The upland water supply from Loch Katrine to Glasgow was unfiltered at the time of investigations reported here. Continuous monitoring of Cryptosporidium oocysts in water treatment works final water entering supply was a regulatory requirement from 2000, using a sampling and analysis protocol for low turbidity water that filtered 1,000 litre samples at site. Cryptosporidium surveillance was extended across the supply by developing a continuous monitoring programme for operational surveillance of raw waters, using the same sampling and analysis protocol. During 2001–2004 six final water samples exceeded 0.1 oocysts per 10 litres, but none exceeded 1 oocyst per 10 litres. Cryptosporidium 90th and 95th percentiles for source waters from three catchments were significantly lower for April–June than each of the other quarters (p<0.001). Higher Cryptosporidium counts were generally observed in autumn and winter. The first two years following sheep removal from two catchment areas produced no clear impact on source water Cryptosporidium counts. High intensity rainfall in a 2002 storm led to marked increase in source water Cryptosporidium counts. A new water treatment works will provide a barrier to Cryptosporidium oocysts from late 2007.

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