus guidelines on the management of community-acquired pneumonia in adults. Clin Infect Dis. 2007;44(Suppl 2):S27–72.