LEGIONELLA - World Health Organization

3.1 Environmental exposure and disease
There is no established dose–response relationship for Legionella infections, and the concentration
of legionellae necessary to cause an outbreak is unknown. Transmission may occur through
inhalation, aspiration or directly from contaminated water (e.g. wound infections), from a
wide variety of sources, as discussed in Chapter 1. Data on population dynamics indicate that
legionellae are not distributed normally within the aquatic environment, and that even when
high concentrations of the bacteria are detected, this may not be related to health risk (Kool
et al., 1999; Bentham, 2000). Gathering information on population dynamics of Legionella is
difficult, because there have been relatively few outbreaks in which the source has been investigated
while it was still infectious, and in which no intervention has occurred before sampling.
Of the many reports of Legionella outbreaks caused by cooling towers, few provide details of
the numbers of legionellae present in the water at the time the tower was infectious. Often,
the tower was examined long after the infectious period, and the bacterial population may
have changed dramatically in the interim, as shown by the example in Box 3.1.
Box . Hosp tal outbreak n wh ch water sampl ng was neffect ve
In 1985, an outbreak of legionellosis occurred at Stafford District General Hospital in the
United Kingdom. The investigation team was able to isolate L. pneumophila from a piece
of sealant in an air handling unit, but not from any water sample (O’Mahony et al., 1990).
Presumably, at the time of the outbreak, the cooling water contained high levels of
L. pneumophila. However, between the time of the outbreak and the arrival of the
investigation team, the cooling water had been shot dosed (given a brief, high-level
treatment) with biocide at least twice, and had been diluted by fresh make-up water.
Numbers of legionellae would have been reduced considerably by both the biocide
and the dilution with fresh water. Low numbers of L. pneumophila were isolated from a
sample of the cooling water collected between the shot doses. The sealant from which
the investigation team isolated L. pneumophila came from around a chiller battery within
the air-conditioning ducting. The position of the sealant meant that it could have been
contaminated by aerosols from the cooling tower, but would not have been affected by
the biocide additions.
3.1.1 Cooling tower outbreaks
In cooling tower outbreaks in the 1980s, numbers of bacteria were often estimated by
immunofluorescence rather than by culture. These results may have been unreliable, because
the reagents used were polyclonal antibodies, which have questionable specificity; also, the
technique detected both dead and live legionellae. However, isolation of legionellae by culture
tends to underestimate the numbers of legionellae by at least an order of magnitude.
Table 3.1 shows the results of various cooling tower outbreaks in which the towers were sampled
while probably still containing infectious legionellae.
0 LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS