Study: Ecological divergence of wild birds drives avian influenza spillover and global spread. Image Credit: hedgehog94/Shutterstock

Characterization of spillover dynamics and global transmission patterns of avian influenza A viruses

In a recent study published in the PLoS pathogens magazine, researchers assessed the impact of wild bird ecological variations on avian flu spillovers.

Study: Ecological divergence of wild birds drives avian flu spread and global spread† Image credit: hedgehog94/Shutterstock

Forecasting spillovers from wildlife to pets and humans is becoming increasingly important, especially after the 2019 coronavirus disease (COVID-19) pandemic. Therefore, it is essential to thoroughly investigate the different transmission dynamics between bird reservoirs.

About the study

In the present study, researchers examined the contribution of avian hosts to viral overflow and global transmission to assess the overflow pathways at the interface of wild and domestic animals.

The team took samples from wild birds captured across North America between 2008 and 2018, generating 888 flu samples. The collected samples were compared to the reference sequences corresponding to the three influenza subtypes, namely H13, H16 and highly pathogenic avian influenza (HPAI) H5. Furthermore, the team developed the global phylogeography corresponding to the three subtypes using a trait model generated from 11 geographic clusters of the world.

Influenza screening was performed by extracting viral ribonucleic acid (RNA) from swabs obtained from the subjects. The team then used a real-time reverse transcription polymerase chain reaction (rtRT-PCR) to detect influenza RNA. The presence of influenza antibodies was detected via an enzyme-linked immunosorbent assay (ELISA).

In addition, the samples considered seropositive after ELISA were selected to assess cross-reactivity between viral strains and subtypes via hemagglutination inhibition (HI) assays. The degree of overlap and interaction between the H13 and H16 subtypes was estimated by quantifying the transitions of the polymerase basic protein 2 (PB2) gene segments between the subtypes.


The study results showed that the phylogeny of the influenza hemagglutinin (HA) gene had clear evolutionary dynamics across the H13, H16 and HPAI H5 subtypes. Molecular dating of the subtypes found in gulls revealed that H16 has been persistent since 1969 and H13 since 1973. This indicated that H13 and H16 may have similar origins. On the other hand, HPAI 5 has been persistent since 1996.

For the H13 subtype, viruses were from three main clades, while H16 viruses were found in only one dominant clade. The team noted that the evolutionary rate of H13 is 5.81 x 10 . used to be−3 replacements per site per year, while that of H16 is almost 4.61 x 10 . used to be−3 replacements per location per year. This suggested that the evolutionary features of the subtypes might differ from each other even if they share the same avian reservoir host.

Phylogeography of the three subtypes showed that the H13 subtype originated mainly from the Mississippi-Atlantic flight path in the US, which acted as an H13 source for the North Atlantic and South America, while the Black Sea-Mediterranean region in Eurasia was the source of H13 for East Asia and continental Europe. In addition, H13 was found for 24.2% of its time in the Mississippi-Atlantic Ocean, 15.5% in the Black Sea-Mediterranean Sea, 12.3% in Continental Europe, 11.3% in South America and 10, 3% in East Asia (10.3%). H13 subtype viruses spent the least time in Alaska and the US Pacific-Central flight path.

In the case of H16 subtypes, related viruses were mainly found in Northern Europe and the North Atlantic region. The team noted that Alaska was the source of H16 for regions in the southern Pacific and central US flight paths. In addition, H16 spent 35.9% of its time in Northern Europe, 25.2% in Continental Europe and 16.0% in Alaska.

In addition, the geographic distribution of HPAI H5 was mainly in the Eastern Hemisphere, while transmission rates were highest in the Black Sea Mediterranean region with 29.45% Markov jumps, East Asia with 17.77% Markov jumps and Central Asia jumps with 17.37% Markov jumps. The team noted a positive viral flow from East to Central Asia, with 17.5% Markov jumps in this region. Moreover, East Asia with 33.1% Markov jumps was identified as the main site of HPAI H5 circulation.

Serological testing of the Alaskan ducks and gulls showed interaction between H16 and H13 subtypes, resulting in antigenic cross-reactivity between the subtypes according to geographic origin. This was noted because the sera obtained from North America against an H16 clade showed strong cross-reactivity with an H13 clade from South America.

In addition, the sera against H13 samples from South America also identified an H13 clade originating from East Asia and an H16 clade from the Atlantic-Mississippi region. This indicated that the observed antigenic overlap was responsible for the low number of H13 detected in South America. Taken together, this showed that host barriers exist between ducks and gulls, which may lead to an increased susceptibility of immunologically naive populations to antigenically different viruses.


Overall, the study results showed the highly complex transmission of HPAI across different geographic areas and populations. The researchers believe that the current study could enable the advancement of global surveillance, as well as the prediction of future outbreaks.

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