Global patterns of avian influenza A (H7): virus evolution and zoonotic threats

Abstract

Avian influenza viruses (AIVs) continue to impose a negative impact on animal and human health worldwide. In particular, the emergence of highly pathogenic AIV H5 and, more recently, the emergence of low pathogenic AIV H7N9 have led to enormous socioeconomical losses in the poultry industry and resulted in fatal human infections. While H5N1 remains infamous, the number of zoonotic infections with H7N9 has far surpassed those attributed to H5. Despite the clear public health concerns posed by AIV H7, it is unclear why specifically this virus subtype became endemic in poultry and emerged in humans. In this review, we bring together data on global patterns of H7 circulation, evolution and emergence in humans. Specifically, we discuss data from the wild bird reservoir, expansion and epidemiology in poultry, significant increase in their zoonotic potential since 2013 and genesis of highly pathogenic H7. In addition, we analysed available sequence data from an evolutionary perspective, demonstrating patterns of introductions into distinct geographic regions and reassortment dynamics. The integration of all aspects is crucial in the optimisation of surveillance efforts in wild birds, poultry and humans, and we emphasise the need for a One Health approach in controlling emerging viruses such as AIV H7.

Keywords: H7; One Health; avian influenza; influenza A virus; virus evolution; zoonosis.

Figure 1.

Seasonal waves of human infections with low pathogenic AIV H7N9 in China from 2013 to 2018. Data taken from Flutrackers.com (FluTrackers 2019), collated by Ian Mackay (Mackay 2019).

Figure 2.

Phylogenetic relationship of the HA gene segment of H7 AIVs isolated from different geographical regions. The whole HA gene segment was used from viruses reported from 2008 to 2018, with ancestral virus A/chicken/Brescia/1902 (H7N7).The locations of virus isolation are classified into different regions. Branches are distinguished by different colours based on the region of viruses.

Figure 3.

Hierarchical transmission network of H7 AIVs in the period from 2008 to 2018. Based on available sequences, three viral reservoirs located in Europe (pink), East Asia (blue) and North America (yellow) were identified (as shown in Fig. 2) and indicated here. Statistically supported pathways among regions on the global level and among subregions within two viral reservoirs are shown by arrow lines. The corresponding pairwise locations are shown using circles. Pathways with the highest significance and locations with the highest probability as transmission origins are highlighted by thick arrow lines and red circles.

Figure 4.

Reassortment patterns with H7 low pathogenic AIVs. Co-circulation of 21 different genome constellations of H7N9 in humans and LBMs of China in 2016–2017. Genome segments are arranged by HA and NA, followed by the ‘internal’ segments in order of size. Each colour corresponds to a unique genetic lineage. Adapted from (Qi et al. 2018a).