Department of the Environment,
Transport and the Regions

Cryptosporidium in Water Supplies

11 Review of research published since publication of the Second Report of the Group of Experts

11.1 Introduction

11.1.1 There have been several hundred research publications worldwide since the Second Report of the Group of Experts appeared in 1995. The abstracts of some 150 of these reports are reviewed in Appendix A8 and key findings are summarised in this Chapter. By far the greatest proportion of the research output has been funded in North America and the UK. More recently, research publications from mainland Europe and Australia have started to appear, reflecting a greater awareness of the significance of Cryptosporidium in water supplies.

11.2 Detection of the organism

11.2.1 Significant advances have been made both in filtration and separation technologies and in the procedures for concentration of the organism prior to identification. The combination of improved cartridge or membrane filters and concentration on immuno-magnetic beads has brought about significant improvements in the level of recovery and precision in recovery. Flow cytometry and laser scanning techniques now offer the potential to automate the processes of sorting, detection and confirmation of the organism. Current research promises further advances using continuous centrifugation, compressed foam filtration media or vortex flow filtration.

11.3 Viability and infectivity studies

11.3.1 There has been controversy over the role of surrogate tests for infectivity as applied to environmental isolates. Nevertheless, there is a consensus that such tests are essential for the interpretation of disinfection studies. Recent research funded by DETR and the American Water Works Association Research Foundation (AWWARF) has compared dye inclusion/exclusion techniques, in vitro excystation and neo-natal mouse infection in controlled disinfection studies. The results indicate that all surrogates gave unreproducible results and provided a poor basis for assessing efficiency of disinfection.

11.3.2 An insight into possible reasons for these findings has been provided by DETR funded research on the effects that cleaning and concentration procedures have on the surface properties of oocysts. It appears that the relatively extreme chemical environment and physical conditions cause unpredictable reactions at the oocyst surfaces and also influence the results of PCR analysis.

11.3.3 These findings, along with evidence for distinctive genotypes with different infection potential towards animals and humans (see 11.3.4 and Appendix A7), calls into question the reliability of published disinfection studies. While some studies may provide a general indication of disinfection capability, reliance on the data for treatment design or for modelling interaction of disinfectants appears unwise. Developments involving tissue culture may offer a more reliable surrogate for human infectivity. However, it is likely that such approaches will also be sensitive to oocyst cleaning and concentration procedures.

11.3.4 Appendix A7 considers the evidence for identifiably distinct 'strains' or sub-types (genotypes or lineages) of C. parvum and concludes that this is very strong. One such sub-type appears to be restricted to man. There is no evidence that this results from the parasite changing within the host during the infection. Indeed, in a further outbreak in the UK where there was evidence of animal (sheep) contamination of the water, isolates from cases were of the animal genotype. The evidence of a high prevalence of 'human genotype' in three waterborne outbreaks has considerable significance in relation to transmission by the water route, suggesting that sewage effluent may have been the major source of these outbreaks.

11.4 Water treatment

11.4.1 There is now wide acceptance that one key to minimising exposure to Cryptosporidium is the consistent production of low turbidity water and the avoidance of peaks in turbidity. Much research has concentrated on the optimisation of turbidity and particle counts and also on the identification of a suitable surrogate for Cryptosporidium removal during treatment. The most promising tool appears to be measurement of bacillus spores and this approach has been validated in a number of pilot studies in North America and the UK.

11.4.2 Interest in removal of protozoan cysts has prompted the development of a variety of innovative filtration technologies, some of which have now been approved under regulation 25 of the Water Supply (Water Quality) Regulations 1989. A number of disinfection technologies based on high intensity UV radiation have also been introduced as treatment for Cryptosporidium. The exposure characteristics and design of these systems are such that they are only applicable to small water supply systems presently.

11.5 Risk assessment

11.5.1 DETR funded studies of microbiological risk assessment have addressed specifically the risk of exposure to Cryptosporidium via drinking water. Modelling and pilot scale work has confirmed that water treatment processes impose extremely non-random characteristics on the distribution of oocysts, probably through absorption of oocysts onto particulate matter. These findings may shed some light on the initiation of outbreaks, with a small number of consumers becoming exposed to extremely localised high concentrations of the organism.

11.5.2 An American Waterworks Association Research Foundation study has demonstrated the possibility of using seroprevalence of antibodies as an indication of a history of exposure to Cryptosporidium via drinking water. A study of serum from consumers receiving surface water derived drinking water showed a higher prevalence of antibodies than in a matched population receiving water from a deep groundwater source.

11.6 Cryptosporidium in the environment

11.6.1 The Ministry of Agriculture, Fisheries and Food and Scottish Office Agriculture and Food Department (MAFF - SOAFD) study on pathogens in the farm environment has contributed greatly to the understanding of the significance of agricultural activity as a source of the organism in environmental waters. The study has confirmed that good agricultural practice in relation to disposal of animal manures and slurries should not pose a particular pollution hazard. However, a related Environment Agency-MAFF study on wild animals in the farm environment has indicated that rats, mice and other animals carry the organism and may play a significant role in its dispersion in the environment. (See Appendix A2).

11.7 Recommendations for research

11.7.1 The Group has identified the following as areas requiring further research:

11.7.2 The following recommendations for research relate to groundwater (see Chapter 4):

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Updated 11 July 2001
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