LEGIONELLA - World Health Organization

• Serologic testing can be used for diagnosis, but is not the most helpful diagnostic tool,
because a fourfold rise in antibody titre from specimens obtained 3–6 weeks apart is necessary
to make a clinical diagnosis of legionellosis; a diagnosis is rarely made from a single high titre.
• DFA testing can be used for diagnosis; however, testing using this method must be regular
so that changes in results can be detected immediately.
The consequences of failing to regularly test patients with health-care acquired pneumonia
were identified in a study by Lepine et al. (1998), who reported a cluster of cases of legionellosis
in a hospital soon after the introduction of urine antigen testing. The hospital had experienced
an outbreak of nosocomial legionellosis 16 years earlier and, as revealed by molecular subtyping
methods, the isolates from the two outbreaks were identical. There was no increase in the
hospital’s overall rate of nosocomial pneumonia. The study suggested that persistent transmission
of Legionella infections may have been occurring over a long period, without being recognized.
Several investigations report underuse of diagnostic testing. Fiore et al. (1999) published a
survey sponsored by the Centers for Disease Control on surveillance systems for health-care
acquired infections. Of the 192 hospitals that responded, only 60% could provide in-house
testing for legionellosis, and only 21% had established routine testing procedures that included
legionellosis for respiratory specimens from patients with nosocomial pneumonia. This study
highlights the importance of surveillance for legionellosis and infection control in hospitals,
residential institutions and other such buildings.
Health-care facilities must have policies in place to test for legionellosis in patients with nosocomial
pneumonia. Effective diagnosis and evaluation of results are crucial for the adequate and
prompt management of incidents and outbreaks, for the control of clusters of infections, and
for the protection of other patients.
11.3 Analysing environmental samples for Legionella
11.3.1 Standards for Legionella detection and recovery
There are a number of manuals and laboratory procedures for the recovery of legionellae from
environmental samples. In 1998, an international standard (International Organization for
Standardization ISO 11731) was developed to incorporate the different strategies used by a number
of institutions for efficient recovery and detection of legionellae (ISO, 2004). The following
sections provide an overview of methods for detecting the bacterium in water samples,
according to the ISO standard.
11.3.2 Ensuring safety during environmental sampling
Environmental samples of Legionella should be collected by people with knowledge of Legionella
ecology and general risk assessment (see Chapters 2 and 9). People taking environmental
samples require training to ensure that they select samples containing the highest numbers of
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS