Havelock North's disastrous gastro outbreak could have infected thousands more than first thought, a major new DNA analysis has found.
The researchers behind the just-published study say a "multi-barrier" strategy around drinking water is needed to stop the catastrophe happening again - with one expert drawing a parallel to today's Covid-19 crisis.
At the time of the August 2016 outbreak, more than 5000 people were reported to have fallen ill, including more than 40 who were hospitalised, and at least four who died partly as a result.
The outbreak cost about $21m - around half of which was borne by local households, who each lost an estimated $2440 from time off work and buying bottled water - and prompted public protests and a lengthy Government inquiry.
Its cause was eventually traced to heavy rain that sent water contaminated with sheep dung into a pond, before it entered an aquifer from which a bore drew it into the Hawke' Bay town's water supply.
Now scientists have revealed the true scale of the disaster – finding that campylobacteriosis case numbers could have been as high as 8320, with potentially more than 2000 of these living outside of Havelock North.
ESR scientist Dr Brent Gilpin, who worked with a team from Hawke's Bay District Health Board, the Eastern Institute of Technology and Otago and Massey universities, said that fuller picture was created using whole genome sequencing to link cases.
"New Zealand has been reporting around 5500 cases from this outbreak for some time but that was based upon a telephone survey of people living in Havelock North so didn't include any cases from the surrounding areas," Gilpin said.
"We always knew there were a significant number of illnesses among people from Hastings and the wider Hawke's Bay."
There were also at least 20 people from as far away as Auckland and Christchurch who visited Havelock North when the water was contaminated and were infected.
Genome sequencing – effectively decoding an organism's full genetic make-up, so it could be inspected – allowed the team to confirm that cases from outside Havelock North were really part of the outbreak, and not the result of some other cause.
"We were able to define the outbreak more fully and link cases based on the genetic similarity of campylobacter from people with other cases with campylobacter from the water and with sheep on the paddocks."
The researchers estimated that between 6260 and 8320 cases of illness - including up to 2230 who lived outside the reticulation area - were linked to the contaminated water supply.
Of these, 953 cases reported by doctors, 42 were hospitalised, three developed Guillain-Barré syndrome – a rare disorder in which the body's immune system attacks the nerves – and campylobacter infections contributed to at least four deaths.
An unknown number of people continue to suffer health complications.
Of the 12 genotypes the researchers observed in cases, four were also observed in water, three were also observed in sheep and one was also observed in both water and sheep.
Study co-author Professor Michael Baker, of Otago University, said he wasn't surprised the actual number of infected was higher than first thought.
"You have to remember that quite a few people with this illness don't present with symptoms," he said.
"Generally speaking, only about one in 10 cases of campylobacter actually become identified and notified. But this outbreak was different because there was so much awareness of it."
Gilpin said the approach used in the study could be applied to much smaller outbreaks and help with future responses.
"This is a similar approach to that being used to sequence viral genomes from positive Covid-19 cases where the sequencing helps define who is part of each outbreak," he said.
"It is important not to focus too narrowly when investigating disease outbreaks as impacts can be widely felt.
"This study reinforces the importance of nationwide approach to safe drinking water. The decisions made in Havelock North regarding drinking water don't just affect that town, but clearly have impacts far beyond."
The study also suggested extra safeguards were needed to protect populations from such drinking water outbreaks, in light of expected increases in heavy rainfall events with climate change, and intensification of agriculture.
They suggested more work needed to be put into source protection, suitable water treatment, regular monitoring of water, and epidemiological surveillance of disease incidence.
Baker said the "multi-barrier" approach needed was similar to that being used at the country's borders today, to keep Covid-19 out.
The Government has already taken a raft of actions, including setting up an independent national regulator of drinking water supplies, and last week investing more than $760m to help councils upgrade run down water services.
The new study has been published in The Journal of Infection.