IJE Advance Access originally published online on March 27, 2007
International Journal of Epidemiology 2007 36(4):873-880; doi:10.1093/ije/dym029
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Breaks and maintenance work in the water distribution systems and gastrointestinal illness: a cohort study
1Division of Infectious Disease Control, Norwegian Institute of Public Health, Oslo, Norway.
2Norwegian Food Safety Authority, District office of Trondheim, Trondheim, Norway.
3Division of Environmental Medicine, Norwegian Institute of Public Health, Oslo, Norway.
4Trondheim City Waterworks, Trondheim, Norway.
5Norwegian Water and Wastewater BA (NORVAR BA).
6Division of Epidemiology, Norwegian Institute of Public Health, Oslo, Norway.
* Corresponding author. Norwegian Institute of Public Health, P.O Box 4404, Nydalen N-0403, Oslo, Norway. E-mail: kany{at}fhi.no
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Abstract |
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Background During maintenance work or breaks on the water distribution system, water pressure occasionally will be reduced. This may lead to intrusion of polluted water—either at the place of repair or through cracks or leaks elsewhere in the distribution system. The objective of this study was to assess whether breaks or maintenance work in the water distribution system with presumed loss of water pressure was associated with an increased risk of gastrointestinal illness among recipients.
Methods We conducted a cohort study among recipients of water from seven waterworks in Norway during 2003–04. One week after an episode of mains breaks or maintenance work on the water distribution system, the exposed and unexposed households were interviewed about gastrointestinal illness in the week following the episode.
Results During the 1-week period after the episode, 12.7% of the exposed households reported gastrointestinal illness in the household, compared with 8.0% in the unexposed households [risk ratio (RR) 1.58, 95% confidence interval (CI): 1.1, 2.3]. The risk was highest in households with higher average water consumption. The attributable fraction among the exposed households was 37% in the week following exposure.
Conclusion Our results show that breaks and maintenance work in the water distribution systems caused an increased risk of gastrointestinal illness among water recipients. Better data on the occurrence of low-pressure episodes and improved registration of mains breaks and maintenance work on the water distribution network are needed in order to assess the public health burden of contamination of drinking water within the distribution network.
Keywords Drinking water, gastrointestinal illness, waterborne, water pressure, water distribution
Accepted 7 February 2007
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Introduction |
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For the last decades, the main emphasis on preventing waterborne illness in industrialized countries has been on upgrading water treatment plants, improving source water protection and improving regulations of the water supply. In this area, there has been a great progress in improving the quality of water leaving the treatment plant. Increasingly, concern has been raised on contamination occurring within the distribution system. This can occur through cross-connections, contaminated storage facilities, backflow and during low and negative pressure incidents. There are many causes of pressure transients, such as turning on and off a pump, opening and closing valves, power failures, flushing of the network, fire fighting and anything that causes a sudden change in demand. Mains breaks, maintenance work and repair can cause loss of water pressure lasting for hours. Studies performed in the United States have shown that low-pressure incidents in otherwise satisfactory water distribution pipes can cause aspiration of microorganisms from the surrounding soil.1
In recent years, a substantial proportion of waterborne outbreaks have been attributed to failures in the distribution system. Distribution system deficiencies accounted for 36% (17/47) of waterborne outbreaks in community water systems reported in the United Staes during 1991–982 and this increased to 50% (9/18) during 1999–2002.3,4 Since these outbreaks often affect a smaller proportion of the population, they may be more difficult to detect. Fewer outbreaks caused by source water contamination or failure in disinfection may also have contributed to the relative increase.
To our knowledge, there have been no studies conducted on the association between breaks or maintenance work in the water distribution system and incidence of gastrointestinal illness in the community. The primary objective of the present study was to assess the association between mains breaks or maintenance work in the water distribution system with presumed pressure loss and gastrointestinal illness among recipients the following week. The secondary objective was to investigate if some factors related to the episodes, such as weather conditions or measures to prevent contamination, were associated with an increased or decreased risk in the affected households.
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Methods |
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We conducted a cohort-study among recipients of water from seven larger waterworks in urban areas in Norway during a 1-year period starting on September 15, 2003. The waterworks each serve between 35 000 and 460 000 people, with a total of around 1 100 000 people.
Power
With a sample of 600 exposed and 600 unexposed households and estimated frequency of households with gastrointestinal illness of 4% among unexposed households during a 1-week period, we estimated that the study had a power of 80% to detect a risk ratio (RR) of 2 given a two-sided alpha-level of 0.05.
Selection of episodes
Each waterwork was asked to identify up to two low-pressure episodes per month in the 1-year period. A low-pressure episode was defined as an incident where a part of the water distribution network was closed off due to mains breaks or maintenance work with presumed loss of water pressure in the distribution system. The episodes were either planned, i.e. related to routine maintenance work, or unplanned, i.e. caused by spontaneous pipe-breakage or accidents during construction work. The waterworks were asked to select the first planned and the first unplanned episode occurring each month that affected at least 10 households. For each episode, the following information was registered: time and place, climatic conditions, reason for the low-pressure episode, measures taken to prevent contamination, location of sewage pipe in relation to water pipeline and the water work personnel's own evaluation of the risk of contamination.
Selection of households
For each episode, the waterworks selected 10 exposed households at random from the customer register among all households affected by the low-pressure episode. Ten unexposed households were selected at random from the customer register among unaffected households in the same area as the exposed household. In a letter to all selected households, we informed them about the study and that they would be contacted by telephone and asked to participate in the study. The same information-letter and questionnaire were used both for the exposed and the unexposed households so as not to reveal the household's exposure status.
Interviewers who were unaware of the households’ exposure status interviewed one person (>16 years) in both the exposed and unexposed households 8–14 days after the episode using a standard questionnaire and an interview guide. The households were informed about the interview by letter at the time of the episode, so the interviewee could prepare to answer the questions on behalf of all household members. The following information was collected: age and gender of all household members, average tap water-intake at home per person in the household, any travel abroad within the last month, children in day-care centre, employment in kindergarten, pets in the household or other regular animal contact. In addition, they were asked if they had noticed any discolouration or strange taste of the tap water within the last 14 days, or if they thought there had been any work done on the water pipes recently.
The person interviewed was also asked if there had been any episodes of acute gastrointestinal illness in the household during the week after a certain date that corresponded to the low-pressure episode for exposed households (‘the observation period’). An episode of gastrointestinal illness was defined as an episode of vomiting and/or diarrhoea with at least three loose stools during a 24-h time-period. Information about age, gender and symptoms of acute gastrointestinal illness of all household members was collected at the individual level.
Ethics, data handling and analysis
The study was reviewed and approved by the Regional Committee for Medical Research Ethics.
We entered and analysed data with Microsoft Office Excel (Microsoft Corporation) and STATA 8.0 (Stata Corporation, College Station, TX, USA).
The main analysis was conducted at the household level. A case household was defined as a household with at least one person with an episode of gastrointestinal illness during the observation period. We estimated the attack rate of gastrointestinal illness among exposed and unexposed households, respectively, the RR and the risk difference with 95% confidence intervals (CI). The attributable proportion among the exposed households was computed according to method described by Rothman,5 Stratified analyses with calculation of Mantel–Haenszel adjusted RRs were performed in order to assess possible confounders. Interaction was assessed by the likelihood-ratio test between logistic models with and without the interaction term.
To include the effect of household clustering and possibly secondary transmission within households, a second analysis was conducted on the individual level where we calculated attack rates stratified by age and gender among exposed and unexposed household members. To account for the effect of household clustering we used the xtlogit procedure in STATA with household as the panel ID variable.
We assessed possible effect modifiers in a separate logistic regression model in the exposed group of households only. Variables with P-value <0.2 were evaluated in the model. The final model retained all variables with P-value <0.1.
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Results |
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Description of study material
A total of 88 low-pressure episodes were included in the study, varying from 2 to 24 per waterworks. The main reasons for not including more episodes were lack of interviewing capacity or lack of episodes. Mains breaks or leaks were the most commonly reported causes of the registered episodes, accounting for 63% (55/88). Change of equipment (valves, pipes, etc.) accounted for 26% (23/88) and other causes such as cleaning of pipes, construction work close to the water pipes, defective valves, etc. accounted for the last 11% (10/88). Fifty of the episodes were not planned, of which 48 were caused by leaks or mains breaks. The water was shut off for an average of 6.6 h per episode (median 5 h, range 1–33.5 h). In almost half (47%), the water shut-off was limited to ordinary working hours (0800–1600).
Only one waterworks chlorinated the affected section of the pipe after work/repair and this was done in 12 of the 14 episodes registered by this waterworks. Flushing was done in 77 (87%) of the episodes. Boiling advice to the recipients was not given in any of the reported episodes. Water samples were obtained in only 18 of 62 episodes where this information was given (29%) and only one sample was positive for Escherichia coli.
The total number of affected households in the 88 episodes was 5935, with an average of 67 households per episode (Table 1).
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A total of 616 exposed and 549 unexposed households were interviewed in the 88 episodes, thereby giving a response rate of 70% (616/880) and 62% (549/880), respectively. The main reasons for dropout were inability to reach the households by telephone (37%), that they had moved or that the phone number could not be obtained (21%), or that they declined to participate in the study (20%). For 15%, the reason for non-participation was not given. Four exposed and two unexposed households were excluded because they could not time their gastrointestinal illness in relation to the episode.
The exposed and unexposed households were similar with respect to known risk factors investigated (Table 2).
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Analysis of gastrointestinal illness
During the observation period after the registered episode, 12.7% of the exposed households reported gastrointestinal illness in the household, compared with 8.0% in the unexposed households (Table 3). The RR was 1.58 (95% CI: 1.1, 2.3) and the risk difference 4.7% (95% CI: 1.2, 8.2). The attributable fraction among the exposed households was 37%. Stratified analysis for foreign travel or employee in kindergarten did not change the crude estimate (Table 3). The RR calculated for each waterworks varied between 1.3 and 2.2 for five of the waterworks. For the waterworks that routinely chlorinated, the RR was 1.1 and for the last waterwork, only two episodes were included, giving a very imprecise estimate (RR = 0.9, 95% CI: 0.1, 5.3).
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Three quarters of exposed households believed there had been work/repair on water pipes vs 25% among unexposed households (P < 0.001). To assess information bias regarding non-blinding of the exposure, we conducted stratified analysis on whether the households thought there had been work/repair done on the water distribution system; whether they noticed discolouration or bad taste of the tap water. The Mantel–Haenszel adjusted RRs were 1.38, 1.37 and 1.54, respectively (Table 3).
The proportion of households reporting gastrointestinal illness was highest in the winter months (10.9%; 41/375) and lowest in the summer months (9.1%; 13/143). Stratified analysis by season did not change the crude estimate of the RR (Table 3) and there was no strong interaction (likelihood-ratio test of interaction P = 0.82)
In the exposed households, a higher average daily water consumption (>1 glass water per person per day) was strongly associated with gastrointestinal illness compared with a lower average daily water consumption (
1 glass water per person per day) (RR = 4.9, 95% CI: 1.6, 15.2). In the unexposed households, the amount of water consumed was not strongly associated with gastrointestinal illness (RR = 1.1, 95% CI: 0.5, 2.4) (Table 4). The analysis for interaction showed a strong positive interaction between exposure and amount of water consumed in the household (P = 0.029).
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The interviewed households included a total of 3020 household members. The attack rate of gastrointestinal illness during the observation period after break/maintenance work of the water mains was 7.5% and 3.9% in the exposed and unexposed households, respectively, giving an odds ratio of 2.0. The highest attack rate was in the youngest children (0–5 years) in both the exposed and unexposed households; however, the highest RR was observed in adults 20–39 years, where the attack rate was 10.2% and 1.8% among exposed and unexposed, respectively (Table 5).
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Clinical symptoms and medical care was similar in the exposed and unexposed households (Table 6). The median duration of illness was 2 days. Twenty-three per cent had to stay away from work or school, with a median of 2 days absent.
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Factors influencing the risk of illness associated with pipe breaks/maintenance work
The following factors seemed to increase the risk in exposed households: cleaning pipes by swabbing, rain during break or maintenance work and longer duration of water shut-off (Table 7). Flushing the water pipes and use of chlorination indicated a decreased risk. Only flushing the pipes, use of chlorination and duration of water shut-off had a P-value <0.1 in the multivariate logistic regression model.
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The water work personnel conducting the work on the water mains were asked to make an evaluation of the risk of contaminated water reaching the consumer and classify into low, medium and high risk. None of the episodes were classified as high-risk and only seven were classified as medium risk (8.8%). The episodes classified as medium risk were associated with a higher risk of illness than episodes classified as low risk (RR = 1.8; 95% CI: 1.0, 3.2).
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionConclusionAcknowledgementsReferences |
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We have found an increased risk of acute gastrointestinal illness in households affected by work on the water distribution network with presumed pressure loss. The risk of experiencing gastrointestinal illness was almost twice as high for persons living in an exposed household as compared with persons in unexposed households. In none of the registered episodes did the waterworks personnel consider that there had been a high risk of contaminated water reaching the consumers and no boiling advices were given.
It has been suggested that a substantial proportion of endemic acute gastrointestinal illnesses may be attributed to problems within the distribution system rendering the water unsafe when it reaches the customers’ taps. Several investigators have studied the effect of drinking tap water vs drinking bottled water or water treated by in-house water treatment units on the incidence of gastroenteritis. An intervention trial by Payment et al.6 in Canada suggested that 14–40% of gastrointestinal illness were attributable to tap water meeting current standards and that the distribution system appeared to be partly responsible for this increased risk. Modelling of the distribution system showed that it was very prone to negative pressures.7 A similar blinded intervention trial in the US8,9 did not reveal any differences in risk of gastrointestinal illness between households that received the intervention and the control households. However, the study was limited to only one waterwork, which was rated among the best 2% in the country and no negative pressure events occurred during the study period.1 A similar randomized double-blinded trial was conducted in Melbourne, with similar results of no effect of treatment of tap water.10
In a case-control study of sporadic cryptosporidiosis in the UK, risk-factors for diarrhoea in the control group were investigated.11 The researchers found a strong association between self-reported diarrhoea and low water pressure at the faucet. However, the study was relatively small and due to study design they were unable to confirm that the loss of pressure events preceded the diarrhoea.
An ecological study of environmental risk factors and campylobacteriosis in Sweden showed an increasingly higher risk of infection associated with longer average length of the water distribution system. The authors suggested that this could be caused by intrusion of contaminants in the distribution network,12 emphasizing the problem with contamination in the water distribution system.
The discrepancies between the results from studies investigating risk of illness caused by contamination in drinking water distribution systems are not surprising, since a variety of factors may influence the results. Differences in study design, especially regarding blinding of the participants, could lead to a placebo effect, thereby giving a higher relative risk in the non-blinded studies. However, differences in the quality of the water supplies and distribution systems in the study areas are also likely to influence the study results, including the technical condition of the pipeline system, amount of leakage, the presence of pathogens in the surroundings of the water pipes and the occurrence of pressure transients in the distribution systems.
During episodes of maintenance work or repair of breaks, there are several possible modes of external contaminants reaching the interior of the water pipes. During normal operation, the water in the distribution network is subject to overpressure. This prevents intrusion of external contaminants through leaks or cracks. In Norway, 20–50% of the water is lost through leakage in the distribution system,13,14 and therefore it can be anticipated that there is a high potential for water intrusion when the pressure is reduced or even reversed. When the water is closed off in order to conduct work on the distribution system, a negative pressure may occur in parts of the network, especially parts located on a higher level and this may lead to intrusion of water surrounding the pipe.
In a study in the US investigating the presence of microbial contaminants in soil and water samples collected immediately adjacent to drinking water pipelines, faecal coliform bacteria were detected in 43% of the water samples and 50% of the soil samples indicating the presence of faecal contamination.15 The same study found 56% of the samples positive for viruses; predominantly enteroviruses, but also norovirus and hepatitis A virus were detected, providing clear evidence of human faecal contamination immediately exterior to the pipe. Also in Norway, sewer lines are often located in the same ditch as water pipelines and similar microbiological findings as in the US study may be expected.
To reduce the risk of intrusion, sewer pipes should be located below the water pipes; however, in saturated soil conditions, it has been shown that microbes can move several meters in short periods of time.16
An episode causing loss of water pressure in the water pipe may thus lead to intrusion of pathogens present in the surroundings of the pipes, possibly caused by leakage from sewer pipes located nearby. In our study, the clinical symptoms of gastrointestinal illness were generally mild and were similar in the exposed and unexposed households. This can be explained by an intrusion event causing pathogens from leaking sewer pipes entering the water pipes. The kinds of gastrointestinal illnesses caused by contamination during breaks or maintenance work in the water distribution system in the exposed group would therefore reflect the gastrointestinal infections that are endemic in the nearby population.
In the exposed cohort, a higher average water intake in the household increased the risk 4-fold compared with households with a lower water intake, supporting the main results. While the highest rate of gastrointestinal illness was observed in children as expected, the highest relative risk of illness was observed among young adults—the age-group that generally consumes most water.17 Exposure data were not collected at the individual level and therefore it was not possible to evaluate the effect on individual consumption.
Flushing of the pipelines and use of chlorination after an episode seemed to reduce the risk of illness. General professional guidelines in Norway for water pipeline operations recommend chlorination of water pipelines after maintenance operation, loss of water pressure or both and prior to repressurizing, in order to protect pipeline water from contamination.18 However, the recommendations are specified to incidents where it is considered to be a risk that contamination has occurred and thus it is often not done. Only one of the seven participating waterworks chlorinated routinely in all episodes of work on the water distribution pipeline. The procedure involved adding calciumhypochlorite [Ca(OCl)2] to the pipe segment when refilling it with water, disinfection for two hours followed by flushing. The amount of calciumhypochlorite in grams was equivalent to the diameter of the pipe in millimetres, in accordance with the guidelines from Norwegian Institute of Public Health. The whole process of chlorination takes a few hours, and the main reason for omitting this step is to minimize the duration of affected households being deprived of pipeline water. However, since the study was not designed to investigate the effectiveness of these measures specifically, the results need to be interpreted with care.
In order to make evidence-based recommendations, further studies are needed to investigate which protective measures the waterworks should implement to reduce the risk of illness most effectively. A ‘boil water’ notice to the general public on short notice is considered to be ineffective, but may be appropriate to people at special risk, for instance in hospitals or other institutions.
To be able to estimate the disease burden that can be attributed to loss of water pressure associated with mains breaks or maintenance work in the water distribution network, we would need to know the prevalence of exposure. Although there are some figures on registered breaks or scheduled maintenance operations, these are often not complete. In addition, unnoticed pressure transients may occur also during normal distribution system operations, that can lead to intrusion of contaminated water into the water pipe.1 If we anticipate 20% of the 4.5 million Norwegian inhabitants exposed to one low-pressure episode every year, with an absolute risk difference at the individual level of 7.5% – 3.9% = 3.6% (Table 5), this would cause an estimated 33 000 cases of acute gastrointestinal illnesses. However, if we anticipate that pressure transients in the distribution system to be a more common occurrence, causing frequent, smaller intrusion contamination episodes, the estimated disease burden could be large.
Limitations
Some caution is needed in interpreting the results. Our study was based on data from large waterworks supplying mainly urban areas. The results may therefore not be generalizable to smaller waterworks in rural areas, where longer distribution pipelines may be more prone to leaks and pressure transients. As was shown by Kirmeyer et al.1, the distribution system studied by Payment et al. 6,19 was very prone to negative pressures. Analysis of the data also showed that the people living far from the treatment plant had the highest risk of gastroenteritis.1
Recreational water exposure is another important risk factor for waterborne disease. During summer, use of private small plastic pools filled with tap water can pose another risk for exposure of contaminated water after low-pressure episodes. This exposure was not assessed in the present study, but may explain some of the higher risk in the exposed households.
For some pathogens, such as Giardia and Cryptosporidium, the incubation period may be longer than one week. Since we used only 1-week follow-up period, we would not include the effect of contamination with pathogens with longer incubation period. This may have reduced our calculated risk estimates to some degree. However, the endemic level of these pathogens in Norway is considered to be low.
Although we tried to accomplish blinding of the participants regarding the exposure, this was not completely successful. This may have led to some recall bias among the participants and therefore may have influenced our results. However, when stratifying on whether the households believed they had been exposed, the adjusted RR was only slightly lower than the unadjusted, thereby indicating that this did not have a large influence on the results.
In our study, we included several medium to large-sized waterworks, from different parts of Norway. This gives a more representative picture of the risk, and makes the results more generalizable than studies involving only single waterworks. Even if the study was too small to provide a precise risk estimate for each waterwork separately, the estimates pointed in the same direction. The increased risk associated with higher average daily water intake also supports our conclusions that the association is causative.
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Conclusion |
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To our knowledge, this is the first study to assess the risk of gastrointestinal illness following breaks or maintenance work on the water distribution system and our results indicate an increased risk of acute gastrointestinal illness in affected households. The risk was highest in households with higher average water consumption. The clinical symptoms were generally mild and of short duration. Intrusion of polluted water related to loss of water pipeline pressure has been suggested as a potential risk to public health, but has to our knowledge not been directly addressed in analytical epidemiological studies. Results and conclusions from our study support the hypothesis of such an association.
This study needs to be followed up to establish effective preventive measures in order to prevent illnesses associated with contamination in the distribution network. Better data on the occurrence of low-pressure episodes and improved registration of mains breaks and maintenance work on the water distribution network in urban and rural areas are needed in order to better assess the public health burden of contamination in the water distribution network.
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Acknowledgements |
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The authors gratefully acknowledge the contributions of the participating district Food Safety Authorities and the waterworks in Oslo, Skien, Kristiansand, Sandnes, Stavanger, Bergen and Trondheim without whom this study could not have been completed.
The project was organized by the Norwegian Water and Wastwater BA (NORVAR BA), which also was the main financer.
Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the Norwegian Food Safety Authority.
Conflict of interest: None declared.
KEY MESSAGES
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