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.