Water quality’s murky number of sick

How many people are made sick by their drinking water? The question is harder to answer than you might think.

The future of the country’s water infrastructure is the topic of heated public debate.

The Government has pushed for structural change. One of its core arguments is that our drinking water makes people sick – around 34,000 people a year, in fact.

Where did this number come from?

It originates from a preliminary Ministry of Health study published in 2007, which was part of a wider investigation into the burden of waterborne illness in New Zealand. It estimated 18,000 to 34,000 cases of sickness a year caused by drinking water.

The data it used is uncertain, though, and an accurate answer would be nearly impossible to find.

Gastrointestinal disease

(GID) is commonly caused by bacteria and other bugs (known as pathogens) consumed through food and water. Drinking water, particularly if it is untreated, can contain pathogens that make us sick. Common symptoms are diarrhea, stomach cramps and vomiting.

Contaminated drinking water can cause serious illness. In 2016, thousands of people in Havelock North got sick with campylobacter and dozens went to hospital after the town’s water supply was contaminated.

But most people with GID do not go to the doctor. Even if they do, it is difficult to link their illness to a specific cause, such as drinking water.

To arrive at the estimate of 34,000, the author of the ministry study connected several dots.

Awide-scale 1999 UK study had estimated the ratio of reported and unreported intestinal disease cases.

For campylobacteriosis – the most commonly reported gastro illness in New Zealand – the UK authors estimated 7.6 unreported cases for every reported case there.

Including figures for cases caused by other pathogens – as well as cases with no clear cause – the ministry study came to a figure of 823,000 cases a year when applying the same rates to New Zealand.

The author acknowledged in the report that this figure is based on many significant assumptions.

This is not enough information to answer the question, though. We want to know how many of those were caused specifically by drinking water.

The author turned to EpiSurv, which collects data on cases of notified diseases in New Zealand. Drinking water was listed as a risk factor in 37% of GID cases, and was deemed the most likely or only risk factor in 4.1% of cases.

This is a very wide range, and the author did not think it could be relied on. Risk factor information is incomplete to this day.

The author used the bottom estimate of 4.1% – likely to be conservative – and applied it to the estimated total of 823,000 GID cases. The result was around 34,000 (accounting for population growth since then, the figure today would be closer to 44,000 cases).

The lower estimate of 18,000 came from a different method, which used less reliable data and only considered campylobacteriosis, meaning the real number would likely be higher.

In 2010, there was another attempt to quantify the number of people made sick from drinking water. It came to a much higher figure – around 93,000 cases per year, with a range between 51,000 and 204,000.

The report for the

Department of Internal Affairs, produced by global consultancy LEG, sought to calculate the economic cost of drinking water that did not meet national standards.

It drew on both the 2007 report and overseas studies.

It used a higher estimate for GID cases with drinking water as a risk factor – around 27% for campylobacteriosis, hence the larger figures.

No major assessment of this question has happened since.

This range of uncertainty is significant, and illustrates the difficulty in attaching concrete figures to complex health problems.

Without more complete data, the number of people made sick from drinking water will remain an extremely broad estimate.

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This story was reviewed by Dr Belinda Cridge (Technical Lead, Drinking Water at ESR).