Role for migratory wild birds in the global spread of avian influenza H5N8

Abstract

Avian influenza viruses affect both poultry production and public health. A subtype H5N8 (clade 2.3.4.4) virus, following an outbreak in poultry in South Korea in January 2014, rapidly spread worldwide in 2014-2015. Our analysis of H5N8 viral sequences, epidemiological investigations, waterfowl migration, and poultry trade showed that long-distance migratory birds can play a major role in the global spread of avian influenza viruses. Further, we found that the hemagglutinin of clade 2.3.4.4 virus was remarkably promiscuous, creating reassortants with multiple neuraminidase subtypes. Improving our understanding of the circumpolar circulation of avian influenza viruses in migratory waterfowl will help to provide early warning of threats from avian influenza to poultry, and potentially human, health.

Figure 1

Maximum clade credibility (MCC) time scaled phylogenetic tree of multi-subtype HA sequences colored by subtype, region and host-type traits. The clades marked a and b contain H5N8 sequences, and c and d contain sequences from Europe and North America, respectively. The displayed MCC tree was obtained from a posterior set of trees inferred using BEAST (13) with the SRD06 nucleotide substitution model, uncorrelated relaxed clock model and constant population size tree prior. The branches are colored according to the most probable ancestral trait, and ancestral traits were inferred by a symmetric (subtype and region) or asymmetric discrete trait model (host-type) upon the posterior tree set (14). Host-types are Dom-Ans (red): domestic anseriform birds, Dom-Gal (green): domestic galliform birds, Wild-Long (blue): long-distance migratory wild birds, Wild-Short (purple): short-distance migratory wild birds.

Figure 2

Reconstruction of the transmission route using phylogenetic data only from H5N8 HA sequences. At each time slice, the host-type and location coordinates on the branches of the posterior set of phylogenetic trees are inferred and plotted as a cloud of points. The host-type was inferred by discrete trait model (as Figure 1) (14), and the continuous location coordinates were inferred using a homogeneous Brownian motion diffusion model (15). The map projection used is the azimuthal equidistant projection, centred on the North Pole, which is marked with a + sign. In this projection, all points on the map are at proportionately correct distances and at the correct direction from the North pole. Color key as for Fig. 1; see also Movie 1.

Figure 3

Posterior distributions of time to most recent common ancestor (TMRCA) of HA sequences from Europe and North America with H5N8 subtype only, including Host type reconstructions, based upon a posterior set of phylogenetic trees generated as in Fig 1. Color key as for Fig. 1.