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NOTE: A surveillance case definition is a set of uniform criteria used to define a disease for public health surveillance. Surveillance case definitions enable public health officials to classify and count cases consistently across reporting jurisdictions. Surveillance case definitions are not intended to be used by healthcare providers for making a clinical diagnosis or determining how to meet an individual patient’s health needs.

Clinical Description

Legionellosis is associated with two clinically and epidemiologically distinct illnesses: Legionnaires’ disease, which is characterized by fever, myalgia, cough, pneumonia, and Pontiac fever, a milder illness without pneumonia.

Laboratory Criteria For Diagnosis

  • Isolation of Legionella from respiratory secretions, lung tissue, pleural fluid, or other normally sterile fluids, OR
  • Demonstration of a fourfold or greater rise in the reciprocal immunofluorescence antibody (IFA) titer to greater than or equal to 128 against Legionella pneumophila serogroup 1 between paired acute- and convalescent-phase serum specimens, OR
  • Detection of L. pneumophila serogroup 1 in respiratory secretions, lung tissue, or pleural fluid by direct fluorescent antibody testing, OR
  • Demonstration of L. pneumophila serogroup 1 antigens in urine by radioimmunoassay or enzyme-linked immunosorbent assay

Case Classification


A clinically compatible case that is laboratory confirmed


The previously used category of "probable case," which was based on a single IFA titer, lacks specificity for surveillance and is no longer used.

Related Case Definition(s)