LEGIONELLA - World Health Organization

2.4 Sources of Legionella infection
It is not possible to predict whether a source will cause infection based solely on the Legionella
count. The likelihood that a source will cause an infection depends on the load of bacteria, the
effectiveness of dissemination, the way in which it multiplies, and its ability to form aerosols.
2.4.1 Disease spread via aerosols and inhalation
The role of aerosols from contaminated potable water distribution systems in leading to legionellosis
is well established. Other chapters of this publication (see Chapters 4–8) discuss the many
aerosol-generating systems that have been linked with transmission, such as cooling towers,
building water systems, respiratory therapy equipment and hot tubs.
Showers are often mistakenly thought to be the only source of aerosols linked to nosocomial
legionellosis (Woo, Goetz & Yu, 1992); however, water outlets, humidifiers, respiratory devices
and nebulizers that have been filled or cleaned with tap water can also spread Legionella and have
been reported as a source of infection in several cases (Arnow et al., 1982; Moiraghi et al., 1987;
Brady, 1989; Mastro et al., 1991; Woo, Goetz & Yu, 1992). Toilet flushing is also a potential
source (Albrechtsen, 2002).
As discussed in Chapter 1, community-acquired cases of legionellosis can almost always be
attributed to inhalation of aerosols from devices such as cooling towers, hot tubs, industrial
equipment and indoor fountains (Heng et al., 1997; Den Boer et al., 2002; Greig et al., 2004).
The largest outbreaks of disease to date have all been associated with transmission of aerosols
from these types of equipment (Den Boer et al., 2002; Garcia-Fulgueiras et al., 2003; Greig et
al., 2004). Cooling towers are a particular problem, with one report suggesting that cooling
towers account for at least 28% of all sporadic cases of legionellosis (Bhopal, 1995).
Other systems implicated in the spread of legionellosis via aerosols include domestic plumbing
systems (Singh, Stout & Yu, 2002; WHO, 2004; see Chapter 4); misting devices associated with
food displays (Mahoney et al., 1992), natural thermal springs (Sommese et al., 1996; Alim,
Hakgudener & Poyraz, 2002) and thermal spas (Brady, 1989; Martinelli et al., 2001; Vogiannis
et al., 2004).
As discussed in Chapter 1, nasogastric tubes have been included in several studies of nosocomial
legionellosis, with microaspiration of contaminated water presumed to be the mode of entry
(Marrie et al., 1991; Blatt et al., 1994; Stout & Yu, 1997). However, a recent study failed to
detect colonization of the oesophageal tract by Legionella in this situation (Pedro-Botet et al.,
2002). Patients suffering from nosocomial legionellosis are significantly more likely to have
undergone endotracheal tube placement, or to have been intubated for significantly longer,
than patients with other causes of pneumonia (Strebel et al., 1988; Kool et al., 1998; Winston,
Seu & Busuttil, 1998).
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS