Bovine spongiform encephalopathy (BSE) is believed to be transmitted by the ingestion of proteinaceous agents called prions which accumulate in the brain and spinal cord of infected bovines. Concern has been expressed about the risks of transmission of BSE to humans through BSE prions discharged to the aquatic environment from rendering plants, abattoirs and landfills. The disease‐related form of the prion protein is relatively resistant to degradation, and infectivity decays rather slowly in the environment. Levels of disinfection used for drinking water treatment would have little effect. This paper presents the assumptions which were used to model the risks from a rendering plant disposing of cull cattle carcasses in the catchment of a chalk aquifer which is used for a drinking water abstraction. The risk assessment approach focused on identifying the hydrogeological and physical barriers which would contribute to preventing BSE infectivity gaining entry to the aquifer. These barriers included inactivation of BSE agent by the rendering process, removal from the effluent by treatment at the plant, filtration and adsorption in the clay and chalk, and dilution in the ground water. The importance in environmental risk assessment of the cow‐to‐man species barrier is considered. Two key conclusions about the environmental behaviour of the BSE agent are that prion proteins are ‘sticky’ and bind to particulates, and that the millions of BSE prion molecules comprising a human oral ID50 are subject to some degree of dispersion and hence dilution in the environment. Assuming the rendering plant processes 2000 cull cattle carcasses per week, the risks to drinking water consumers were estimated to be remote. Indeed, even using worst case assumptions an individual would have to consume 2 l d−1 of tap water for 45 million years to have a 50% chance of infection through drinking water drawn from the aquifer.

Aims:The aim is to determine the risk of transmission of BSE to humans and cattle through the application of sewage sludge to agricultural land. Methods and Results: A quantitative risk assessment based on the Source±Pathway±Receptor approach is developed. Central to the model is the estimation of the arithmetic mean concentration of BSE agent in sewage sludge. The main sources of uncertainty in the risk assessment are the degree to which sewage sludge treatment destroys BSE agent, whether there is a threshold dose for initiation of BSE infection in cattle, and most importantly, the amount of brain and spinal cord material which enters the sewer from the abattoir. Assuming 1% of brain and spinal cord is lost to the sewer from abattoirs, the model predicts a risk of BSE transmission of 7á1´10 ±5 cow ±1 year ±1 for cattle grazing on land to which sewage sludge has been applied. Conclusions: The risks to humans through consumption of vegetable crops are acceptably low. Although the risks to cattle are higher, because of higher exposure to soil and greater susceptibility, the model demonstrates that sewage sludge alone cannot sustain the BSE epidemic in the UK cattle herd. Furthermore, the model suggests that recycling of BSE agent through sewage sludge will not sustain endemic levels of BSE in the UK cattle herd. Signi®cance and Impact of the Study: The conclusions are consistent with the ®ndings from epidemiological studies which so far have not detected horizontal transmission of BSE (which would include transmission from contaminated pastures). The model demonstrates the importance of containment of brain and spinal cord within the abattoir.

Monitoring programs often underestimate the total oocyst loading and, thus, the risk of infection.Counts of Cryptosporidium oocysts in 100‐L volume “spot” samples of treated water have been simulated for conditions representative of an outbreak. The counts agree well with those reported in outbreaks in the United Kingdom and the United States. Whereas 33 percent of 100‐L samples contained no oocysts, a small proportion contained hundreds and even thousands of oocysts. The risk of Cryptosporidium infection is directly related to the total number of oocysts entering the distribution system in the treated water as represented by the arithmetic mean oocyst density. Monitoring programs based on taking single spot 100‐L volume samples will tend to miss the “all important” high‐count samples and thus underestimate the arithmetic mean and the risk to public health. Spot samples recording 0 oocysts/100 L therefore provide very little reassurance. To detect temporal variation in the form of infrequent high‐count spikes, the Cryptosporidium regulations in the United Kingdom require continuous monitoring of treated water as it leaves the treatment facility.

With funding from the European Commission, a consortium of members of the European Water Research Institutes is carrying out a programme of work with the objective of optimising and standardising a method for determining the presence in water of (oo)cysts of Cryptosporidium and Giardia. Each of the stages of the conventional analysis procedure (initial concentration, recoveryand identification and enumeration) are being investigated and the relative merits of existing and new methods are being assessed. Newly developed filters (Envirochek and Filta-Max) have been shown to be more efficient for initial recovery of (oo)cysts from water than the previously used Cuno cartridge filters. In addition, for the analysis of raw waters, flocculationwith ferric sulphate has been shown to give recoveries similar to the Envirochek and Filta Max. Modern purification systems such as immunomagnetic separation have also been assessed and found to offer some advantages over flotation although optimisation of the latter has brought improved efficiency. Preliminary assessment of solid phase cytometry has indicated that this technique could offer significant time savings compared to conventional microscopic counting. The results of the study will be used to propose a revised standard method to CEN.

Drinking water should not contain disease-causing microorganisms or harmful chemicals. It should be clear, colourless and odourless. To ensure that drinking water meets the above criteria, Canada has negotiated a set of guidelines with the provinces.