Legionnaires Disease on Rise in US--2016 Update

Laura A. Cooley, MD, MPHTM


June 13, 2016

Editorial Collaboration

Medscape &

Diagnosis and Testing

Clinical features of Legionnaires disease include cough, shortness of breath, high fever, muscle aches, headaches, and radiographic pneumonia. Legionnaires disease can also be associated with other symptoms, such as diarrhea, nausea, and confusion. For Pontiac fever, clinical features include flu-like illness (ie, fever, chills, malaise) without pneumonia.

Indications that warrant testing for Legionnaires disease include the following[5]:

  • Patients who have failed outpatient antibiotic therapy for community-acquired pneumonia;

  • Patients with severe pneumonia—in particular, those requiring intensive care;

  • Immunocompromised patients with pneumonia;

  • Patients with pneumonia in the setting of a legionellosis outbreak; and

  • Patients with a travel history (patients who have traveled away from home within 2 weeks before the onset of illness).

Clinicians should test patients with healthcare-associated pneumonia for Legionnaires disease. This is especially important among patients at increased risk of developing Legionnaires disease, patients with severe pneumonia (in particular those requiring intensive care), or if any of the following are identified in your facility:

  • Patients with Legionnaires disease, no matter where they acquired the infection;

  • Positive environmental tests for Legionella; and

  • Changes in water quality that may lead to Legionella growth (such as low chlorine levels).

The preferred diagnostic tests for Legionnaires disease are the Legionella urinary antigen test and culture of lower respiratory secretions (sputum or bronchiolar lavage) on selective media. Isolation of Legionella by culture is confirmatory and an important method for diagnosis, despite the convenience and specificity of urinary antigen testing. If your patient has pneumonia and the urinary antigen test is positive for Legionella, then your patient is considered to have Legionnaires disease. This test is designed to detect the most common cause of legionellosis (L pneumophila serogroup 1). However, all species and serogroups of Legionella are potentially pathogenic, so a patient with a negative urinary antigen result may have legionellosis caused by another species or serogroup of Legionella. Therefore, if urinary antigen testing is negative but Legionnaires disease is still suspected, a respiratory culture is required.

Furthermore, molecular techniques can be used to compare clinical isolates obtained from culture with environmental isolates to confirm the source of an outbreak. Thus, best practice for detection of Legionella and for public health surveillance is to also obtain respiratory specimens for culture at the time that urinary antigen testing is ordered, preferably before the administration of antibiotics.

Most cases of Pontiac fever are diagnosed in association with an outbreak, on the basis of clinical signs and symptoms, often along with cases of Legionnaires disease. The urinary antigen test can be used to confirm the diagnosis; however, owing to the low sensitivity of this test in the setting of Pontiac fever, it cannot be used to rule it out.[6]