Drinking water quality reliant on murky estimate

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 – about 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 were 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 diarrhoea, 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.

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.

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

A wide-scale 1999 United Kingdom 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 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