The Niigata Municipal Government is appealing a court ruling that recognized late-onset Minamata disease, which is caused by mercury poisoning from industrial pollution.

The Niigata District Court ruled May 30 that clear symptoms of the disease can appear long after the intake of methyl mercury and urged the city to grant seven plaintiffs with the government help that goes to officially recognized victims.

The municipal government filed the appeal Monday with the Tokyo High Court.

Minamata disease was confirmed in Niigata in 1965 after people who ate fish from the Agano River developed symptoms such as loss of sensation in limbs and tunnel vision. A Showa Denko chemical plant had released mercury-tainted waste water into the river.

The disease was first discovered in Kumamoto Prefecture, and it was officially recognized by Japanese health authorities in 1956.

Niigata Mayor Akira Shinoda said in a statement released Monday that opinions are still divided over the recognition of late-onset Minamata disease, and that accepting the ruling could lead to "confusion" in recognizing official patients of the disease.

The Niigata District Court said in its ruling that in milder cases, people with Minamata disease only exhibit symptoms of impaired sensation and urged the city to accept the seven plaintiffs as patients of the disease under the pollution-related health damage compensation act.

At the same time, the district court rejected the claims of two other plaintiffs fighting for such recognition. These two people filed an appeal on Friday.