CHANGSHA - Fifteen students at a Central China school have been diagnosed with hepatitis A, and investigators suspect water pollution is to blame, the local government said Monday.

The patients are all 12th graders at No 4 High School of Qiyang county in Yongzhou city of Hunan province, the county government said in a statement.

It said the first case was reported on Feb 17 and the latest was diagnosed on Sunday. All patients are being treated at the People's Hospital in Qiyang county.

A joint investigation has been launched by disease control and prevention authorities at county, city and provincial levels. Investigators found the students had drunk water from a well on campus, and suspected contaminated well water was to blame for their illness.

But the exact cause has yet to be confirmed, the county government said.

School authorities confirmed all 12th graders had resumed classes in mid-February, at least a week before the spring semester began, to cram for the college entrance exam scheduled for early June. As tap water supply was cut during the winter break, well water was temporarily used.

School authorities had organized health checks for every student and teacher who drank from the well, and vaccinated the other 12th graders against the virus.

The school has more than 4,000 students, ranging from 7th to 12th grade. About 1,200 are in 12th grade and all of them are boarders.

The county government is watching the situation closely as the hepatitis A virus can be latent for 15 to 45 days.