KOBE - The number of swine flu infections in Japan rose to 228 on Wednesday, most of them school students, as the outbreak spread to a third western prefecture, officials said.

The western commercial hub of Kobe remained the worst affected, announcing more than 20 new infections of the (A)H1N1 virus among roughly three dozen new cases confirmed in Japan in the morning.

The western prefecture of Shiga became the third locality to confirm a swine flu infection in Japan, with a man in his 20s, who returned from a weekend trip to Kobe, testing positive for the virus.

Japan's first domestic cases of the virus were confirmed last weekend in Kobe and Osaka, where they spread quickly in and between two high schools that had met for a volleyball tournament.

Kyodo news agency earlier reported that 86 percent of cases detected so far in Japan were between the ages of 10 and 19 -- making it more prevalent among youth in Japan than it is in the United States.