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Legionellosis (Legionnaires' Disease and Pontiac Fever)

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Legionellosis Case Classification

CDC describes case classification for suspected and confirmed Legionellosis based on signs and symptoms of disease (i.e., clinical compatibility), travel history (for travel-associated cases), and laboratory testing.

Employers, occupational safety and health professionals, and others involved in preventing and responding to Legionellosis outbreaks can use these definitions to help recognize and classify suspected and confirmed cases of disease.

Legionellosis is not contagious, meaning it is not spread from person-to-person. Disease transmission primarily occurs through inhaling Legionella-contaminated, aerosolized water. While rare, exposure is also possible from breathing in (i.e., aspirating) Legionella contaminated soil or while drinking water.1 An exposed person's physical condition (general health), the Legionella concentration in an aerosol (bacteria suspended in air, including in droplets), and contamination levels at a source all contribute to whether an exposure results in disease and how severe the disease is.

Legionellosis Case Classification

CDC describes case classification for suspected and confirmed Legionellosis based on signs and symptoms of disease (i.e., clinical compatibility), travel history (for travel-associated cases), and laboratory testing.

Employers, occupational safety and health professionals, and others involved in preventing and responding to Legionellosis outbreaks can use these definitions to help recognize and classify suspected and confirmed cases of disease.

This page discusses symptoms and other characteristics of Legionellosis, and compares the two forms of disease resulting from Legionella exposure, Legionnaires' disease and Pontiac Fever.

Symptoms and other Disease Characteristics

Legionnaires’ disease and Pontiac Fever each have distinct symptoms, attack rates (the rate at which exposed persons become infected), and disease severity. In their early stages, Legionnaries’ disease and Pontiac Fever have similar flu-like symptoms. However, Legionnaires’ disease progresses to pneumonia, whereas Pontiac Fever does not. When Legionnaires’ disease progresses to pneumonia, it is frequently severe and can be fatal. It is important to recognize Legionnaires’ disease symptoms because early medical treatment can save lives.

Comparison of Legionnaires' disease and Pontiac Fever2

  Legionnaires' disease Pontiac Fever
Symptoms

Early symptoms may include low-grade fever, headache, aching joints (arthralgia) and muscles (myalgia), tiredness, and appetite loss

Later symptoms include high fever ranging from 39° to 41°C (102°-105°F), cough (dry at first, later producing phlegm), difficulty in breathing, chills, and chest pain

Gastrointestinal symptoms may include vomiting, diarrhea, nausea, and abdominal pain

Influenza-like (flu-like) symptoms that may include low-grade fever, headache, tiredness, appetite loss, muscle and joint pain, chills, nausea, and a dry cough

Pneumonia is not present

Incubation period (time between exposure and appearance of symptoms) 2-10 days, but, rarely, up to two weeks3 24-72 hours
Attack rate (rate at which exposed persons become infected) <5% >90%
Death rate (general population) 10% 0%
Reason for illness Bacterial infection Reaction to endotoxin production by Legionella bacteria3
Disease Diagnosis
Legionnaires' disease
  • Diagnosing Legionnaires’ disease usually begins with a pneumonia diagnosis, confirmed by chest x-ray, and verified by one or more positive diagnostic tests for Legionella.
  • The preferred diagnostic tests for Legionnaires’ disease are the Legionella urinary antigen test and lower respiratory secretion (i.e., sputum) culture.
Pontiac Fever
  • Pontiac Fever resolves (goes away) spontaneously and often goes undiagnosed.
  • A urine or blood test may diagnose Pontiac Fever; however, a negative test does not definitively rule out disease.
  • Pontiac Fever is most often diagnosed when there are other known, laboratory-confirmed Legionnaires’ disease or Pontiac Fever cases with possible exposure at the same time or place.

The CDC's Legionella (Legionnaires' Disease and Pontiac Fever) web page summarizes the diagnostic tests for Legionella and their advantages and disadvantages. It also provides detailed information for physicians and other healthcare professionals on indications for Legionella testing and preferred diagnostic tests.

Treatment

Legionnaires’ disease is treatable with antibiotics. The earlier a patient receives antibiotic therapy, the more likely it will be successful. Examples of common antibiotics appropriate for treating Legionnaires’ disease include fluoroquinolones such as levofloxacin and newer macrolides such as azithromycin. The Infectious Diseases Society of America-American Thoracic Society (IDSA-ATS) guidelines offer the most updated information about treatment recommendations.

Antibiotic therapy is not recommended for Pontiac Fever. The illness will go away without antibiotic treatment. Patients typically recover from Pontiac Fever within a week.


1 Cramp, GJ, Harte, D, Douglas, NM, Graham, F, Schousboe, M. and Sykes, K. An outbreak of Pontiac fever due to Legionella longbeachae serogroup 2 found in potting mix in a horticultural nursery in New Zealand. Epidemiology and Infection 138(1), 15-20 (2010).

2 Legionella (Legionnaires' Disease and Pontiac Fever). Centers for Disease Control and Prevention (CDC), U.S. Department of Health and Human Services (HHS).

3 Edelstein, P.H. Urine Antigen Tests Positive for Pontiac Fever: Implications for Diagnosis and Pathogenesis, Clin. Infect. Dis., 44(2), 229-231 (2007).

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