QUANTITATIVE BSE RISK ASSESSMENT - RELATING EXPOSURES TO RISK

There is currently no epidemiological evidence to identify the route(s ) of transmission of BSE from cattle to humans, although consumption o f infected bovine offals, prior to their use being banned in human foo d in late 1989, is the most likely source. Quantitative risk assessmen t methods have been applied to estimate the risks through various pote ntial routes of exposure to BSE, including those from environmental di sposal of BSE-infected residues. The risks predicted for the consumpti on of beef-on-the-bone and for the drinking of water from an aquifer p otentially contaminated with effluent from a cattle rendering plant ar e similar at 10(-9) and 10(-8) person(-1) year(-1), respectively. It i s suggested here that while the risks predicted for beef-on-the-bone a re realistic, the risks through drinking water could be over-estimated by a factor of about 10(20). First, the risk assessment methodology d oes not take into account the very different natures of the exposure. Thus, through beef-on-the-bane an unfortunate consumer map ingest a hi gh dose (approaching an ID50) in a single exposure, while because of d ilution in water, drinking water consumers would never be exposed to h igh doses, even cumulatively over the period of a human lifetime. Seco nd, the risk assessment for drinking water does not allow for the poss ibility of a threshold effect such that a minimum number of BSE prions is needed to initiate infection. It is concluded that direct comparis ons of predicted risks from BSE should be avoided unless the natures o f the exposure are similar. Information on whether there is a threshol d effect is more critical for quantitative BSE risk assessment through environmental routes of exposure than the exact magnitude of the cow- to-man species barrier.