Occurrence of Mycobacterium avium subsp. paratuberculosis in raw water and water treatment operations for the production of potable water.

Mycobacterium avium subsp. paratuberculosis (Map) causes Johne's disease of cattle and is implicated as a cause of Crohn's disease in humans. The organism is excreted in animal faeces and can contaminate water catchment areas. This coupled with Map's survival in the environment means that water destined for domestic use may be a source of exposure. This work was designed to determine the occurrence of Map in Lough Neagh (the largest freshwater lake in the British Isles), used as a reservoir, and in two water treatment works (WTW1 and WTW2) which abstract from the lough and which have slow sand filtration (SSF) and dissolved air flotation respectively as their principal treatment regimes. The organism was not detected in lough water samples by culture (n=70) but 29% (20/70) were positive by PCR. In the raw water to WTW1 and WTW2 no culture positives were detected but 54% (13/24) and 58% (14/24) respectively were PCR positive. In WTW1 there were no culture positives at the SSF or final water but 31% (8/26) and 45% (9/20) respectively were PCR positive. In WTW2 similar results were obtained with 26% (6/23) and 48% (11/23) in the floccules and final water respectively. At WTW2 however one culture positive was detected in the final water. This latter finding is of concern. The inability to reach definitive conclusions indicates the need for further research, particularly in the detection methods for viable Map.