VIENNA, Nov 25 (Reuters) - Austria has found a case of bird flu on a small chicken farm near Vienna airport and is ordering poultry farms with more than 350 birds to keep them indoors, public health agency AGES said on Thursday. The spread of highly pathogenic avian influenza in Europe and Asia has put the poultry industry on alert. Past outbreaks, which usually occur in the autumn, have led to the culling of tens of millions of birds and can lead to trade restrictions. Known as bird flu, the virus is attracting the attention of epidemiologists too as it can be transmitted to humans. China has reported 21 human infections with the H5N6 subtype of avian influenza so far this year, more than in the whole of 2020. "The affected farm"s chickens died or were slaughtered under the authorities" supervision. The farm was closed," AGES said in a statement, adding that the case was confirmed on Wednesday. The farm is in the town of Fischamend, east of Vienna and near the borders with Slovakia and Hungary, both of which have reported outbreaks of bird flu in poultry in the past week. read more Bird flu was last detected in Austria early this year, on another small farm, AGES said. It said the virus was being spread across Europe by migratory birds and that local wild birds such as ducks and geese usually also play a role. The Health Ministry plans to issue a decree on Thursday ordering poultry farms of more than 350 birds in "risk areas" to keep them inside, AGES said. It did not elaborate on where the risk areas might be but said all contact between farm birds and wild birds should be avoided and farms should implement protective measures such as only feeding their birds in areas with a roof.