Assessment of water supply and associated
Matters in relation to the incidence
of Cryptosporidiosis in West Herts.
and North London in February and March 1997
A Report by
M J GRAY PhD C Chem FRSC C Biol MIBiol
Principal InspectorExecutive Summary
An outbreak of cryptosporidiosis with 345 confirmed cases of the illness occurred in West Herts and North London in February/March 1997. An Incident Management Team (IMT) was convened to manage the outbreak, comprising representatives of the relevant Health Authorities, one Local Authority, Public Health Laboratories, Three Valleys Water (the Company) and General Utilities Projects, and a Regional Epidemiologist. Outbreak Control Teams (OCT) were set up in each of the four affected Health Authorities to gather local epidemiological and microbiological evidence, make recommendations to the IMT, and to implement local control measures agreed with the IMT.
The IMT carried out a detailed descriptive epidemiological study into the possible causes of the outbreak and a case control study to determine if there was an association between specific risk factors and infection with cryptosporidium. The IMT published a report which concluded that the outbreak was associated with the consumption of unboiled water supplied from Clay Lane Water Treatment Works. There are therefore grounds for concluding that the Company supplied water unfit for human consumption.
The report is critical of the Company in that:
(a) there were deficiencies in the Company's risk classification of the water source at Eastbury Pumping Station;
(b) it accordingly had an inappropriate level of treatment at Clay Lane Water Treatment Works;
(c) in some respects the monitoring and control provisions of the raw water sources feeding Clay Lane WTW were deficient;
(d) there was a delay in issuing advice to boil water by a minimum of 24 hours although the Company's response thereafter to the recognition of the outbreak was timely, efficient and appropriate;
(e) there were deficiencies in the provision of accurate information on the location of potentially affected properties but once the decision was taken to issue advice to boil water, the Company took appropriate steps to ensure that all affected persons were made aware of the advice at the earliest opportunity;
(f) the Company did not implement fully all the recommendations of the two Reports of the Expert Group on Cryptosporidium in Water Supplies; and
(g) there were some deficiencies in the notification of relevant bodies but thereafter liaison and communication were generally satisfactory.
In the light of advice from leading Counsel, it is considered that a prosecution which would have to rely solely upon the evidence contained in the report of the IMT would not be successful.
The Company has already taken action to install membrane filtration at Eastbury Pumping Station. The risk is thus being met with appropriate treatment and enforcement action is therefore not required.
Recommendations are made:
(a) to the Company on matters concerned with water quality monitoring, emergency procedures, risk assessment of ground water sources, and implementation of recommendations contained in the two Reports of the Expert Group on Cryptosporidium in Water Supplies;
(b) to all water Companies on matters concerned with risk assessment of ground water sources and water quality monitoring; and
(c) to the IMT on matters associated with communications.
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Updated 10 July 2002
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