Health Stream Literature Summary - Issue 48 - December 2007

Gastroenteritis associated with accidental contamination of drinking water with partially treated water.

Fernandes, T.M., Schout, C., De Roda Husman, A.M., Eilander, A., Vennema, H. and van Duynhoven, Y.T. (2007) Epidemiology & Infection, 135 (5); 818-26.

In the Netherlands there are several newly built residential areas that have a dual water distribution system where partially treated surface water (known as grey water) is supplied for household purposes such as toilet flushing, washing machines and garden taps. This paper describes an outbreak of gastrointestinal illness that occurred in a new housing estate in the central part of The Netherlands, where 30,000 households were served a by dual water system. On the 3 rd of December 2001 , two people living in this estate complained about an unusual odour and taste of the tap water. Tap water samples taken of the 4 th of December revealed an abnormal count of total coliform bacteria. Previously on the 29 of November 2001, the drinking water system had been connected to the grey water system in order to flush and clean it after maintenance work. This cross-connection was not removed when the grey water system was operational again and accidental higher pressure in the grey water system caused grey water to circulate into the drinking water pipes. One general practitioner in the affected area on the 6 th of December informed the local public health service of an excessive number of patients with nausea, vomiting and diarrhoea attending his practice over the previous days.  

A retrospective cohort study was commenced on the 20 th of December 2001 where the affected area (area A) was compared to a reference area (area B) in terms of the incidence of households reporting gastrointestinal complaints. The study population included 938 possibly exposed households from area A and 1613 non-exposed households from the adjacent non-exposed area B. Area B also had a dual water system and was similar to the exposed population in regard to socioeconomic status, age distribution and time of residence in the area. In both areas questionnaires collected information on: number of household members; clinical symptoms reported in the household including: diarrhoea, vomiting, nausea, abdominal pain, abdominal cramps, blood in stool, fever, headache, muscle pain, cold chills, itching and coughing and/or sneezing; for each symptom, date of onset in the first ill individual of the household; having one or more of the above symptoms after 9 December; consultation with GP and absence from work or school. Questionnaires also collected information on regular daily water consumption for each individual in the household. In the exposed area another questionnaire assessed compliance with the water boiling advice which was given by the water company on 5 and 6 December.  

There were two hundred random households from area A and B who were asked to send in a stool sample, preferably from one person in the household that had symptoms of GE recently. All samples were tested for norovirus (NoV), Giardia lamblia and Cryptosporidium parvum . Environmental investigations where preformed to compare pathogens in stools and water. Samples of drinking water leaving the treatment plant were tested as part of routine analyses and included testing for E. coli and coliform bacteria, faecal streptococci and spores of Clostridium perfringens . On the 20 th of December a 1000 L sample of grey water was tested for NoV.  

The incidence of general practice consultations of GE in area A was compared with the incidence of consultations in two control areas (B and C) of the same housing estate during 29 November to 9 December. Two local health centres were selected: one situated in area B that receives mainly patents from areas A and B and the other situated in area C (a more distant area in the same new housing estate, also with dual water system but no exposed to contamination), that mainly receives patients from area C. The incidence of consultations for GE by day and by area of residence was calculated and compared between the three areas.  

Of the initial households there were 921 exposed and 1529 non-exposed included in the study. In area A, 223 (54.1%) case-households occurred compared with 117 (24.1%) in area B (RR 2.3, 95% CI 1.9-2.7). The daily incidence of case-households increased during 29 November to 9 December in both areas A and B. All of the symptoms of GE were reported at least twice as often in area A than in area B. In area A, the proportion of case-households increased with the average daily amount of water consumed per individual in the household with a clear does-response relationship found. A similar trend was also found in area B. There were 31 households from area A that returned stool samples and completed questionnaires and 33 from area B. In area A one sample was positive for NoV genogroup 1, genotype Birmingham and one was positive for G. lamblia both samples were from case households. In area B, one sample was positive for NoV genogroup 11 which was collected from a household that reported GE symptoms after 9 December 2001. The drinking water samples taken in area A yielded isolates of coliform bacteria and faecal streptococci. The E. coli strains isolated were non-pathogenic. The grey water samples were positive for NoV RNA genogroup 1 at a concentration of 1600 RNA-containing particles per litre.  

The general practice study included 1866 inhabitants of area A, 2875 inhabitants of area B and 5788 inhabitants of area C. During the study period, 37 individuals were diagnosed with GE in area A (19.8 cases/1000 inhabitants) compared with 20 (7.0 cases/100 inhabitants) in area B (RR AB 2.8, 95% CI 1.7-4.9) and 19 (3.3 cases/1000 inhabitants) in area C (RR AC 6.0, 95% CI 3.5-10.5).

The cohort and the general practice study both suggest an outbreak of GE complaints and GP consultations for GE in the area exposed to contaminated drinking water with a clear dose-response relationship with drinking tap water. The results suggest that consumption of drinking water contaminated with grey water increased the risk of acquiring GE. The microbiological evidence was inconclusive however circumstantial evidence suggests NoV may have been responsible for the outbreak with other pathogens involved to a less degree. An outbreak of GE as well as a clear dose-relationship with water also was found to occur in the adjacent reference area B although drinking water samples taken in this area showed no evidence of contamination. The authors suggest that this may be explained by some Area B residents visiting or working in Area A, although reporting bias in response to media publicity may also have played a role.  

In 2003 the Dutch environmental authorities banned the use of grey water (with the exception of rainwater use for flushing toilets) based on extensive environmental studies and risk assessments. Further studies are being conducted to assess the possible health risk associated with using rainwater.  

Comment This outbreak was previously reported in Health Stream Issue 30 p5. The use of the term “grey water” to describe partially treated but undisinfected surface water in this paper differs from its normal usage in Australia where grey water means wastewater from indoor uses except for toilet waste.