Genetic Changes in Vhsv-Ivb Across Time in the Great Lakes and Pathogenicity Modifications

Viral Hemorrhagic Septicemia virus (VHSV) first appeared in the Great Lakes basin as a novel strain (IVb) in 2003, killing ~32 fish species in outbreaks between 2005-09. After the initial outbreaks, many new haplotypes have evolved approximating a “quasi-species” evolutionary pattern.Thirty-seven sites were surveyed in 2015-16, from which 2561 individuals of 55 fish species were screened for VHSV-IVb. Seven species and 21 individuals (2 in 2015, 19 in 2016) tested positive from Lakes Erie (76%) and Michigan (24%); notably, all lacked evident clinical signs. All 2012-16 isolates were genetically distinctive and the original haplotypes appear to have been eradicated, indicating that the virus has continued to evolve.A pairwise examination of the VHSV-IVb G-gene haplotypes in three time groups of early (2003-06, N=58), mid (2007-10, N=57), and late (2011-16, N=61), revealed significant differences in the genetic composition of isolates seen between the late and early groups (FST =0.159), as well as between the late and mid (FST=0.132) groups. Comparative virological studies examined 3 new Lake Erie isolates to the original -IVb isolate (MI03GL) from muskellunge (Esox masquinongy) from Lake St. Clair: from round goby (Neogobius melanostomus) from Fairport, OH, May 2015; from gizzard shad (Dorosoma cepedianum) from Sandusky Bay, OH, April 2016; from largemouth bass (Micropterus salmoides) from Ashtabula, OH, May 2016. New isolates were purified upon propagation on BF-2 cells. Infection of EPC, RTG-2 and RTgill-W1 cell lines revealed differential pathogenicity, kinetics of replication and profiles of antiviral gene transcription (using Type I IFN pathway as marker).In summary, our results show that VHSV-IVb remains present in the Great Lakes ecosystem but is continuing to mutate, still representing an emerging threat for aquaculture and to immunologically naïve wild fish populations in uninfected waterways.