H5N1 HPAI virus has emerged in Thailand at the end of 2003, then disappeared in May 2004, the disease reappeared from July-Mars 2005 and then again in July 2005, following a regular most likely seasonal cycle. Local practices and human behaviour (poultry transport, live bird markets, fighting cocks) have been identified as the potential reason for viral diffusion. The 3rd epizootic wave (2005) was less virulent, probably due to the application of strict sanitary measures and a decrease in the pathogenicity of the viral strain for certain type of birds. The reappearance of the disease happened in the same region as the first reported cases. These regions are not the most active ones in term of poultry production but the environmental conditions might be favourable for virus persistency which remains to be better understood.
Indeed, since 2003 virus re-emergence always occurs within some specific environmental conditions: rice paddies with 2-3 harvest/year, shallow pounds, and temperatures lower than the average.