One of the statutory responsibilities of the Medical Officer of Health is to investigate the source of notifiable infectious diseases. Another responsibility is to remove the conditions favourable to such infection. So, if drinking water might be the source of the infection, it must be investigated - if there is a contaminated drinking water source, it can then be managed appropriately. Alternatively, contaminated drinking water should be ruled out so that we can be sure that future users of the drinking water are not at risk. The investigation will take into account the epidemiology, the environment and the microbiology.
Distribution - the number of cases, whether O/E>1, demographic details of cases etc
Causes - exposures relevant to the organism. For drinking water, all the drinking water sources during the incubation period need to be considered - home, work school, pre-school, food premises, relatives' houses etc.
Environmental investigation usually requires consideration of source, pathway & receptor - where drinking water is concerned the receptor and pathway are obvious, but the source requires consideration of many questions including the following:
- Is the supply regulated?- if so the supply will be monitored and monitoring results will be helpful- though keep in mind that samples are only snapshots and may not represent the worst quality water to which the users may be exposed. As some pathogenic organisms have a small infectious dose, even minor intermittent contamination might cause infection.
- Is the source from groundwater? If so, is the borehole built according to Institute of Geologists of Ireland guidelines?
- See EPA Advice Note 14 on Borehole Construction and Wellhead Protection for Guidance on the assessment of the construction of existing drinking water supply boreholes
- What is the quality of the raw water? Contaminated raw water will need a robust treatment system that cannot be overwhelmed or have safety cut-outs to protect the users of the water.
- Is the treatment system always sufficient? Does it protect against the pathogen of concern?
- Is there evidence of a problem with the distribution system?
Risk assessment tools such as used by the Drinking Water Inspectorate in the UK may be helpful.
Microbiological results from human and environmental samples can be very helpful. However, for negative results, consider:
- How representative is the sample of the conditions to which cases may have been exposed?
- How sensitive and specific is the sampling?
- How sensitive and specific is the test?