Health authorities in the west have warned that a bug which can cause kidney failure may be present in some substandard private water schemes.
Mr Cathal Kearney, principal environmental health officer for Mayo, said: "My greatest fear for public health is that there would be an outbreak of E.coli O157 caused by one of the unsatisfactory private schemes".
There are about 130 private group water schemes in the county, more than 40 per cent of which were recently deemed in breach of EU regulations.
E.coli O157 is common among farm animals and water supplies can become infected, as happened in Canada recently.
The reported incidence of the so-called "superbug" in the Western Health Board region (Galway, Mayo and Roscommon) is the highest in Ireland. In 1998 there were 19 reported cases.
Mr Kearney said: "My concern is that if there was a discharge of animal waste into a private supply we could have a very significant and serious outbreak. The worry from our point of view is that the organism only needs to be present in very small numbers to cause somebody to become ill.
"With E.coli 0157, certain strains produce a chemical toxin that causes kidney failure in a significant number of those infected. This may mean that they are on dialysis for the rest of their life."
Those most at risk are babies, young children, the elderly, sick, or pregnant.
There have been a number of cases of E.coli O157 in the west in recent years but there was no conclusive evidence to link them with contaminated water.
"I would like to see the operators of more private water schemes opting to join a regional water supply. Secondly, I would like to see unsatisfactory private schemes upgrading their supplies and installing good disinfection equipment," said Mr Kearney.
Farmers spreading slurry also had a responsibility to protect the sources of water schemes, he said. He added that anybody in doubt about the safety of their supply could eliminate risk by boiling the water.