Natural history of highly pathogenic avian influenza H5N1

Abstract

The ecology of highly pathogenic avian influenza (HPAI) H5N1 has significantly changed from sporadic outbreaks in terrestrial poultry to persistent circulation in terrestrial and aquatic poultry and potentially in wild waterfowl. A novel genotype of HPAI H5N1 arose in 1996 in Southern China and through ongoing mutation, reassortment, and natural selection, has diverged into distinct lineages and expanded into multiple reservoir hosts. The evolution of Goose/Guangdong-lineage highly pathogenic H5N1 viruses is ongoing: while stable interactions exist with some reservoir hosts, these viruses are continuing to evolve and adapt to others, and pose an un-calculable risk to sporadic hosts, including humans.

Keywords: Clade; Lineage; Reservoir; Virus evolution; Wild bird.

Figure 1. Wild waterfowl represent the seeding reservoir of influenza A viruses

Three reservoir groups of influenza A viruses are shown: wild waterfowl, poultry, and mammals. Arrows indicate transmission of influenza A viruses between different host reservoir groups and circulation among species within each group. Dotted lines represent sporadic transmission, solid lines represent sustained transmission between different species. Question marks indicate unknown or unresolved transmission patterns. The hemagglutinin subtypes endemic in one or more species in each group are indicated. For wild waterfowl, endemic neuraminidase types are also indicated.

Figure 2. Changing ecology of avian influenza viruses

a) stable period of LPAI-host interaction. Arrows indicate transmission of avian influenza virus (AIV) between different host reservoir groups and circulation among species within each group. Dotted lines represent sporadic transmission, thin solid lines represent frequent transmission, thick solid lines represent sustained transmission. Question marks indicate unknown or unresolved transmission patterns. Green color indicates transmission of LPAI. b) sporadic transition of H5 and H7 subtypes from LPAI to HPAI. Red color indicates transmission of HPAI. Inverted chicken or goose indicates commonly observed mortality in terrestrial poultry or aquatic poultry, respectively. c) transient adaptation period of HPAI H5N1 in aquatic poultry. d) transient expansion period of HPAI in wild waterfowl.

Figure 3. Timeline of major events of goose/Guangdong-lineage H5N1 evolution

Shown are times of major changes in the evolution of highly pathogenic H5N1, goose/Guangdong-lineage. Expansion into different geographical areas is depicted, as is status in various hosts in different locations: solid lines depict stable interactions between virus and hosts and dashed lines depict transient interactions. Blue lines represent aquatic poultry hosts, green lines terrestrial poultry hosts, and red lines wild birds hosts. SE Asia – South East Asia, Gs/GD - goose/Guangdong, HK – Hong Kong.