The number of people infected in the Havelock North campylobacter crisis in 2016 was much higher than previously estimated, new research suggests.
Thousands of people were infected by drinking water from contaminated bores. Four people died and others were left permanently disabled.
A joint study using gene-sequencing helped scientists discover the true scale of the outbreak.
The study suggests the campylobacteriosis case number could be as high as 8320, with up to 2230 of these living outside of Havelock North.
The Institute of Environmental Science and Research (ESR) science leader Dr Brent Gilpin said the study used DNA 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 in a release.
"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 with campylobacter."
"Genome sequencing allowed us 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 genetic similarity of campylobacter from people with other cases, with campylobacter from the water and with sheep on the paddocks."
Gilpin said they found almost 3000 other related cases.
"Some people who became ill reported that they were in the area during the time that the water was known to be contaminated," he said.
"But we were able to get the campylobacter from the people who were ill and genome sequence those campylobacter and through that we were able to demonstrate that they were the same types of campylobacter that were found in the other sick people that were also found in the water and in the sheep that were the ultimate reservoir of this contamination event."
Gilpin said the same approach 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. It's important to not narrow focus too much 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."
Just last week in Havelock North, Prime Minister Jacinda Ardern announced a $761 million investment to upgrade New Zealand's water program.
Councils would only reap the benefits if they signed up to the nationwide water program.
The study was conducted by ESR, Massey University, the University of Otago, the Hawke's Bay District Health Board and the Eastern Institute of Technology and was published in the Journal of Infection.
The work was supported by the Ministry of Health, Health Research Council, ESR Strategic Science Investment Fund, and the Royal Society Te Apārangi.