European legislation
Regarding poultry:
Regulation (EU) 2016/429 of the European Parliament and of the Council of 9 March 2016 on transmissible animal diseases and amending and repealing certain acts in the area of animal health (Animal Health Law).
Commission Delegated Regulation (EU) 2020/687 of 17 December 2019 supplementing Regulation (EU) 2016/429 of the European Parliament and the Council, as regards rules for the prevention and control of certain listed diseases.
Commission implementing decision (EU) 2023/2447 of 24 October 2023 concerning emergency measures in relation to outbreaks of highly pathogenic avian influenza in certain Member States.
Regarding captive birds, other than poultry:
Commission Delegated Regulation (EU) 2020/692 of 30 January 2020 supplementing Regulation (EU) 2016/429 of the European Parliament and of the Council as regards rules for entry into the Union, and the movement and handling after entry of consignments of certain animals, germinal products and products of animal origin.
Regarding monitoring:
Commission Delegated Regulation (EU) 2020/689 of 17 December 2019 supplementing Regulation (EU) 2016/429 of the European Parliament and of the Council as regards rules for surveillance, eradication programmes, and disease-free status for certain listed and emerging diseases.
Belgian legislation
The Belgian legislation regarding avian influenza is laid down in the Royal Decree of 5 May 2008.
You can find more information on avian influenza on the WOAH website and on the website of DG SANTE of the European Commission.