Legionellosis

Description

Legionellosis encompasses two diseases, Legionnaires' disease and Pontiac fever, caused by bacteria in the genus Legionella. The bacterium grows in warm, freshwater environments. Under specific conditions, it can be inhaled into the lungs, the principal site of infection.

Occurrence

Legionellae are ubiquitous worldwide in freshwater environments. However, concentrations of the organism do not reach sufficient levels to cause disease unless three environmental conditions are met. First, the aquatic environment must be somewhat stagnant. For example, plumbing systems in large buildings (e.g., hotels) sometimes have sections that are infrequently used. Second, the water must be warm enough (25°-42°C, 77°-108°F) to promote bacterial growth to sufficient numbers to cause disease. Third, 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.

Risk for Travellers

Travellers who visit developed settings (e.g., hotels, even in developing countries) and 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 travellers, such as those being treated for cancer, are at higher risk. Exposures can occur during activities such as 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. 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.

Clinical Presentation

The first sign of Legionnaires' disease is pneumonia which usually requires hospitalization and can be fatal in 10%-15% of cases. Symptoms occur 2-10 days after exposure. In outbreak settings, fewer than 5% of persons exposed to the source of the outbreak actually develop Legionnaires' disease. Pontiac fever is different 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. Infrequently, simultaneous outbreaks of Legionnaires' disease and Pontiac fever can be traced to the same source. Person-to-person transmission does not occur with either Legionnaires' disease or Pontiac fever.

Prevention

Travellers 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, travellers should be advised to seek medical attention promptly if they develop symptoms of Legionnaires' disease or Pontiac fever. There is no vaccine for legionellosis and antibiotic prevention is not effective.

Treatment

For travellers with Legionnaires' disease, specific antibiotic treatment is necessary and should be administered promptly while diagnostic tests are being processed. Appropriate antibiotics include quinolones 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.

Bibliography
  • Benin AL, Benson RF, Arnold KE, et al. An outbreak of travel-associated Legionnaires disease and Pontiac fever: the need for enhanced surveillance of travel-associated legionellosis in the United States. J Infect Dis. 2002;185:237-43.
  • Benin AL, Benson RF, Besser RE. Trends in Legionnaires disease, 1980-1998: declining mortality and new patterns of diagnosis. Clin Infect Dis. 2002;35:1039-46.
  • Fields BS, Benson RF, Besser RE. Legionella and Legionnaires' disease: 25 years of investigation. Clin Microbiol Rev. 2002;15:506-26.
  • Jernigan DB, Hoffman J, Cetron MS, et al. Outbreak of Legionnaires' disease among cruise ship passengers exposed to a contaminated whirlpool spa. Lancet. 1996;347:494-9.
  • Joseph CA. Legionnaires' disease in Europe 2000-2002. Epidemiol Infect. 2004;132:417-24.
  • Joseph C, Morgan D, Birtles R, et al. An international investigation of an outbreak of Legionnaires disease among UK and French tourists. Eur J Epidemiol. 1996;12:215-9.

- Matthew Moore

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