Biotech Laboratories is a nationally accredited
biological testing laboratory that has been analysing samples of
water-cooled air-conditioning systems since 1990. The laboratory
analyses approximately 8,000 samples per year and of these, some 5,500
are for Legionella bacteria.
The detection limit for Legionella by the Australian Standard method is
10 colony forming units per millilitre (CFU/mL) which is basically the
same as bacteria per millilitre. The industry guidelines for acceptable
results, which is also quoted in the Australian Standard AS 3666,
Air-handling and water systems of buildings - Microbial control also
state an acceptable limit of less than 10 CFU/mL.
In this laboratory over the 10-year period from 1993 to 2003, the
average isolation of Legionella bacteria occurred at a rate of once in
every 9 samples, i.e. 10.7%. The monthly isolation rate varies with the
seasons and peaks in late summer each year. During the 10-year period
the lowest monthly isolation rate was 1.9% and the highest was 23.8%
(nearly once in every 4 samples). This is despite the fact that it would
be expected that these samples came from air-conditioning systems that
were being maintained by reputable and efficient water treatment
specialists and being maintained to the requirements of AS 3666. It is
important also to remember that in that 10-year period although 10%
water samples were positive for Legionella, none resulted in a
Legionnaires’ Disease outbreak. This is despite the fact that on average
5% of all positive samples has a Legionella count exceeding 1,000
CFU/mL.
The
data presented would suggest that reliance on maintaining cooling towers
to Australian Standard AS3666 does not prevent contamination of the
system with legionellae. It would be expected that at some time or
another all water-cooled air-conditioning systems would show some level
of contamination with Legionella. This does not necessarily reflect on
the quality of the system maintenance.
The
concentration of Legionella present is important however, and a level
exceeding 1,000 CFU/mL requires immediate decontamination. A level of 10
to 100 CFU/mL is commonly found and although some remedial action is
indicated it can often be regarded as insignificant so long as proper
maintenance practices are being maintained. By maintaining the correct
maintenance procedures and testing at a monthly interval, the risk of
Legionella concentrations reaching the high risk level (>1,000 CFU/mL)
before detection is extremely low. This tolerance level does not apply
to spa pools, warm water, reticulated domestic supplies, water used for
cooling drill bits or other machinery where aerosols are produced. In
these cases even low Legionella levels are highly significant and
immediate disinfection is required. Irrespective of the water source
after a positive detection follow up testing for Legionella is required
3 to 5 days after remedial action is taken to ensure that the process
was effective.
It
must be remembered that the species of Legionella detected, be that
Legionella pneumophila serogroup 1, Legionella pneumophila
serogroup 2-14, or Legionella species has no effect on the actions that
are required as a result of a positive detection. It is the total number
of Legionella present per millilitre water that is important. If
a water sample had a Legionella pneumophila count of 200 CFU/mL
and a Legionella species (not pneumophila) count of 900 CFU/mL,
then the total Legionella count for the water would be 1,100
CFU/mL which requires immediate shutdown and disinfection. This fact is
supported by AS/NZS 3666.3 which classifies actions required based on
the level of Legionella and makes no mention for a diminished risk
status based on the speciation of the microorganism.
It is true that all cooling tower outbreaks to date of Legionnaires’
Disease have been due to Legionella pneumophilia, but at the same
time you must remember that 98% of all Legionella found in
cooling towers are Legionella pneumophilia. It is therefore not
surprising that all outbreaks have been due to this species and not one
of the other Legionella species. It is important to remember that
the group Legionella species includes L. micdadei, L. longbeachae,
L. bozemanii, L. feeleii, and L. dumoffii.
All these have been associated with serious human infections and in many
cases have led to death.
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