The Kanagawa Prefectural Government on Saturday inspected cattle at a dairy farm in Isehara for signs of mad cow disease.
Prefectural officials said the fifth cow in Japan found to have been infected with the disease -- formally known as bovine spongiform encephalopathy, or BSE -- had been raised at the farm.
Officials also examined paperwork to determine the dates and places of birth of 47 other cows at the farm.
The farmer was questioned about the dates he started using a milk substitute suspected of causing the disease. The officials hope to determine by the end of this month the chances of additional cases of infected cows.
The fifth infected cow was born in December 1995, three to four months earlier than the cows in the first four cases. It was the first such cow to be found in Kanagawa, prompting fears that the infection may have spread over a larger area than previously believed.
The Holstein was fed the milk substitute six days after its birth, the same brand of milk substitute the third infected cow had been fed, according to the prefecture officials.
Japan's first infected cow was found in September in Chiba Prefecture. In late November, two cows raised in Hokkaido and Gunma prefectures were also found to have the disease. The fourth infected cow was found May 11 in Hokkaido.