Investigation and control of a cluster of cases of Legionnaires disease in western Sydney

This report published in Communicable Diseases Intelligence Volume 25, No 2, April 2001 contains information on three cases of Legionella pneumophila infection that were identified in Sydney's west in November 1998. Epidemiological investigations identified an association with one workplace.

Page last updated: 28 May 2001

A print friendly PDF version is available from this Communicable Diseases Intelligence issue's table of contents.


Julianne Brown,1,2 Krishna Hort,1 Ron Bouwman,1 Anthony Capon,1,3 Narinder Bansal,4 Ian Goldthorpe,1 Kerry Chant,5 Subramanyam Vemulpad1

Introduction | Methods | Results | Discussion | Acknowledgements | References

Abstract

Three cases of Legionella pneumophila infection were identified in Sydney's west in November 1998. Epidemiological investigations identified an association with one workplace. Environmental sampling revealed that the cooling towers in the workplace, and at 2 other premises within a 1 km radius of the workplace, were positive for L. pneumophila serogroup 1 (LP1) which was indistinguishable from clinical isolates of 2 of the cases on DNA fingerprinting. Our report highlights limitations of the current control program for Legionella in cooling towers, including the finding of unregistered cooling towers, cooling towers positive for LP1 despite satisfactory results on inspection, and cooling towers potentially linked to disease with counts of LP1 below the current protocol requirements for immediate decontamination. Commun Dis Intell 2001;25:63-66.

Introduction

Legionnaires' disease was first identified in 1976 after an outbreak occurred at an American Legion convention.1 Legionnaires' disease is generally caused by the environmental pathogen Legionella pneumophila serogroup 1 (LP1) or Legionella longbeachae.2 Outbreaks of LP1 have been associated with cooling towers,3-9 evaporative condensers,10 domestic hot water systems,11,12 and excavation and construction activity.13 On average one LP1 outbreak is reported annually in Australia.2

In western Sydney outbreaks of Legionnaires' disease occurred in 1992,14 1993,8 199415 and 1995.15 We report here on a cluster of cases of Legionnaires' disease associated with a cooling tower in a workplace in Sydney's west.

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Methods

Epidemiological investigation

Two cases of Legionnaires' disease16 were notified by laboratories to the Western Sector Public Health Unit on 12 November 1998. Both cases satisfied definitions of confirmed cases in the NSW Health Department Infectious Diseases Manual.16 When initial investigations revealed that the 2 cases were employed in the same workplace (Premises A) an outbreak was suspected and active surveillance was initiated and continued for a 10-day period. All emergency departments and intensive care units were contacted daily to ascertain whether there were any probable cases of Legionnaires' disease. Laboratories were also contacted daily to ensure test results were available as soon as possible. Additionally, we requested that the workplace alert us of staff members suffering prolonged respiratory illness.

All cases were interviewed by Public Health Unit staff using a standard questionnaire.17 Information on the case's movements, including potential exposures to Legionella, during the 10-day period prior to onset of illness was obtained for each case. Where the case was unconscious, proxies were interviewed and electronic work diaries and time sheets were used as sources of information. Clinical specimens were processed using standard microbiological techniques. Putative isolates were identified as L. pneumophila by growth, colony characteristics and serogrouping.

Environmental investigation

Environmental investigations commenced immediately on receipt of notifications, which was between 3 and 5 weeks after the likely exposure. Potential environmental sources of Legionella, such as cooling towers, at or near sites of exposure reported by cases, were identified from registers held by local government authorities.

These sources were then inspected and sampled in sequence, commencing at locations of common exposure to more than 1 case, and progressing to those where only 1 case reported potential exposure. At sites of common exposure, potential sources up to 500m from the site were investigated. When a third case was reported with exposure 800m from Premises A, the radius of investigation around Premises A was increased to 1 km. The Environmental Health Officers made efforts to identify any unregistered premises or cooling towers within that radius by visual inspection from suitable vantage points.

Molecular investigations

Environmental samples were tested for the presence of Legionella by the Legionella Reference Laboratory of the Division of Analytical Laboratories (DAL), Lidcombe, New South Wales. This testing involved the use of standard culture procedure, using heat treatment at 50C for 30 minutes and Glycine-Vancomycin-Polymixin D-Cycloheximide agar. Cultural confirmation involved the use of Buffer-charcoal-yeast extract and Tryptocase-soya broth agar, Gram staining and serological characterisation.

Molecular investigations were carried out by the Molecular Microbiology Laboratory of the DAL. Isolates were sent for molecular testing from cooling towers which tested positive for LP1 at premises where there was an epidemiological link to the cases. In situations where more than 1 cooling tower at the premises tested positive for LP1, samples from the cooling tower with highest count were examined. At least 2 different isolates from each cooling tower and all available clinical isolates were subjected to molecular testing. Isolates were characterised by two methods - randomly amplified polymorphic DNA18 and restriction fragment lengths polymorphism.19 The DNA fingerprint profiles were then visually compared.

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Results

Epidemiological investigation

Three cases with onset dates 6 days apart were notified to the Public Health Unit and characterised as confirmed cases (Table 1). Active surveillance identified a fourth case that met initial probable case definition criteria, but was subsequently excluded on the basis of negative Legionella antibody tests in acute and convalescent sera.

Table 1. Cases by date of onset, age, case criteria and risk factors

Case number
Date of onset
Age (years)
Case criteria
Risk factor
Case 1 23/10/98 44 Urine positive for Legionella pneumophila antigen smoking
Case 2 25/10/98 60 L. pneumophila serogroup 1 isolated smoking
Case 3 29/10/98 66 L. pneumophila serogroup 1 isolated smoking


All cases survived. One case developed acute renal failure. In-depth interviews (via proxy in Cases 1 and 2 ) revealed 3 epidemiological links between the cases. These were as follows: Cases 1 and 3 were employed at the same premises, (Premises A); Case 2 lived 800m south of Premises A; and Cases 1 and 3 attended a day long seminar in the same premises during their incubation periods, which was 6km north of Premises A.

Environmental investigation

Intensive investigation identified 8 premises with 30 cooling towers within 1 km of Premises A, including 3 premises with a total of 6 cooling towers which had not been registered with the local government authority. Four cooling towers in premises within 500m of Premises A were found to be positive for LP1; 2 of these towers were in Premises A, and the remaining 2 were at Premises B located within 100m of Premises A. All of these cooling towers had been registered and were satisfactory on inspection (Table 2).

Table 2. Summary of Legionella pneumophila 1 (LP1) counts by the number of premises and cooling towers in each distance range

Distance from Premises 1 (metres)
Number of premises Number of registered premises Number of premises LP1 positive Number of cooling towers Number of registered cooling towers Inspection satisfactory Number of cooling towers LP1 positive
< 500
5
4
2
21
20
19
4
500-1000
3
1
2
9
4
5
5
1000 - 2000 *
6
6
0
10
10
8
0
> 2000 *
11
11
3
32
32
32
4
Total
25
22
7
72
66
64
13

* Only premises linked to case exposures were investigated at these ranges

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A further 5 cooling towers in 2 premises and D) between 500 and 1000m of Premises A were found to be positive for LP1. Four of these towers were at one of the unregistered premises (Premises D) and were unsatisfactory on inspection. The remaining towers, including those at Premises C and the other unregistered premises, were satisfactory on inspection. (Table 2)

Outside this range, only cooling towers at sites reported as visited by cases were inspected and sampled. LP1 was isolated from a further 4 cooling towers at 3 premises at sites from 2 km to 20 km from Premises A; this did not include the premises attended by 2 of the cases for a seminar.

Overall 72 cooling towers at 25 premises were inspected and sampled during the investigation, and 13 cooling towers were found to be positive for LP1.

Following inspection and sampling, immediate decontamination was undertaken for all cooling towers in Premises A and B between 13 and 15 November, which was 2 and a half weeks after the onset of the last case. At other premises, action was taken in accordance with the New South Wales Code of Practice for the control of Legionnaires' disease.20 All cooling towers that were positive for L. pneumophila were re-tested following decontamination.

Molecular typing was undertaken on LP1 isolates from 4 cooling towers from the 4 LP1 positive premises within 1km of Premises A and compared with clinical isolates obtained from 2 patients (Cases 2 and 3). The DNA profiles of the 2 clinical isolates were identical and matched at least 1 isolate from 3 of the cooling towers (at Premises A, B and C, Table 3).

Table 3. Cooling towers where LP1 was isolated and results of molecular typing

Location
Cooling tower* On register Inspection satisfactory Legionella count† Molecular typing done DNA fingerprint match
Y/N Y/N LP 1 Other
< 500m
A.1
Y
Y
330
70
Y
Case 2, 3
A.2
Y
Y
40
0
N
 
B.1
Y
Y
130
50
Y
Case 2,3
B.2
Y
Y
40
0
N
 
500 - 1000m
C.1
Y
Y
370
0
Y
Case 2,3
D.1
N
N
40
30
Y
None
D.2
N
N
10
0
N
 
D.3
N
N
30
10
N
 
D.4
N
N
10
0
N
 
> 2000m
E
Y
Y
150
70
N
 
F.1
Y
Y
20
0
N
 
F.2
Y
Y
10
0
N
 
G
Y
Y
100
0
N
 

* Cooling tower notation as premises cooling tower number.
Colony forming units per millilitre (cfu/mL).


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Discussion

We identified 3 cases of Legionnaires' disease, which we viewed as a sentinel to initiate active surveillance and a comprehensive environmental investigation. The epidemiological links among the 3 cases were supported by the finding of indistinguishable DNA profiles in the 2 cases where clinical specimens were available.

Environmental investigation identified cooling towers positive for LP1 in 4 premises which were potentially linked to the cases. Molecular typing was unable to distinguish between LP1 isolates from cooling towers at 3 premises and clinical samples from 2 cases. However, the lack of information on the frequency with which this particular DNA profile of LP1 is found in cooling towers prevents a definite conclusion on the source of the cluster.

The environmental investigation identified that 50 per cent (4/8) of the premises and 30 per cent of the cooling towers (9/30) within 1 km of Premises A were positive for LP1 on sampling. This proportion is higher than that found in a survey of registered New South Wales clubs in 1997 (6% of 126 clubs tested were positive for Legionella spp)21 and higher than that found among the premises and cooling towers investigated which were distant from Premises A.

This could be due to the more intensive investigation within the 1 km radius of Premises A, which identified 3 premises and 6 cooling towers which had not been registered. Four of these cooling towers were positive for LP1 and were unsatisfactory on inspection. This suggests that action by cooling tower owners to register towers with their local councils may be associated with improved cooling tower maintenance and reduced detection of LP1. However, even if the unregistered towers are not included, the rate of isolation of LP1 in this area is high and other factors, such as the preponderance of industrial rather than commercial premises, and the time of the year, need to be considered. (Our Environmental Health Officers anecdotally report that production schedules in some industrial premises have created difficulties in adhering to regular maintenance regimes for cooling towers).

The New South Wales Code of Practice has a response protocol based on stratified counts of L. pneumophila and it recommends immediate shutdown and decontamination where counts of L. pneumophila in cooling tower water exceed 1000cfu/mL.20 For counts between 100 and 999 cfu/ mL a re-evaluation of maintenance procedures, including the current disinfection process is recommended.20 None of the premises within a 1 km radius of Premises A had counts over 370 cfu/mL. Although these samples were taken 2 to 3 weeks after the onset dates for the cases, these cooling towers were associated with disease.

The response protocol based on stratified counts of L. pneumophila first appeared in an internal Department of Housing and Construction report in 1987.22 The report suggested that the use of stratified Legionella counts should only be used as a guide and as an interim tool. The evidence for the stratified counts was based on the consensus of experts, primarily from the United States of America, rather than specific quantitative studies (personal communication, Mr Clive Broadbent (AM), Legionella Consultant, Canberra, 20 December 2000). These stratified counts were then adopted into the New South Wales Code of Practice in 1991, based on the 1987 internal Department of Housing and Construction report (personal communication, Mr Tony Burns, Senior Environmental Health Officer, Wagga Wagga, 21 December 2000). The findings of this investigation, and results of recent research in South Australia23 which indicate that health risks from cooling towers cannot be reliably based upon single or infrequent Legionella tests, suggest that a review of the response protocol is required.

Seventy percent (9/13) of L. pneumophila positive cooling towers were assessed as compliant with the Public Health Act 1991 at the time of inspection and sampling. The questionnaire includes a visual assessment of cleanliness, water turbidity, presence of slime, type of biocide used and recent cleaning history. We suggest that a broader questionnaire may be needed, as legislative compliance is a proxy for risk of Legionella contamination. The emphasis in the Code of Practice is on correct maintenance of cooling towers and occupiers are not required to sample cooling towers and test for Legionella.20 Our findings suggest that either the questionnaire does not reflect the true maintenance of cooling towers or current maintenance procedures do not ensure the elimination of Legionella from cooling towers.

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Acknowledgements

We gratefully acknowledge the contributions of Hannah Herrod, Oanh Nguyen and Roderick Reynolds from the Western Sector Public Health Unit, Stephen Crone from the South Western Sydney Public Health Unit; Professor Gwendolyn Gilbert and staff from the Institute of Clinical Pathology and Medical Research; Anna Smith from the Division of Analytical Laboratories; and Mahomed Patel and Mary Beers from the National Centre for Epidemiology and Population Health.

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Author affiliations

1. Western Sector Public Health Unit

2. National Centre for Epidemiology and Population Health, Australian National University

3. Department of Public Health and Community Medicine, University of Sydney

4. Division of Analytical Laboratories

5. South Western Sydney Public Health Unit

Corresponding author: Krishna Hort, Wentworth Area Population Health Unit, PO Box 63, Penrith NSW 2751. Telephone: +61 2 4734 3312. Facsimile: +61 2 4734 3300. E-mail:hortk@wahs.nsw.gov.au

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References

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This article was published in Communicable Diseases Intelligence Volume 25, No 2, April 2001.

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