Towards a quantitative risk assessment for BSE in sewage sludge

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 7.1 x 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. Significance and Impact of the Study: The conclusions are consistent with the findings 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.