"Human faecal matter" in Queenstown's water supply is likely the cause of the current cryptosporidium outbreak afflicting the city.
Residents have been under a boil water notice for the last two weeks after people became sick with the parasite.
Te Whatu Ora's findings mean the tourism hub will remain under a boil water notice while the Queenstown Lakes District Council (QLDC) works to put protozoa barriers in place at its 3.2km treatment plant.
"Based on all the gathered information to date, our most likely hypothesis is that the outbreak started through human faecal contamination of the source water in Lake Wakatipu, which has then been collected into the Two-Mile water treatment plant's intake pipes, then gotten into the water reservoir," medical officer of Health Dr Michael Butchard explained.
"From there, it has then been distributed around the Two-Mile water distribution network to homes and businesses, most likely over a number of days."
He said that genotyping showed all cases are linked since they are all of the same hominis genotype, "which is also evidence of a human rather than an animal source".
As of today, there are 62 confirmed cases of cryptosporidium, 18 probable cases, and 24 under investigation.
The parasites can cause a respiratory and gastrointestinal illness that primarily involves watery diarrhoea, sometimes with a persistent cough, according to the Center for Disease Control (CDC).
Butchcard said there could be "a number of possible contamination sources" as only a "very small" amount of faecal matter can cause an outbreak.
"Possible sources could have included sewage dispersal from watercraft near the Two-Mile Lake Wakatipu drinking water intake, contamination from swimmers or other lake users, wastewater overflows or other undetected faults in the wastewater system. But we are unlikely to ever know."
He said that while water testing hadn't shown any positive results for cryptosporidium, the testing is "complex and not always conclusive", meaning that negative results "cannot rule out water contamination being the source".
The investigation has not flagged any food, beverage or event as the source of the outbreak — which is why authorities believe water contamination is the likely cause.
"We know the boil water notice in particular, along with other public health measures, has been very effective in halting the spread of crypto, and we thank everyone across the community who has played their part in following the advice to help limit the spread" he said.
"However, crypto is still very much present in the community, with a high risk of re-infection and person-to-person spread. This means the risk of further spread of crypto is still very real and higher than before the outbreak began.
"This is why we fully support the direction from Taumata Arowai and QLDC that the boil water notice remains in place for the Two-Mile Water Treatment Plant distribution zone until such time as a protozoa barrier is put in place," Butchard said.
"In the meantime, we continue to strongly encourage the community to boil water, wash and dry hands thoroughly and take care to avoid swimming pools and facilities where crypto could be spread."
QLDC responds
QLDC has acknowledged Te Whatu Ora's findings.
"This has clearly helped limit the spread," general manager of property and infrastructure Tony Avery said in a statement.
"In line with guidance from both agencies, the boil water notice for parts of Queenstown, therefore needs to remain in place until this work is completed to the regulator's satisfaction."
He acknowledged the "ongoing impact" the outbreak has had on local buisnesses, residents and visitors and said: "Our focus remains on completing work at our Two Mile water treatment plant as soon as possible."
The areas still under a boil water notice are Queenstown town centre; Gorge Rd out to and including Industrial Place; Fernhill; Sunshine Bay; and Queenstown Hill above Frankton Rd out to and including Goldridge Way and Goldrush Way.
"Given that we have also seen cases of secondary (person-to-person) infection, it's also extremely important for everyone to take extra measures to avoid unknowingly passing the bug on," Avery said.
"As well as boiling water, the best way to prevent this is to wash your hands really well regularly with soap and dry them thoroughly. You should also not use any swimming or hot pool if you have symptoms or for two weeks after they've stopped."
He "strongly advised" anyone in the area to follow hygiene advice.
"As many people as possible taking these precautions will help reduce the chances of cases increasing or spreading through secondary contamination to other parts of the district. It's been reassuring to see the number of cases stabilise over the last few days so huge thanks to everyone who's played their part."
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