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
Confirmed: a clinically compatible
case that is laboratory confirmed
Comment
The previously used category of "probable case," which
was based on a single IFA titer, lacks specificity for surveillance and is no
longer used.