Water from Lake Wakatipu is used for Queenstown's main water supply. Photo: RNZ / Belinda McCammon
One swimmer in Lake Wakatipu with cryptosporidium could have caused the disease outbreak in Queenstown, says a medical officer.
A Te Whatu Ora/ Health New Zealand investigation has found human faecal matter in the town's water supply, which comes from Lake Wakatipu, was the most likely source of the outbreak.
But the scale of wastewater breaches and the cause of the outbreak remained unclear, says Medical Officer of Health Michael Butchard.
A tiny amount of contamination could have led to about 60 cases in Queenstown since early September, he said.
"You need very few number of the organism to cause an infection in people or to contaminate a water source," Butchard said.
The number of cases had dropped since Queenstown Lakes District Council issued a 'boil water' notice on 18 September, he said.
Cryptosporidium is a tiny, parasitic type of protozoa that can infect people through water contaminated with human or animal faeces. It can spread between people if they touch their mouths or handle food with contaminated hands.
Symptoms of sickness caused by cryptosporidium can include diarrhoea, nausea, vomiting, cramps, bloating, headache, lack of appetite, and a light fever.
Anyone suffering these symptoms should go to their doctor for testing.