Health researchers are investigating whether extreme rainfall events - growing worse under climate change - may be increasing the risk of enteric disease outbreaks in some parts of the country. Photo / George Heard
Health researchers are investigating whether extreme rainfall events - growing worse under climate change - may be increasing the risk of enteric disease outbreaks in some parts of the country. Photo / George Heard
Can climate change make some Kiwis sicker?
Otago University health researchers have been investigating whether the risk of nasty gastro diseases has been rising in some areas, as extreme weather events like last summer’s become more frequent.
While they’ve found answering that question to be unsurprisingly complex, early analysis has at least pointed to some “hot-spots” where risk is higher – with local livestock numbers a key factor.
As our planet warms, researchers expect the danger from enteric diseases like gastroenteritis to increase – namely from heavy rainfall events and flooding flushing sewage and livestock waste into water supplies.
That was just what happened in August 2016, when heavy rain sent water contaminated with sheep dung into a Hawke’s Bay pond, before it entered an aquifer from which a bore drew it into Havelock North’s water supply.
The resulting campylobacteriosis outbreak was later estimated to have infected as many as 8320 people – of whom 40 became so sick they needed hospital care, and four died – while costing some $21m.
For our national health system and budgets, cimate change could pose “significant” risks and demands, said Dr Farnaz Pourzand, a research fellow at Otago University’s Health Environment Infection Research Unit.
But could we calculate the possible health impact it had already caused?
A new research effort by Pourzand and her Otago colleagues, part of a wider, ongoing Health Research Council-funded study, marks one of our first attempts to identify links - something far from straight-forward.
Assessing shifts in regional extreme rainfall itself was notoriously complex – namely because of the naturally high variability and low frequency of such events – although recent studies and reports have identified some broad trends.
Earlier this year, for instance, scientists reported extreme deluges in those regions hit hardest by Cyclone Gabrielle could now involve around a third more rainfall – and occur up to four times more frequently – than before humans made the planet some 1.2C warmer.
In their project, the Otago researchers are drawing on a daily time series of average temperature and rainfall over two decades, collected by Niwa’s Virtual Climate Station Network.
Pourzand said most studies to date have reported links between increased temperature or rainfall and enteric infections, after a variable lag period of a few days to a few weeks.
“Moreover, literature on the relationships between groundwater contamination and outbreaks of enteric disease shows several large outbreaks were attributable to contaminated groundwater consumption.”
Of course, there were myriad other variables in the mix: a given area could be more vulnerable to extreme climate events depending on demographic factors, local land use, or the standard of its water supply.
In August 2016, heavy rain sent water contaminated with sheep dung into a Hawke’s Bay pond, before it entered an aquifer from which a bore drew it into Havelock North’s water supply. Photo / NZME
“These vulnerability factors modulate the relationship between extreme rainfall and temperature, and enteric disease outbreaks.”
The research team were trying to untangle those variables using machine-learning to crunch national data on everything from notifications of pathogens to climate, water supply characteristics, livestock densities and population statistics, and collected by from sources such as ESR, Niwa, Stats NZ and the Ministry for the Environment.
Pourzand said the wider study’s aim was to understand why some locations reported more disease than others.
“We expect people without treated water community supply to be at higher risk of getting sick, particularly in rural areas with high livestock densities.”
While the team planned to complete their study within a year, an initial analysis including district-level data – and comparing trends between urban and rural areas, along with the North and South islands – had shown there were indeed spots with a higher chance of more people being infected.
“Initial findings suggest that some ‘hotspots’ are related to livestock densities, and that the source of water supply is also important,” Otago environmental epidemiologist Professor Simon Hales said.
They’d also found some seasonal patterns and that – perhaps unsurprisingly – rural areas had significantly higher rates than urban ones.
The same modelling was further able to confirm that heavy rainfall was a major factor in the Havelock North outbreak.
As for contrasts between the country’s two main islands, the data showed how campylobacteriosis rates fell considerably from 2006, on the back of industry interventions – although that drop was more evident in the south than north.
Pourzand stressed these early results came with important caveats.
“One limitation of our study is that we do not know whether people got sick from water or other sources, such as direct contact with animals or food,” she said.
“So, it makes it more challenging to identify the relationship between enteric diseases and water supply.”
Another was data gaps about water supplies around New Zealand, Hales added.
“We don’t yet have accurate boundaries of water supply distribution as there is no centrally maintained national data in New Zealand, but colleagues in our research group are working on this.”
Eventually, though, Pourzand hoped the findings of the completed study might be incorporated into water safety plans.
“The results could inform local climate thresholds for use in early warning systems and national maps of both current and future vulnerability to health impacts of extreme rainfall,” he said.
“These maps can inform geographic targeting of vulnerable water supplies.”
