Table 1. Key observed changes in epidemiological characteristics of the H5N1 HPAI virus.

Year Reported changes in virulence, infectivity, or environmental resilience of H5N1 HPAI virus

1996

pathogenic H5N1 outbreak recorded from domestic geese in southern China (Webster et al. 2006)

1997

duck H5N1 reassorts with quail H9N2, becomes highly pathogenic strain (Guan et al. 2000); acquires capability for causing fatal disease in humans, and human-to-human transmission (Tam 2002)

1999

NS genes of post 1998-strains enhance virus replication in mammalian cells (Twu et al. 2007); increased lethality in mammals demonstrated by laboratory trials in mice (Guan et al. 2002)

2000

infectivity for domestic ducks increased, although domestic ducks remain asymptomatic carriers (Li et al. 2004)

2001

increase in infection duration and length of virus shedding period in domestic ducks (Hulse-Post et al. 2005); increased virulence in mammals (Lipatov et al. 2003); infections among swine discovered in Fujian, China (Shu et al. 2006)

2002

acquires capability to cause fatal disease in wild and domestic ducks (Sturm-Ramirez et al. 2004); further increased infectivity to mammals (Chen et al. 2004); fatal infections of tigers and cats recorded in China (Xia et al. 2003).

2003

decreased environmental stability in water (Brown et al. 2007); fatal infections of tigers and leopards in Thailand (Keawcharoen at al. 2004)

2004

further increased lethality in mammals and humans (Maines et al. 2005); case rate mortalities of 70-80% recorded for confirmed human H5N1 cases in Vietnam and Thailand (Tran et al. 2004, Areechokchai at al. 2006)

2005

oseltamivir resistant strain emerges in Vietnam (de Jong et al. 2005); amantadine/rimantadine resistant H5N1 strain recorded in northern China (He at al. 2007); H5N1 infections of village swine documented in Indonesia (Cyranoski 2005)

2006

vaccine-resistant “Fujian strain” attains dominance in China (Smith et al. 2006); first human H5N1 cases reported from Europe, Central Asia, Africa, and the Middle East (Turkey, Iraq, Azerbaijan, Egypt, Djibouti: WHO 2008)

2007

oseltamivir-resistant strain emerges in Egypt (WHO 2007); H5N1 infections detected among vaccinated poultry in Egypt (WHO 2008a)