Risk of introduction of BSE into Japan by the historical importation of live cattle from the United Kingdom.

All cattle imported from the United Kingdom to Japan since 1980 and slaughtered before 2002 were traced (n=33), and the number of cattle that were possibly infected with BSE and entered the animal feed chain was calculated. Because there was no effective system to avoid recycling of the BSE agent via animal feed until the early 1990s, of the 33 cattle imported from the UK into Japan, most probably 7 or 8 were infected and entered the animal feed chain, 2 of which entered the animal feed chain in each of 1992 and 1993. In terms of infectivity, 400-550 cattle oral ID(50) of the BSE agent entered the feed chain in each of these years. The amount of infectivity that entered the feed chain in 1989, 1991 and 1995 was smaller but still substantial, suggesting that the BSE agent might have entered the Japanese feed chain in any of these years.