Officials do not know why cryptosporidium levels have increased
About 45,000 people in north Wales have been told to boil their drinking water after an increase in bacteria in reservoirs supplying the area.
The warnings apply to the Bangor, Menai Bridge and Llanfairpwll areas, as well as the village of Capel Curig.
Welsh Water said any tap water used for drinking or food preparation should be boiled until further notice.
The company said the warning is likely to remain in place for at least two weeks while investigations take place.
This is the latest in a series of problems with cryptosporidium affecting water supplies in north Wales.
AREAS ADVISED TO BOIL DRINKING WATER
Bangor, Bangor side of Felinheli (marina), Beaumaris, Bethesda, Capel Curig, Cwm y Glo
Gaerwen , Gallt y Foel, Gerlan, Llanddaniel, Llanddeiniolen, Llandegai
Llandegfan, Llanfaes, Llanfairpwll, Llangoed, Menai Bridge, Mynydd Llandygai
Pentir, Rachub, Rhiwlas, Sling, Talybont, Tan y Foel, Tregarth
The area's tap water comes from the Mynydd Llandegai water treatment works, which takes supplies from the Marchlyn Bach and Ffynnon Lligwy reservoirs.
Welsh Water said an increase in levels of cryptosporidium was found after "routine water quality sampling," which "prompted us to take this precautionary measure to protect public health."
The company said "extensive" further water sampling was needed, and it did not know why there was an increase in cryptosporidium levels.
Welsh Water's head of water quality Tim Masters said: "I have to stress we wouldn't do this unless we felt it was the right thing to do on a precautionary basis.
"I know it has happened three times in the last three years but all the incidents are actually different.
"In this one I hope we're acting before there could be any possibility of illness in the community."
Cryptosporidium is a parasite found in humans and animals and can cause temporary diarrhoeal illness. Its eggs are killed by boiling water.
The National Public Health Service for Wales (NPHSW) has asked local GPs to report any cases of diarrhoea and to take samples to be tested for gastrointestinal infections such as cryptosporidium.
It also advised people to maintain good personal hygiene including washing hands with soap and warm water if they had vomiting and diarrhoea or were looking after someone who was ill.
The NPHSW statement said: "If you have gastroenteritis, don't return to school or work until you have been symptom free for 48 hours. Don't visit patients in local hospitals and long-term care facilities.
Welsh Water said: "We apologise for inconvenience caused, but protecting public health is our highest priority."
For several days in mid-August, almost 5,000 people in the Tywyn, Aberdyfi, Bryncrug, Rhyd-yr-Onen and Brynglas areas of Gwynedd were also advised to boil their tap water after the quality was affected by heavy rainfall.
It followed tests which showed some cryptosporidium parasites in the water.
In November 2005 hundreds of people were left ill in Gwynedd and Anglesey after cryptosporidium affected affected a Snowdonia reservoir.
Dr Masters said that in the latest case that as far as officials were aware the treatment plant at Mynydd Llandegai was not at fault, and they were also investigating the reservoirs whose catchments were grazed by sheep.
The treatment works were about to be replaced, he said.
He said they did not how long people would have boil their water but said they would look at reducing the numbers of affected, for instance by using another source of supply on Anglesey.
"We've got to be sensible. We've got to be precautionary," said Dr Masters.
"We need to see the cryptosporidium levels fall back to where they normally are which is generally zero or we need to be able to put some additional treatment in place. We're looking at both of those at the moment. "
Any customers who want more information can call 0800 052 0130.
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