LEGIONELLA - World Health Organization

Other causes of infection
In Europe, approximately 70% of Legionella infections are caused by L. pneumophila serogroup 1,
20–30% are caused by other serogroups, and 5–10% are caused by non-pneumophila species
(Joseph, 2002a).
The majority of human infections with species other than L. pneumophila are pneumonic, and
occur after exposure to Legionella (Fang, Yu & Vickers, 1989). Of the reported non-pneumophila
infections, the causes of infection are (Reingold et al., 1984; Fang, Yu & Vickers, 1989):
• L. micdadei (60%)
• L. bozemanii (15%)
• L. dumoffii (10%)
• L. longbeachae (5%)
• other species (10%).
L. longbeachae has been associated with exposure to potting composts in Australia, the USA
and Japan (Steele, Lanser & Sangster, 1990; Steele, Moore & Sangster, 1990). The mechanisms
of infection from potting compost are not fully understood. Outbreaks of legionellosis
associated with construction or maintenance are likely to be the result of sloughing of biofilms
(the slimy matrices produced and inhabited by bacteria, which enable them to adhere to a
surface) or descaling of plumbing systems caused by changes in water flow or pressure (Storey,
Ashbolt & Stenstrom, 2004b; see Chapter 2 for more information). Recently, it has been
suggested that there may be a spectrum of illness from a single source, with several reports of
outbreaks involving cases of both Legionnaires’ disease and Pontiac fever.
Under appropriate conditions, most Legionella that can grow at body temperatures may be able
to cause human infections (Fields, 1996). Infections due to species other than L. pneumophila may
be underdetected, because of a lack of appropriate diagnostic tests (Fields, Benson & Besser, 2002).
The Legionella count alone cannot be used to predict whether a source positive for the bacterium
will cause infection. The likelihood that a source will cause an infection depends on several
factors: the load of bacteria, the effectiveness of dissemination, the way in which the bacteria
multiply and the source’s ability to form aerosols.
Leg onella-like amoebal pathogens
Some legionellae cannot be grown on routine Legionella culture media, and have been termed
Legionella-like amoebal pathogens (LLAPs) (see Chapter 11). These organisms have been
isolated and maintained by co-cultivating the bacteria with their protozoan hosts. One LLAP
strain was isolated from the sputum of a pneumonia patient by enrichment in amoebae and
is considered to be a human pathogen (Fry et al., 1999; Marrie et al., 2001). Additional LLAP
strains may be human pathogens, but proving this is difficult, because they cannot be detected
by conventional techniques used for legionellae. Recently, three LLAP strains were named
Legionella species (Adeleke et al., 2001; La Scola et al., 2004).
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS