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

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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.


Poster presentation at the 18th International Conference on Diseases of Fish and Shellfish, Belfast, UK.