Record year for crypto in water supplies

Water companies found the stomach bug cryptosporidium in water supplies 14 times in 1996, probably the highest number ever. Five incidents are still being investigated by the Drinking Water Inspectorate (DWI) - among them a disease outbreak in the Wirral which North West Water dismissed as unrelated to drinking water despite epidemiological evidence to the contrary.

Cryptosporidium is a protozoan parasite which can be spread in water supplies and causes a severe diarrhoeal illness. The infective oocysts are insensitive to chlorine disinfection and water companies must rely on efficient filtration and source protection.

Water companies are obliged to report suspected contamination with cryptosporidium to the DWI. The Department of the Environment revealed in March that the Inspectorate was still investigating 15 of 31 such events reported since January 1993 (ENDS Report 266, pp 6-7 ). A total of 14 events occurred during 1996, but only six were classified as "incidents" likely to have resulted in a deterioration of water quality (see table ).

Each incident involved either oocysts being found in treated water and/or a rise in cryptosporidiosis cases in the community. The DWI has concluded its investigation of Yorkshire Water's incident at Elvington, but the others are still under investigation.

In both the Wirral and Itchen incidents, oocysts were found in treated water and cryptosporidiosis cases in the community increased. The Wirral outbreak was declared after 22 confirmed cases were reported in a month - the normal background level is less than ten. An outbreak control team found an association with water supplied from North West Water's Sutton Hall water treatment works, which takes water from the river Dee.

The team's report concluded that, following Public Health Laboratory Services (PHLS) guidelines, the Wirral outbreak should be classified as "strongly associated with drinking water". But the DWI's latest annual report reveals that the company told the Inspectorate that it did not consider the outbreak to be connected with the water supply.

North West Water may have been encouraged to take this line because the outbreak control team expressed reservations about the strength of the association. Only one oocyst was found in each of the positive drinking water samples and there was no evidence of difficulties at the treatment works.

However, in ascribing so much significance to the number of oocysts detected, the team adopted a trigger-value approach which has been criticised by PHLS experts. A report following an outbreak in Torbay in 1995 noted that oocyst levels in treated water cannot be regarded as an indication of the risk of infection because of the poor sensitivity of detection methods and the uneven distribution of oocysts in the supply (ENDS Report 253, pp 11-12 ).

Reports of cryptosporidium in water supplies are continuing at a high level this year with six cases up to 5 June. Following a large outbreak in Hertfordshire and north London (ENDS Report 266, pp 6-7 ), former Environment Secretary John Gummer announced plans to reconvene an expert committee on cryptosporidium. The new Government has confirmed the re-establishment of the committee under the chairmanship of Professor Bouchier of the University of Edinburgh.

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