Conclusion and final remarks

This study reports eight new whole genomes of TiLVs from Thailand, collected between 2014 and 2019. Their regularly spacing sampling dates provided us crucial temporal information for the estimation of TiLV early origin and epidemiological dynamics. Together with publicly available whole genomes of TiLVs, our analyses revealed evidence for reassortment among different virus isolates, and provided new insights into the early origin of TiLV.
Our ability to infer evolutionary history of TiLV was limited by data availability. Given the global presence of the virus across multiple continents, the small samples we analysed herein were unlikely to capture the true scale of the epidemic. Our results supported that TiLV can spread very rapidly across multiple continents, and reassortment is a common evolutionary feature of this virus that can happen even among viruses that are geographically very far apart. This was perhaps facilitated by cross-country importation network of tilapia. Furthermore, our analyses consistently showed long and deep branch structures for many lineages of TiLVs, including those from Thailand, Bangladesh, and Peru. These structures were indicative of long unsampled histories and perhaps hidden genetic diversity of TiLVs in many parts of the world, including Asia and South America, highlighting the lack and the need for systematic surveillance of this virus. Indeed, our results suggested that TiLV likely originated around 2003–2009, 5–10 years prior the first report of the virus. More complete TiLV genomes from these and other geographical regions will help improve the estimation of how the virus evolves and spreads. This information will in turn enable better management, control, and surveillance of the virus, reducing its global social and economic burden.