OSAKA, Jan. 16-- A cull of some 200,000 chickens began on Friday at a farm in Okayama Prefecture in western Japan, following confirmation of a highly pathogenic H5 strain of bird flu virus, local media reported.

The cull is expected to take four days, with roughly 50,000 birds killed each day. The dead birds will be incinerated.

The case was reported to the local livestock health center by the farm on Thursday morning, after 28 of its birds were found dead beginning on Wednesday.

Four of the dead chickens and one living one tested positive, and a further genetic test confirmed the virus.

The prefectural government has imposed a ban on the movement of chickens and eggs at six farms within a 3-km radius of the farm.

Fifteen other farms in a 10-km radius have been ordered not to ship their products. The 21 farms raise nearly one million birds all together.

This is the fourth avian influenza case detected in Japan this winter. The previous three cases, two in southwestern prefecture Miyazaki and one in western prefecture Yamaguchi, have resulted in the killing of chickens ranging from 4,000 to 42,000 per instance.

Okayama is the fourth largest chicken egg producing prefecture in Japan, with around 10 million chickens.