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 of infected bovine offals, prior to their use being banned in human food in late 1989, is the most likely source. Quantitative risk assessment methods have been applied to estimate the risks through various potential routes of exposure to BSE, including those from environmental disposal of BSE-infected residues. The risks predicted for the consumption of beef-on-the-bone and for the drinking of water from an aquifer potentially contaminated with effluent from a cattle rendering plant are similar at 10(-9) and 10(-8) person-1 year-1, respectively. It is suggested here that while the risks predicted for beef-on-the-bone are realistic, the risks through drinking water could be over-estimated by a factor of about 10(20). First, the risk assessment methodology does not take into account the very different natures of the exposure. Thus, through beef-on-the-bone an unfortunate consumer may ingest a high dose (approaching an ID50) in a single exposure, while because of dilution in water, drinking water consumers would never be exposed to high doses, even cumulatively over the period of a human lifetime. Second, the risk assessment for drinking water does not allow for the possibility of a threshold effect such that a minimum number of BSE prions is needed to initiate infection. It is concluded that direct comparisons of predicted risks from BSE should be avoided unless the natures of the exposure are similar. Information on whether there is a threshold 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.