"There is a possibility that they were infected from this case but a positive antibody (result) is not enough to reach a conclusion... In any case there is no risk that they will develop symptoms," the ministry said.

"Since the five people ... have not developed symptoms of bird influenza and there is no risk of them developing symptoms in the future and no possibility of new infections to others, the Health Ministry thinks this is not a problem for public health," it added.

The ministry said it was conducting a thorough assessment of the five positive antibody test results, adding that it planned to make a report around Wednesday.

If confirmed the cases would mark the first human infections of bird flu in Japan, which reported several cases of bird flu earlier this year.

All of those cases have been identified as the H5N1 virus, the strain that has hit other countries in Asia and been blamed for human deaths in Vietnam and Thailand.

FIRST INFECTION?

Nobuhiko Okabe, a director at the National Institute of Infectious Diseases, said the chances seemed high that at least one of the five, a worker at a poultry farm, had been infected.

"That's what we think based on the results of testing," he said, but added that there could be changes in the assessment.

Okabe said it may be difficult to determine for certain whether or not the five had been infected, but added that it was possible to say with a greater degree of certainty in the case of the farm worker, since the person gave blood serum samples twice.

The five people have not exhibited any symptoms typical to influenza, although there had been some light symptoms such as coughing, Okabe said in a brief telephone interview.

"In any case, none of them were serious... There were, at least, no symptoms typical of influenza," he said.

"There is nothing for them or the people around them to worry about."

The Health Ministry said four of the people were workers at a poultry farm while the other person who tested positive was a worker for a livestock hygiene service centre.

In all, 58 people were tested including farm workers and workers for the Kyoto prefectural government, it said.

Okabe said the tests were conducted around March and April only for people who agreed to be tested.

Earlier, Kyodo news agency said four of those reported infected had worked at a farm in the town of Tamba in Kyoto prefecture.

The other person had helped disinfect the farm after a bird flu outbreak among chickens in February, it added.