LEGIONELLA - World Health Organization

Bhopal et al. (1991) studied sporadic cases of legionellosis (i.e. those not associated with known
outbreaks or with travel) in relation to the distance between the person’s home and cooling
towers. The study found that risk of infection decreased with increasing distance. People living
within 0.5 km of any tower were three times more likely to become infected than people
living more than 1 km away.
Bentham & Broadbent (1993) reviewed the common features of some community outbreaks
associated with cooling towers, and found that towers implicated in outbreaks were mainly
those of less than 300 kilowatts. Outbreaks were most frequent in autumn, and often involved
towers that had been operated after a period of shutdown. The study monitored the numbers
of legionellae in systems that had been shut down, with samples taken before, 10 minutes after
and 70 minutes after switching on the circulation. In some cases, switching on the system
raised Legionella concentrations from below the detection limit (4000 CFU/litre) to between
5.0 × 10 4 and 9.5 × 10 5 CFU/litre within 10 minutes.
3.2 Health-based targets
Health-based targets are based on critical evaluation of health concerns; for example, for Legionella
safety, an overall health-based target might be to have “no cases of legionellosis caused by
artificial water systems”. Health-based targets for Legionella safety are normally set nationally
and applied locally. Targets should be set by a senior authority responsible for health, in consultation
with relevant experts, including environmental microbiologists, engineers, system designers
and installers, maintenance staff and contractors, and people responsible for ensuring the health
and safety of systems.
Health-based targets usually focus on controlling the proliferation of legionellae and the production
and release of aerosols, because of the difficulty of determining what represents an acceptable
limit for Legionella. For example, the cooling tower outbreaks listed in Table 3.1, above, all
occurred at levels of at least 10 5 CFU/litre (by culture), but it would be dangerous to assume
that it is safe to set an acceptable limit just below that level, because numbers could increase
rapidly if a system is not adequately controlled. Also, environmental conditions may modulate
the virulence of individual strains (Byrne & Swanson, 1998), and routine culture does not
differentiate between virulent and avirulent strains. Thus, the public health significance of a
culture result from a water sample cannot be determined, because the result is not necessarily
related to virulence, exposure concentration, survival of the organism in an aerosol or the
infectious dose of the organism.
A further issue is that culture methods are biased towards the species currently recognised to
be associated with disease, particularly L. pneumophila, and may not detect all legionellae present
in the environment. In addition, people vary in their susceptibility to infection, making it
difficult to assess generic risk for the population at large. Thus, health risk assessments must
be made without reference to the relationship between dose and response, and with only
limited reference to test results.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS