Japan reported its first case of avian flu in a human on Wednesday - a man who got the disease from birds.

Avian flu has swept through farms across Asia this year, forcing officials to cull more than 100 million birds. The disease has also jumped to humans, killing 12 people in Thailand and 20 in Vietnam.

Although there has been no evidence of human-to-human transmission, experts worry that the virus could mutate into a version easily spread among people, setting off a global pandemic.

The man caught the virus while disinfecting a contaminated poultry farm in western Kyoto during an outbreak in Japan earlier this year, the Health Ministry said.

The ministry took blood samples from more than 80 people who may have been exposed. One tested positive for antibodies for the disease, confirming the infection. Four others may also have been infected but the tests were inconclusive.

It was the first confirmed case of human infection from the disease in Japan, the ministry said in a statement, adding that the man showed no symptoms other than a headache that lasted several days.

It wasn’t clear how the five people were exposed to the virus. One said he hadn’t worn a mask at the farm even after the virus was discovered there, the ministry said.

The operator of the Asada Nosan farm, about 370 kilometres west of Tokyo, was given a one-year suspended sentence for violating livestock hygiene laws and concealing from authorities that the disease was killing chickens at the farm.

The couple who owned the farm committed suicide because of the scandal.