Update on the continuous spread and expansion of H5N1 highly pathogenic avian influenza

H5N1 highly pathogenic avian influenza (HPAI) virus, initially detected in 1996 in China, spread to more than 60 countries or territories on three continents within a ten-year period and has become endemic in poultry in several countries and regions (including Ganges–Brahmaputra Delta, Indonesia, Viet Nam, China and Egypt). The virus infects wild birds and domestic poultry and causes sporadic transmissions to humans raising concerns of a potential pandemic (Guan et al., 2004; Peiris et al., 2007). The recent confirmation of human cases of low pathogenic avian influenza A (H7N9) and bird positive findings across multiple provinces in China since April 2013 in live bird markets highlights the threat posed by existing and newly emerging avian influenza viruses irrespective of their virulence. The economic impact of disease caused by avian influenza viruses is related to losses incurred as a result of high mortality in poultry, to costs associated with control measures including poultry movement restrictions, to disruption of trade and threats to food security of resource poor countries (Horimoto and Kawaoka, 2001; Campitelli et al., 2002; McLeod et al., 2004). The H5N1 HPAI virus has changed and evolved in poultry into genetically distinct virus clades (0–9) and subclades (WHO/ OIE/FAO H5N1 Evolution Working Group, 2012), particularly in Asia. Of the ten H5N1 HPAI virus 1st1 order clades identified globally during 1996–2012, all have been found in China, which is considered to be the source of all H5N1 HPAI viruses identified (Sims et al., 2005; Zhao et al., 2008;