LEGIONELLA - World Health Organization

DFA has been used successfully with expectorated sputum, endotracheal suction aspirates,
lung biopsies and transtracheal aspirate (Stout, Rihs & Yu, 2003). Pleural fluid examination in
patients with legionellosis by culture or DFA rarely yields positive results, but has occasionally
been helpful. Between 25% and 70% of patients with culture-proven legionellosis have positive
DFA for L. pneumophila, and the test’s specificity is higher than 99.9%. Therefore, a negative
result does not rule out legionellosis but a positive result is almost always diagnostic, provided
that the slide is read correctly.
Care must be taken to prevent false-positive results of DFA. These can result from clinical specimens
coming into contact with contaminated water, such as contaminated buffers or organisms
washed from positive control slides (Lück, Helbig & Schuppler, 2002). In addition, skill and
experience are required to interpret the DFA; therefore, laboratories lacking expertise should
be discouraged from using it.
Enzyme immunoassays
Microagglutination or enzyme immunoassay (EIA) methods can be used to serologically
diagnose L. pneumophila serogroup 1 in tissues (Edelstein, 2002). Several EIA serologic
diagnostic kits are commercially available, with sensitivity ranging from 80% to 90% and a
specificity of about 98%. The sensitivity of kits for testing antibody from serotypes 2–6 is still
unknown. The conformity of EIA tests with the immunofluorescence method is about 91%
(Edelstein, 2002).
11.2.3 Diagnosing legionellosis using nucleic acid detection
Overview of polymerase chain reaction assays
L. pneumophila DNA was first detected in clinical samples by a commercial nucleic acid
hybridization assay that used a radioisotopically labelled RNA (ribonucleic acid) probe.
However, concerns about the sensitivity and specificity of the assay led to its subsequent
withdrawal (Fields, Benson & Besser, 2002).
Since then, Legionella polymerase chain reaction (PCR) assays have been used more actively to
detect DNA from environmental samples, but can also be used for analysing clinical samples,
particularly those from the respiratory tract. Detection of Legionella and L. pneumophila
DNA has been reported using PCR assays (with or without confirmation by blot hybridization
or sequencing) (Mahbubani et al., 1990; Lisby & Dessau, 1994; Ko et al., 2003; Liu et al., 2003),
including those targeting:
• ribosomal RNA (rRNA) genes or their intergenic spacer regions
• a gene coding for heat-shock protein (dnaJ)
• the RNA polymerase gene (rpoB)
• the macrophage infectivity potentiator (mip) gene.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS