Legionella pneumophila
serogroup 1 |
Legionella pneumophila
serogroup 2 - 14 |
Legionella species
(not L. pneumophila) |
71% |
27% |
2% |
At present there are over 25 species and 48 serogroups of Legionella
recognised, a substantial proportion are capable of causing disease. The United States
public health authority, Centers for Disease Control, reports that when an isolate is
recovered from a disease outbreak, 90% of cases of legionellosis are due to L.
pneumophila [4]. Of these cases 82% are caused by L. pneumophila serogroup 1
[4]. Legionella micdadei, L. pneumophila serogroups 6 and 3, and L. longbeachae
are the organisms isolated next most frequently from patients with legionellosis [4]. The
Queensland Health department figures for the period 1988 to 1992 showed that 37% of
legionellosis cases were due to L. pneumophila, 35% to L. longbeachae and
16% to L. bozemannii [9]. Based on both these findings it can be noted that the
species of legionellae or the serogroup of L. pneumophila isolated from a cooling
tower should not influence the interpretation of the laboratory findings. Most
laboratories will report isolates as L. pneumophila serogroup 1, L. pneumophila
serogroup 2 to 14 or Legionella species (not pneumophila). As shown,
pathogenic legionellae have been reported from each of these groups and therefore the risk
assessment should, at present, only be based on the concentration of the legionellae
reported. To further influence this method of evaluation, our laboratory classifies
positive legionellae isolates as either L. pneumophila or Legionella species
(not pneumophila).
The monthly isolation rate ranged from 2.4% (October 1993)
to 23.8% (November 1995). As seen in Graph 1, the isolation rate for legionellae peaked in
the summer and autumn months.
Graph 1. Legionella isolation rates from April 1992 to March 1996.
LEGIONELLA ISOLATION RATE
YEAR
|
JUL
|
AUG
|
SEP
|
OCT
|
NOV
|
DEC
|
JAN
|
FEB
|
MAR
|
APR
|
MAY
|
JUN
|
1991/92 |
|
|
|
|
|
|
|
|
|
14 |
14 |
8.8 |
1992/93 |
6.3 |
9.8 |
8.6 |
8.7 |
11.2 |
13.2 |
14.2 |
12.6 |
14.2 |
14.4 |
12.9 |
8.2 |
1993/94 |
5.8 |
5.5 |
3.9 |
2.4 |
4
|
6.8
|
13.4
|
14.6
|
17.1
|
12.6
|
10.3
|
8.5
|
1994/95 |
4.9
|
7.1
|
6.4
|
9.5
|
10.4
|
14.1
|
20.2
|
17.8
|
16
|
13.6
|
10.6
|
7.3
|
1995/96 |
9.5
|
8.9
|
15
|
13.7
|
23.8
|
19
|
17.4
|
18.7
|
17.6
|
|
|
|
RESULTS OF STANDARD PLATE COUNT ANALYSIS
Of the 3,478 cooling tower water samples which had detectable levels of
legionellae; 1,988 had a PC performed. Table 3 shows the distribution of the PC results of
the water samples that had a positive LA and a PC performed.
Table 3. Standard Plate Count results of Legionella
positive samples.
<10,000 CFU/ml |
10,000 - <100,000 CFU/ml |
100,000 - <1
Million CFU/ml |
>=1 Million CFU/ml |
650 isolates |
811 isolates |
418 isolates |
109 isolates |
32.7% |
40.8% |
21.0% |
5.5% |
The Standards Australia publication HB32 [10] states that
an acceptable level for PC of cooling tower water is below 100,000 CFU/ml. As shown in
Table 3, 73.5% of cooling tower water samples with detectable levels of legionellae had a
PC of less than 100,000 CFU/ml. During the review period it was not uncommon to have a Legionella
count higher than the PC for a given sample. It should be noted that legionellae and other
fastidious bacteria are not detectable by the PC method. An example of this was a sample
with a Legionella count of 16,000 CFU/ml and a PC of 2,000 CFU/ml, this has also
been reported by other workers [6]. These findings should not be interpreted as an inverse
relationship between PC and LA. The increase in detection of legionellae at lower PC
levels reflects the sensitivity of LA. The standard method used [2] utilises heat
treatment, dilution procedures and selective agar plates to minimise the adverse affects
of other bacteria present in the water sample. Despite this, the fact remains that these
procedures are not perfect and contaminating bacteria are often grown on the Legionella
culture plates. Where the PC is high, and other bacteria are not suppressed the
sensitivity of the LA is reduced and a false negative result may be reported. It would
therefore be of assistance to have a PC performed in unison with each LA.
LEGIONELLA/PLATE COUNT RATIOS
Miller and Kenepp [5] proposed that the risk status of a
cooling tower be based on the ratio of Legionella to PC rather than solely on the
CFU/ml of Legionella present. This was based on two observations; first, it was
noted that legionellae were rarely observed in environmental specimens in excess of 1% of
the total bacterial population; second, they observed that high levels of legionellae were
occasionally found in towers with very few other detectable bacteria present [6].
Therefore the Legionella/PC ratio may reflect the tendency of the water system to
select for legionellae. This may be due to variations in the resistance of biocides used
to treat the water system. The distribution of the review data using the categories
preposed by Miller and Kenepp. is shown in Table
Table 4. Legionella / Standard Plate Count
ratios.
Legionellae (% of PC) |
<1% |
1 - 10% |
11 - 50% |
>50% |
% of Samples |
54% |
27% |
16% |
3% |
Using this criteria, 3% of samples that had detectable
levels of legionellae and PC performed, would fall into the highest risk group. However,
using the present risk categories based solely on CFU/ml levels, 12.3% of samples in this
same group would be classified into the high risk category (ie >1,000 CFU/ml). On the
basis that legionellae are found in a relatively high percentage of cooling towers in the
absence of disease, it would appear the concentration of viable legionellae alone may be
overestimating the risk associated with high levels. The criteria proposed by Miller and
Kenepp [5] reflects the selection of legionellae in the cooling tower. A high Legionella/PC
ratio (>50%) indicates a reduction of microbial competition which could lead to a rapid
increase in Legionella overgrowth and perhaps this criteria should be considered as
an additional factor when evaluating the risk based on laboratory findings. Using both Legionella/SPC
ratio and the legionellae level risk assessment methods on this subgroup of
samples, only 1.5% would be classified into both high risk categories. The concern with
any laboratory based risk evaluation process is that it may underestimate the importance
of low or moderate levels of legionellae and overestimate the risk associated with high
levels. Also, all in vitro risk assessments ignore the physical characteristics of
a cooling tower such as efficiency of drift eliminators, design, size and position in
relation to air intake ducts or the public access areas.
CONCLUSIONS
During the review period an increase in the Legionella isolation rate for the
summer and autumn months was observed (Graph 1). During the period from December to May
the highest isolation rates were recorded. Further investigation of the variation in
monthly isolation rates is required.
These investigations may further examine the effect of
environmental factors such as temperature, precipitation and humidity on the colonisation
of cooling tower waters with legionellae.
At present the three groups of legionellae reported
routinely by testing laboratories (L. pneumophila serogroup 1, L. pneumophila
serogroup 2 to 14 and Legionella species (not pneumophila) have all been
implicated in disease processes [4,9]. Based on this, the risk assessment of a cooling
tower contaminated with legionellae should, at present, be based solely on the level of Legionella
present and not on the species or serogroup of the isolated microorganism.
The data presented shows that a low or an acceptable PC
level of cooling tower water does not indicate that the system is free of legionellae
(Table 3). That is, a PC test of a cooling tower should not be used as an indicator of the
Legionella risk status of a tower. A PC should be used, however, as an indicator of
the microbial load of a tower in relation to current acceptable standards. A PC should
always be performed in unison with a LA, as a high SPC may reduce the sensitivity of the
LA.
The routine analysis of cooling tower water for the
presence of legionellae remains a controversial topic. On the basis that the bacteria is
found commonly in cooling towers in the absence of an associated Legionnaires' Disease
outbreak, most Australian public health departments do not recommend routine testing for
this microorganism. However, building owners, managers and water treatment companies
routinely have these tests performed. The ever-present threat of a disease outbreak and
the associated legal and financial liabilities warrant routine risk assessments to be
made. The data presented would suggest that reliance on maintaining cooling towers to the
Australian Standard AS3666 and the Standards Australia publication HB32 does not prevent
contamination of the system with legionellae. We would therefore recommend that the
relevant authorities review the present guidelines with respect to including LA in the
preventative maintenance protocol of cooling towers.
On this basis it should be noted that the interval between
analysis must be selected carefully. If quarterly testing is performed, results provide
only a random check for the presence of legionellae. It is our experience that inside a
three month period legionellae levels may increase rapidly to high risk levels and fall to
acceptable levels prior to the next analysis. This is reinforced by the results showing an
equivalent isolation rate between towers tested monthly as compared to quarterly, but a
dramatic decrease in the average Legionella level of a positive sample for the
monthly analyses towers (90 CFU/ml as compared to 972 CFU/ml). To approach a system for
monitoring the presence of legionellae, a monthly LA would be required. As the method
requires a 10 day incubation, testing inside a four week period could be of limited use
except to monitor a known positive cooling tower during the decontamination process. It
should also be noted that a PC should be performed routinely with each LA to enable
accurate interpretation of both negative and positive results.
Further it is proposed that research be initiated to
evaluate the usefulness of Legionella/PC ratios as an additional factor for
evaluating the risk associated with legionellae contaminated cooling towers. This would
require a review of previous laboratory data for cooling towers implicated in
Legionnaires' Disease outbreaks in Australia and overseas.
References
1. Australian Standard: AS3666-1989
"Air-handling and water systems of buildings- Microbial control" (1989).
2. Australian Standard: AS3896-1991
"Waters-Examination for legionellae" (1991).
3. APHA: Standard Methods for the Examination of Water
and Wastewater 17th ed., American Public Health Association, p9-61, (1989).
4. Marston, B.J., Lipman, H and Breiman, R.F.: A decade
of surveillance for Legionnaires' Disease: update on risk factors for morbidity and
mortality due to infection with Legionella. Submitted for publication.
5. Miller, R.D. and Kenepp, K.A.: in Legionella
Current State and Emerging Perspectives, Editors: Barbaree, J.M., Breiman, R.F. and
Dufour, A.P.: American Society for Microbiology, pp 40-43, (1993).
6. Miller, R.D. and Kenepp, K.A.: Aerobiology, Abstr.
Pan-Am. Aerobiol. Assoc. Annu. Meet., p20, (1991).
7. Morris, G.K., and Feeley, J.C.: Abstr. ASHRAE
Annual Meeting, p76, (1990).
8. NSW Health Department: Code of Practice for the
Control of Legionnaires' Disease, New South Wales Health Department, p35, (1991).
9. Pearce, M.: "Legionellosis in Queensland - A
Review" Communicable Diseases Intelligence, Vol.16, No.14, p296. (1992).
10. Standards Australia: HB32-1992 "Control of microbial growth in
air-handling and water systems in buildings" (1992).
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