The Asian lineage of H5N1 avian influenza virus emerged from geese (A/Goose/Guangdong/1/96) in China in 1996, and is continuing its spread infecting domestic poultry, other birds and sporadic infection to humans revealing its pandemic potential. Since late 2003, H5N1 virus has been reported over 60 counties in Asia, Europe and Africa killing millions of poultry and infected humans in 16 countries with a total of 650 confirmed cases including 386 fatal have been reported to WHO. Spread of H5N1 virus has been linked with bird migration, although trade of poultry and poultry products has also been attributed in the spread. During the last one and half decades of circulation in poultry, the H5N1 virus has undergone significant genetic diversification leading to emergence of 10 major virus clades (clades 0-9), with subsequent diversification into 2nd-, 3rd- and 4th- order clades. Vaccination is one of the strategies for control and prevention of H5N1 virus infection; however, some countries still face challenges to eliminate H5N1 virus in poultry. One of the major problems is antigenic drift in viruses that develops due to vaccine pressure, particularly when vaccination is not implemented properly. Two classes of anti-viral drugs (neuraminidase inhibitors and adamantanes) are available for treatment and prophylaxis of influenza; however, there is emergence of drug resistant variants in poultry and humans. Wide-spread circulation and ever changing nature of H5N1 virus make avian influenza control program more difficult. Therefore, educational programs to enhance awareness on avian influenza, animal husbandry practices and biosecurity measures are important in control of the disease.
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