LEGIONELLA - World Health Organization

Traditionally, the rRNA genes have been used for assays targeting the Legionella genus and the
mip gene for L. pneumophila-specific assays. Assays for Legionella and L. pneumophila using
direct (real-time) monitoring platforms have also been described (Ballard et al., 2000; Hayden
et al., 2001; Rantakokko-Jalava & Jalava, 2001; Wellinghausen, Frost & Marre, 2001). With
respiratory samples, Legionella PCR has a reported specificity of ≥99% and sensitivity of 85%
(Edelstein & Meyer, 1994; Fields, Benson & Besser, 2002; Lück, Helbig & Schuppler, 2002;
Uldum & Molbak, 2002).
An important feature of Legionella PCR is that the method can potentially detect all serogroups
of L. pneumophila and is therefore useful in the early diagnosis of infections, particularly in
nosocomial cases (Uldum & Molbak, 2002). Over the past few years, PCR techniques have
improved substantially, particularly those for direct (real-time) monitoring of the generation
of PCR fragments. The use of real-time PCR technique accelerates the diagnostic procedure
for legionellosis and improves the specificity.
PCR methods could have important economic benefits. Their use in outbreaks of legionellosis
could help to rapidly rule out implicated sites, thereby minimizing lost revenue and allowing
resources to be diverted to areas that need further investigation. Until the diversity and distribution
of legionellae are better understood, results from methods other than culture should be
interpreted cautiously.
Limitations of PCR assays
Current data are insufficient for reliably estimating PCR sensitivity and specificity values, or
for comparing PCR to other methods. Broadening the application of PCR requires evaluation
and standardization of sample preparation and PCR protocols, to define primer and probe
specifications and assay sensitivities, and to reduce the effect of PCR inhibitors. Ideally,
internal process controls should also be included, to indicate the presence or absence of PCR
inhibitors or the failure of PCR (Ursi et al., 1992; Hu et al., 2002; Lück, Helbig & Schuppler, 2002).
Few validation data are available for the many assays described, particularly for use in a clinical
setting. L. pneumophila PCR assays appear to be promising, but assays reported to specifically
target all Legionella species should be viewed with caution and carefully assessed. Before a
specific PCR assay is used to diagnose legionellosis, its analytical sensitivity and specificity
should be determined and compared with that of other PCR assays.
11.2.4 Diagnosing patients with health-care associated pneumonia
Nosocomial Legionnaires’ disease is discussed in detail in Chapter 6. This section applies to
diagnostic testing used to evaluate patients with health-care associated pneumonia.
Diagnosing patients with nosocomial pneumonia requires the following:
• Every health-care facility should have access to a laboratory that is proficient in isolating
Legionella from cultures and has urine antigen testing facilities.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS