The phylogenies of avian influenza virus (AIV) genes from avian hosts usually split into Eurasian and North American lineages, reflecting limited bird migration between the Eastern and Western hemisphere. Gene sequences that do not conform to this pattern stem from viruses that recently crossed the hemispheric divide. There is anecdotal evidence that such invading sequences are frequently found in shorebirds (Charadriiformes). The purpose of our study was to provide a comprehensive analysis of the role shorebirds play in the movement of viruses between hemispheres. We analyzed phylogenetic trees of all publicly available gene sequences of AIVs from avian hosts.

Results/Conclusions

Our results show that the proportion of invading sequences varies among gene segments and host orders. The proportion of invading sequences is either roughly the same across host orders or is five – to ten- fold higher in Charadriiformes than in other host orders. We furthermore found evidence for reassortment among invading sequences, i.e. the invading subclades of different segments overlap only partially. This is remarkable since the invaders only make up a very small proportion of sequences within each hemisphere.  For the invading sequences to reassort with each other they either have to pass through the same host population bottleneck or have to be co-adapted to each other. We hypothesize that shorebirds (Charadriiformes) form this bottleneck for AIV movement between hemispheres.