Hawke's Bay gastro disaster – what the experts say

Campylobacter – it can cause a nasty bacterial infection usually associated with chicken and food poisoning.

But for up to 2,000 Hawke’s Bay residents currently ill with the side effects of the infection (think nausea, vomiting and diarrhea) didn’t get it from under-cooked chicken, but from Havelock North’s town drinking water supply.

Prof Michael Baker

On Thursday, a sample of bore water tested positive for campylobacter, but officials suspected it was contamination from the water tanker itself, not the bore. It didn’t go public until the following evening when it advised residents to boil water to kill the bacteria. By then, the damage was done, thousands had consumed the water and the bacteria.

It is unclear exactly how the bore water was infected, but groundwater has been a source of campylobacter in the past.

In this Expert Q&A with Michael Baker, Professor of Public Health, Department of Public Health, University of Otago, we look at waterborne outbreaks of the disease, which has hospitalised 20 people, with two in critical condition.

1: What are the main causes of campylobacter contamination in groundwater?

“Infectious disease outbreaks linked to drinking water can occur in a number of ways. By far the most common contributing factors in New Zealand are use of untreated or inadequately treated water supplies.

“There can also be failures of treatment processes, source water that is more heavily contaminated than usual (eg following a flood) and contamination after treatment. The contributing factors operating in Havelock North will obviously need further investigation.”

2: Are there many communities that have untreated drinking water that might be vulnerable to such contamination?

“While most New Zealanders have drinking water supplies that are considered safe, some communities have untreated drinking water that is potentially vulnerable to contamination. About 15% of the population is not covered by registered community supplies so are particularly vulnerable.

“These are mainly small supplies covering just a few homes, schools or workplaces. Amongst the registered drinking water supplies, most (97%) comply with bacteriological standards (relevant for Campylobacter) but compliance is lower for protozoal standards (relevant for Giardia and Cryptosporidium which are the commonest causes of waterborne outbreaks).”

3: Is it possible to protect our drinking sources from contamination without treatment? Or for best public health outcomes should all drinking water be treated?

“Some water supplies are of very high quality, particularly ground water, so do not need routine treatment.”

4: Is there an overall trend of groundwater quality in NZ? Are such contaminations common, or becoming more/less so?

“There are some long-term data on waterborne outbreaks in NZ. There were 42 such outbreaks reported to ESR in 2014 including 131 identified cases. The most commonly identified pathogens were Giardia and Cryptosporidium followed by Campylobacter which accounted for almost 10% (4/42).

“Over the last 5 years in New Zealand (2010-14) there were typically about 50 reported waterborne outbreaks a year. Around 8 a year were caused by Campylobacter. Most were small, with an average of around 8 cases each.”

5: Other comments

“Waterborne outbreaks of this size are fortunately rare. It is vital that this outbreak is fully investigated to establish its source and contributing factors.

“This kind of event also shows the importance of having high quality surveillance systems to detect such outbreaks as early as possible so they can be stopped. Because Campylobacter infection has quite a long incubation period (up to 10 days) the number of cases caused by this outbreak may continue to increase for a few days.

“There may also be additional transmission in households, so infected people need to be careful not to pass the infection on to others.

“Campylobacter infection remains the highest impact food and waterborne disease in New Zealand. The major source is fresh raw chicken meat (60-90% of which is contaminated by Campylobacter).”

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