Researchers at the University of Tokyo traced the feeding records of 32 of the 33 cattle that were confirmed infected in that period. Thirteen of the animals were born on the northern island of Hokkaido or in the Kanto region near Tokyo between December 1995 and August 1996. All were fed locally manufactured milk replacers containing animal fat produced at a feed plant in the Netherlands. There was no evidence that the animals had been fed contaminated meat and bone meal.

However, the agriculture ministry said the findings were not conclusive, as Dutch and other reports have denied that animal fat can cause BSE.

In September 2003, the agriculture ministry claimed that Japanese BSE may have been caused by meat and bone meal from British and Italian cattle. However, the new report challenges the assumption that all infections were of foreign origin, and suggests that there may be various causes. These include imported animal products, milk replacers, and domestically produced meat and bone meal.

The researchers found that a group of 15 cattle, born in Hokkaido between 1999 and 2001, may have eaten feed containing contaminated meat and bone meal produced by a local rendering plant. The plant is thought to have used local cattle that were infected with BSE.