Absence of adaptive evolution is the main barrier against influenza emergence in horses in Asia despite frequent virus interspecies transmission from wild birds
Zhu, Henan
Damdinjav, Batchuluun
Gonzalez, Gaelle
Patrono, Livia V.
Ramirez-Mendoza, Humberto
Amat, Julien A. R.
Crispell, Joanna
Parr, Yasmin Amy
Hammond, Toni-ann
Shiilegdamba, Enkhtuvshin
Leung, Y. H. Connie
Peiris, Malik
Marshall, John F.
Hughes, Joseph
Gilbert, Martin
Murcia, Pablo R.
Virus ecology and evolution play a central role in disease emergence. However, their relative roles will vary depending on the viruses and ecosystems involved. We combined field studies, phylogenetics and experimental infections to document with unprecedented detail the stages that precede initial outbreaks during viral emergence in nature. Using serological surveys we showed that in the absence of large-scale outbreaks, horses in Mongolia are routinely exposed to and infected by avian influenza viruses (AIVs) circulating among wild birds. Some of those AIVs are genetically related to an avian-origin virus that caused an epizootic in horses in 1989. Experimental infections showed that most AIVs replicate in the equine respiratory tract without causing lesions, explaining the absence of outbreaks of disease. Our results show that AIVs infect horses but do not spread, or they infect and spread but do not cause disease. Thus, the failure of AIVs to evolve greater transmissibility and to cause disease in horses is in this case the main barrier preventing disease emergence.
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