Assessment of the impact of cattle testing strategies on human exposure to BSE agents in Japan.

In Japan, cattle screening tests for BSE are conducted at slaughterhouses for surveillance purposes and as a meat safety measure, but the public health impacts of such testing and the subsequent removal of positive animals from the food chain have not been quantitatively assessed. We evaluated the influence of removing specified risk materials and the alternation of age limits for testing cattle at the slaughterhouse on human exposure to the BSE agent in Japan by constructing a probabilistic risk model. A stochastic model using Monte Carlo simulation was constructed in order to estimate the BSE infectivity destined for the food chain from a single BSE-infected animal at slaughter. The impact of different testing strategies and risk material removal were then compared. Murine intra-cerebral ID(50) (m.i.c. ID(50)) units were used as units for BSE infectivity. Sensitivity analysis was conducted for key input variables by changing values within plausible ranges. The expected fraction of BSE-infected cattle presented for slaughter that would be detected by screening tests was 20%, even if all slaughtered cattle were tested. The removal of risk materials reduced the median value estimate of infectivity destined for human consumption by 95%. Cattle screening tests reduced the infectivity further, but reduction efficacy did not differ among the various testing strategies. Sensitivity analysis indicated that the characteristics of BSE infectivity accumulation during the incubation period, extension of the incubation period, and lowering the detection limit of screening tests had no significant impact on relative infectivity reduction, which remained stable irrespective of testing strategy or changes in these parameters. This study suggests that the impact of changing the age limit for testing cattle on beef safety is small, provided that the removal of risk materials is conducted properly.