Report No DWI0777

Cryptosporidium incidence in private water supplies and correlatory indicators


Feb 1997


Executive Summary

  1. The Department of the Environment's 'Cryptosporidium in Water Supplies - National Research Programme' has carried out extensive work on the detection, aetiology and removal of the organism. One area that deserves further examination is the incidence of Cryptosporidium in private water supplies. This study has therefore tried to establish the extent to which Cryptosporidium occurs in private water supplies.
  2. Fifteen private water sources in the Bradford and Craven local authorities' areas have been sampled over a three month period for Cryptosporidium, Giardia and potential indicators. All the supplies were considered to be reasonably likely to contain faecal material, based on a sanitary assessment of the catchment site and previous sampling results In order to eliminate the possible effects of treatment on the incidence of Cryptosporidium in private water supplies all samples in this study were taken prior to treatment, where fitted and sampling was undertaken at times of greatest precipitation. Cryptosporidium was found in twenty one (14.00%) of the samples taken. This has ranged from no positive results in some of the supplies to 40% positive in the most highly contaminated. Nine of the fifteen supplies (60%) have been found to contain Cryptosporidium during the survey period.
  3. The study also looked at other parameters of water quality to try and establish a link between them and the occurrence of Cryptosporidium. There was a statistically significant correlation to Cryptosporidium for faecal streptococci and Clostridium per fringens.
  4. There was no significant correlation between the detection of Cryptosporidium and coliform bacteria, either total or faecal, turbidity, conductivity or a sanitary assessment score.
  5. Twelve positive results for Giardia cysts were found in a total of 150 samples (8%). This is lower than has been found in North American raw water studies and may indicate a difference in the background incidence between the two countries. Eight of the supplies had positive samples for Giardia (53.3%), which is close to the results found in a UK raw water survey (Gilmour et al., 1991). All these supplies were also positive for Cryptosporidium.
  6. Further study is required to explore the possibility of improving sanitary scoring accuracy in order to obtain a statistically significant relationship with Cryptosporidium perhaps allied to turbidity measurement. The extent of possible Cryptosporidium occurrence in lower risk supplies and at different times of the year also needs investigation.
  7. There was no evidence of Cryptosporidiosis in those consuming the water in any of the supplies.

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