DEVELOPMENT OF A RISK ASSESSMENT FOR BSE IN THE AQUATIC ENVIRONMENT

Bovine spongiform encephalopathy (BSE) is believed to be transmitted b y the ingestion of proteinaceous agents called prions which accumulate in the brain and spinal cord of infected bovines. Concern has been ex pressed about the risks of transmission of BSE to humans through BSE p rions discharged to the aquatic environment from rendering plants, aba ttoirs and landfills. The disease-related form of the prion protein is relatively resistant to degradation, and infectivity decays rather sl ow-ly in the environment. Levels of disinfection used for drinking wat er treatment would have little effect. This paper presents the assumpt ions which were used to model the risks from a rendering plant disposi ng 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 whic h mould contribute to preventing BSE infectivity gaining entry to the aquifer. These barriers included inactivation of BSE agent by the rend ering process, removal from the effluent by treatment at the plant, fi ltration and adsorption in the clay and chalk, and dilution in the gro und water. The importance in environmental risk assessment of the cow- to-man species barrier is considered. Two key conclusions about the en vironmental behaviour of the BSE agent are that prion proteins are 'st icky' and bind to particulates, and that the millions of BSE prion mol ecules comprising a human oral IDS, are subject to some degree of disp ersion and hence dilution in the environment. Assuming the rendering p lant processes 2000 cull cattle carcasses per week, the risks to drink ing water consumers were estimated to be remote. Indeed, even using wo rst case assumptions an individual would have to consume 21 d(-1) of t ap water for 45 million years to have a 50% chance of infection throug h drinking water drawn from the aquifer.