Japan has quarantined 604 cows to prevent the spread of mad cow disease after authorities found a 23-month-old bull with a possible new strain of the illness, a farm ministry official said Wednesday.
The Agriculture, Forestry and Fisheries Ministry ordered the isolation of all cattle raised with the infected animal, the country's eighth case, ministry official Keiji Fushimi said.
Fushimi said the ministry hasn't decided whether to destroy the cattle -- from Fukushima and Tochigi prefectures -- and isn't ruling out the possibility that quarantine numbers could rise.
For now, the cows are being held on the farms where they were bred, he added.
The latest discovery raises questions about tighter screening procedures implemented since Japan's first case was diagnosed in September 2001.
The bull, which tested positive for the disease on Sept. 29, was the youngest animal to be found here with the fatal brain-wasting disease, known formally as bovine spongiform encephalopathy, or BSE. The other infected cattle, the most recent case in January, were 5 years old.
Agriculture minister Yoshiyuki Kamei said Tuesday that followup tests by the National Institute of Infectious Diseases revealed a possible new strain of the disease. Within hours, officials in northern Ibaraki Prefecture, where the cow had been screened before it was to be slaughtered, incinerated the animal.
The case showed heightened resistance to certain enzymes, the health ministry said in a statement.
Since 2001, only three cows younger than 30 months have tested positive for the disease in Europe, underscoring the rarity of the latest discovery.
Japan was the first country outside of Europe to find an infected cow.