Hungry for hosts – gene flow in influenza virus
Wild waterfowl and seabirds are major natural reservoirs of influenza A viruses. Genetic analysis has revealed that influenza A viruses found in all other host species, including humans, were ultimately derived from avian viruses. Geographical separation of host species has shaped the influenza gene pool into largely independently evolving Eurasian and American lineages, although some gene flow between these regions has been documented.
Reassortment between Eurasian and North American lineage viruses have also been documented in wild aquatic bird populations indicating that in these two geographically segregated lineages there is some mixing of viruses. However, the possible effects of virus gene flow between the Eurasian and American gene pools on influenza virus evolution and population structure had not been fully explored until recently. A new study shows that virus gene flow from Eurasia has led to the exclusion of some viruses from North America, most likely mediated by competition for susceptible hosts (Gene flow and competitive exclusion of avian influenza A virus in natural reservoir hosts. Virology, 5 June 2009 doi:10.1016/j.virol.2009.05.002).
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The researchers found that intercontinental gene flow is frequently mediated through seabirds, highlighting the need for increased surveillance of influenza viruses in a broader spectrum of potential host species. Such selection is only likely to occur in cases where the viruses in question are sufficiently antigenically similar to induce a cross-protective immune response.
Population genetics offers a number of concepts and principles to explain the evolutionary behavior of RNA viruses. For example, the competitive exclusion principle states that when two species compete for limited resources one species will eventually outcompete the other and become dominant. In the case of RNA viruses, a combination of high replication numbers and high nucleotide substitution rates makes the prolonged co-existence of two or more genetically distinct virus populations unlikely. In theory, the competitive pressure exerted by an invading influenza virus will select for viruses with increased reproduction and transmissibility. Therefore the adaptive advantage conferred through competition may contribute to influenza disease emergence.
Increased genome surveillance of influenza viruses in bird populations is critical for understanding the effects of gene flow between populations. The extent of virus competition in avian populations infected with influenza remains unknown. In Asia, the long-term endemicity of H5N1 influenza appears to have replaced, most probably through competitive selection, low pathogenic H5 subtype viruses that have been only rarely isolated from poultry in Asia since 2000 when compared to previous surveillance in the 1970′s. This new work provides a possible mechanism for disease emergence and transmission from natural reservoir hosts.
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Tags: Biology, Emerging disease, epidemiology, Health, Influenza, Medicine, Microbiology, Podcast, Science, Virology, virus
