TWO people are in hospital following the cryptosporidium outbreak in South Devon, the Government says.
The UK Health Security Agency is investigating 46 confirmed cases of the parasite after it was found in Brixham's water supply.
Speaking to MPs on Monday, Steve Barclay, the Environment Secretary, warned the number of confirmed cases could rise.
He said: "Given symptoms may take up to 10 days to emerge, obviously that may continue to rise.
"Two people have been hospitalised."
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On Wednesday, South West Water (SWW) urged locals across Brixham, Boohay, Kingswear, Roseland and North East Paignton to boil their tap water.
It came after tests found Cryptosporidium was present in the tap water.
The parasite causes an infection called cryptosporidiosis, which leaves victims with symptoms such as diarrhoea and "childbirth-like" cramps.
Nausea or vomiting, a mild fever, and loss of appetite have also been recorded.
The bug can live in the intestines and faeces of infected humans and animals.
It can spread directly from another person or animal by touching faeces or through swimming in or consuming contaminated water or food.
But experts fear people could fall ill in other parts of the UK.
Mr Barclay said that whilst 16,000 homes served by Alston reservoir were subject to the boil notice, 85 percent (32,000 residents) have had the "precautionary" measure lifted.
It comes as SWW said the "probable" cause of the parasite outbreak is a damaged air valve which may have allowed contaminated water to seep into Hillhead reservoir.
The reservoir was successfully drained, cleaned and was being refilled on Monday, SWW said.
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Conservative MP Anthony Mangnall, whose Totnes constituency includes Brixham, said the 46 confirmed cases is an "underestimation" and blamed SWW for people falling ill.
Blasting the scandal as "totally unacceptable" in the 21st century, he added: "The anger in Brixham is palpable, the frustration is apparent.
"And the sheer inconvenience that has been put on them by South West Water is absolutely abhorrent.
"For 24 hours people were still able to drink the water, SWW continued to say there was no problem.
"The reason that people are ill and 46 is most certainly an underestimation, is because of that time period in which people were able to go on drinking it."
Mr Mangnall demanded an investigation into SWW's handling and said the "damaged reputation that has been suffered by businesses and the community of Brixham needs to be addressed".
Responding, Mr Barclay said: "Issues around drinking water are taken with absolute utmost seriousness within Government.
"And that is why I can assure him that these issues will be looked at extremely closely."