The health ministry plans to impose a regional ban on blood donations if the West Nile virus is detected in people, mosquitoes or wild birds, according to a report released Thursday by a ministry research panel.

No person-to-person transmission of the illness has been documented so far, but there have been cases of infection caused by blood transfusions and organ transplants, the report said.

The prospective ban would be the first regional restriction on blood donation. Previous restrictions include a ban on donations by people who visited Britain over a certain period, which was imposed to minimize the risk of transmitting mad cow disease.

If the West Nile virus is detected, the blood donation ban will be placed on people who live in municipalities where infected people or agents, such as mosquitoes, may have been, the ministry said.

"In reality, restrictions could be expanded from the municipality to the prefecture," a ministry official said, because of the wide area wild birds and mosquitoes move around in.

The ministry is planning to introduce the restriction next spring when the number of mosquitoes carrying the disease could increase drastically.

The virus was first identified in 1937 in Africa and resulted in epidemics in Europe and Russia in the late 1990s. This month a man from Kawasaki, Kanagawa Prefecture who had returned from the United States became the first person in Japan diagnosed with the disease.

In the United States last year the virus infected about 2,500 people and killed 100.

The state is stepping up screening at sea and airports with stricter mosquito controls.

Symptoms of West Nile virus include sudden high fever, headaches and loss of appetite in about 20 percent of infected patients.