LEGIONELLA - World Health Organization

1.2 Prevalence and risk factors
The exact incidence of legionellosis worldwide is unknown, because countries differ greatly
in the methods they use for ascertaining whether someone has the infection and in reporting
of cases. Also, the reported incidence of Legionnaires’ disease varies widely according to the
intensity of investigation and the diagnostic methodology applied (as discussed in Chapter 9).
Table 1.3 provides some useful definitions for epidemiological monitoring, used throughout
this publication. Table 1.4 shows European cases, by category, from 1994 to 2004.
Table 1.3 Useful definitions for epidemiological monitoring
Leg onna res’ d sease Case def n t ons
Confirmed casesa Clinical or radiological evidence of pneumonia and a
microbiological diagnosis by culture of the organism from
respiratory specimens, or a fourfold rise in serum antibodyb levels against L. pneumophila serogroupc (sg) 1, or detection
of L. pneumophila antigend in urine or positive direct
immunofluoresence assay (DFA) test.
Presumptive cases Clinical or radiological evidence of pneumonia and a microbiological
diagnosis of a single high antibody level against L. pneumophila
sg 1 or a seroconversione demonstrated against Legionella
species and serogroups other than L. pneumophila sg 1.
Health-care acquired
(nosocomial) cases
Depending on length of stay in hospital before onset and
environmental investigation results, cases are definitely, probably
or possibly nosocomial (see Box 1.1, below, for details of this
classification).
Travel-associated cases f Case associated with one or more overnight stays away from
home, either in the country of residence or abroad, in the
10 days before onset of illness.
Travel-associated
clusters g
Two or more cases stayed at the same accommodation, with
onset of illness within the same two years (Lever & Joseph, 2003).
Community clustersh Two or more cases linked by area of residence or work, or places
visited, and sufficient closeness in dates of onset of illness to
warrant further investigation.
Community outbreaksi Community clusters for which there is strong epidemiological
evidence of a common source of infection, with or without
microbiological evidence, and in response to which control
measures have been applied to suspected sources of infection.
Domestically
acquired cases
Depending on the elimination of all other sources of exposure,
and the case being known to have used the domestic water
system during the incubation period, and environmental and
clinical results positive for Legionella, cases may be suspected,
probably or definitely domestically acquired.
a When submitted to a Legionella reference laboratory, it is recommended that all positive serum specimens
are examined by the indirect fluorescent antibody test (Boswell, Marshall & Kudesia, 1996) in the presence
of campylobacter blocking fluid, to eliminate cross-reactions between organisms.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS