Coevolutionary Patterns of the VHS-IVb Fish Virus: Still in the Great Lakes and Mutating!

A new, novel, and especially virulent substrain of Viral Hemorrhagic Septicemia virus -VHSv-IVb- first appeared in the Laurentian Great Lakes about a decade ago, resulting in massive fish kills of >30 species. It rapidly spread and has extensively genetically diversified. Recently, the virus has appeared to “go underground”, lacking recent outbreaks. In April and May 2012, our lab analyzed two fishes that were swimming erratically, lacked symptoms, tested positive for the virus, and each had new unique genetic variants. In 2015 we sampled fishes from previous outbreak areas across the Great Lakes, of which just round goby and white perch from central Lake Erie tested positive, lacking classic VHSv symptoms yet with very high levels of the virus. We are sequencing them, and preliminary data indicate other unique variants. Patterns of evolutionary changes coincided among VHSv genes for some of the isolates, but appear independent in others. New viral variants were discerned following the large 2006 outbreak; such differentiation may have been in response to fish populations developing resistance. Rapid evolutionary diversification may allow new viral variants to evade fish host recognition and immune responses, facilitating long-time persistence along with expansion to new geographic areas.