LEGIONELLA - World Health Organization

In the clinical setting, serology is limited in its usefulness as a diagnostic tool for legionellosis
because of the length of time required, the need for paired sera, and the difficulty of obtaining
appropriate convalescent samples (Stout & Yu, 1997). Although diagnosis by antibody detection
from tissues is still useful for epidemiological studies in outbreaks or to establish an infection
retrospectively, it has generally been superseded by the urinary antigen test, as discussed above.
A single high titre with clinical symptoms suggestive of legionellosis gives a presumptive
diagnosis. However, in one study, a single acute-phase antibody titre of 1:256 could not
discriminate between cases of Legionella and non-cases (Plouffe et al., 1995). Cross-reactions
with other bacteria, such as Campylobacter and Pseudomonas species, have also occurred (Marshall,
Boswell & Kudesia, 1994; Boswell, Marshall & Kudesia, 1996; Harrison, 1997).
Indirect IFAT is used to diagnose legionellosis by incubating samples with a hyperimmune
antiserum and then visualizing them by applying a fluorescently tagged anti-Legionella antibody,
fluorescein–isothiocyanate-conjugated immunoglobulin (FITC). A positive control (human
reference serum) and a negative control (human serum from a healthy individual) are required
(Rose et al., 2002). The sensitivity and specificity of IFAT have only been evaluated using
L. pneumophila serogroup 1 antigen; sensitivity and specificity for other serogroups or species
are not known (Muder 2000; Lück, Helbig & Schuppler, 2002). Because of the formation of
cross-reactive antibodies, about 50% of patients infected by L. pneumophila non-serogroup 1
seroconvert with antigens specific to L. pneumophila serogroup 1 (Edelstein, 2002). A negative
result does not exclude legionellosis, and care needs to be taken to confirm a positive result
when low numbers of bacteria are seen (Benson & Ward, 1992).
Antigen preparation differs between laboratories and manufacturers, resulting in different
critical titre levels. For some antigen preparations, specificity could be relatively high for a single
specimen, and low for another antigen (Rose et al., 2002).
A number of companies produce FITC-labelled antibodies for the detection of L. pneumophila.
An FITC-conjugated monoclonal antibody (MAb) directed against L. pneumophila common
outer-membrane protein is commercially available, and is preferred because it is more specific
than polyclonal reagents. The MAb has the advantage of reacting with all L. pneumophila
serogroups, but only identifying L. pneumophila. Genus-specific MAbs are not suitable for
immunofluorescence.
Direct immunofluorescence assays
Direct immunofluorescence assays (DFAs) using antibody conjugated with a fluorochrome
require 2–3 hours to complete the staining procedure. DFAs for Legionella species other than
L. pneumophila should not ordinarily be used. DFA of sputum remains positive for 2–4 days
after the initiation of the specific legionellosis antibiotic therapy, and often for a longer period
in cases of a cavitary pulmonary disease (Lück, Helbig & Schuppler, 2002).
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS