The dog that attacked a dozen people including nine elementary school pupils northwest of Tokyo earlier this week had not been vaccinated for rabies, the local government said Friday.

The pet owner has admitted that he failed to inoculate the 2-year-old Shikoku that went on a biting spree in Isesaki, Gunma Prefecture, on Wednesday, sending five of the 12 to hospital, although their injuries were not life-threatening.

Photo taken Feb. 7, 2024, shows Seibu Chuo park in Isesaki, Gunma Prefecture, where a dog attack occurred. (Kyodo)

The owner has a total of seven Shikoku dogs, which are known as a breed of hunting dog, but only three have been registered with the city and there were no records of any having been vaccinated for rabies for nearly 10 years.

Under Japan's Rabies Prevention Act, owners need to register any dog aged 91 days or more with their municipalities within 30 days of taking ownership of the animal. The law also requires dogs to be vaccinated against rabies once a year.

But the dog involved in the attack, which is about 130 centimeters long and 60 cm tall, was neither registered nor did it have a record of vaccination, according to the city and other sources.

It is currently in the care of a prefectural animal protection facility.

Police are investigating the case on the grounds alleged injuries were caused by negligence and as a violation of the rabies law.

"I am sorry for causing trouble for my bad management," the owner, a 62-year-old company executive from Isesaki, told reporters.

He said he was aware of the need to register and vaccinate his dogs but thought the vaccination "was not good for their health" after one of his previous dogs had died after getting the jab.

The owner registered the remaining four dogs after the city issued a guidance on appropriate pet ownership on Thursday, and he also plans to have them all vaccinated.

Meanwhile, another Shikoku dog ran away from his owner on Thursday afternoon in Kanagawa Prefecture, south of Tokyo, prompting the police to alert local citizens through email the following day.

The dog is still on the loose, but it has been vaccinated and no one has been injured so far, according to the police.

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12 bitten in dog attack northwest of Tokyo, 5 sent to hospital