One canoeist who recently rowed on the Liffey has tested positive for leptospirosis which can be caught from rats and, if untreated, can develop into the potentially fatal Weil's disease.
The Eastern Regional Health Authority is awaiting the result of tests on another canoeist who is in hospital with clinical symptoms of the disease.
Another five canoeists who are not in hospital are being tested although the regional health authority says they may well have the flu, which has similar symptoms to leptospirosis.
The authority's public health specialist, Dr Marie Laffoy, said anybody boating or swimming in the River Liffey on or around October 13th last, especially the Leixlip/Lucan stretch, and who developed flu-like symptoms, should contact their GP.
The flu-like symptoms can disappear and be replaced some time later by Weil's disease. The public health objective is to treat the condition before the second stage develops.
The second stage does not develop in all cases and in many instances, the disease is unpleasant rather than dangerous.
Dr Laffoy said people who had flu-like symptoms and who were not using the Liffey around that time could be assured that they did not have leptospirosis.
Weil's disease is rare here and is usually caused by the urine of rats. A Dublin golfer died of the disease in 1996 after a rat urinated on him. The condition is treated with antibiotics.
GPs in the ERHA region - Dublin, Kildare and Wicklow - have been alerted to the possible outbreak.
Directors of public health throughout the State have also been alerted in case some of those who used the Liffey around the day in question had come from beyond the area.