Approximately 90,000 residents in Galway city and county have been advised to continue boiling their drinking and cooking water, pending further testing for a parasite which causes gastro-intestinal illness.
No positive test has been returned for the cryptosporidiosis parasite in public water supplies, but a further four to eight cases of the associated cryptosporidium illness are believed to have been reported over the weekend.
There have been 48 confirmed cases of the illness in the city and county since January, but the Health Service Executive West's public health department believes this may represent a fraction of the total number of people affected.
The Health Service Executive West said yesterday that it is investigating other possible causes of the diarrheal illness, which can affect normally healthy people for a fortnight and is a serious risk to people with low immunity.
A joint statement issued yesterday by the HSE and Galway city and county councils advised householders, hospitals, nursing homes and catering businesses to continue the precautionary action of boiling water.
The group handling the issue said it had reviewed water samples taken over the weekend and the results had "not identified a problem with water quality".
However, it said that there had been a "slight increase" in the numbers of people affected by the illness.
The alert over water supplies arose last week when HSE West medical advisers noted that there had been an early peak in cryptosporidiosis - normally associated with the lambing season, due to presence of the parasite in animal faeces and sewage.
"Although there is no conclusive evidence to date that the water is the cause of this illness, Galway City Council and Galway County Council continue to strongly advise members of the public in affected areas to boil water," the group said.
All of Galway city and surrounding areas - such as Tuam, Moycullen, Barna, Headford, Athenry and Oranmore - are affected by the notice, and the mayor of Galway, Cllr Niall Ó Brolcháin (Green Party), has tabled an emergency motion on the issue for the next meeting of Galway City Council.
"The water-boil notice will remain in place until next week, at the earliest," the group said, and the situation was being kept under continuous review.
The group said it recognised the measures may cause inconvenience, but thanked members of the public for their continuing co-operation and said that "every effort" was being made to identify the cause of the problem.
Cryptosporidiosis has only become a notifiable illness in the Republic since January 2004, and previous water-associated outbreaks have occurred in the midlands and Co Clare.
There were three drinking water-associated outbreaks in Northern Ireland in 2000 and 2001.
An outbreak among Irish holidaymakers in Spain in July 2003 was traced to a hotel swimming pool.
The parasite is resistant to chlorine and filters in many pools are not sufficient to remove it.
One of the worst outbreaks in Britain in recent years involved approximately 5,000 people in Swindon and Oxfordshire in England in 1989.
