Viral Hemorrhagic Septicemia Virus (Vhsv) Great Lakes Strain Ivb: Viral Detection, Mechanisms of Infection, and Host-Virus Interactions in the Yellow Perch (Perca Flavescens)

Abstract

Viral hemorrhagic septicemia virus (VHSV) is one of the most devastating and problematic viral fish diseases to plague the European aquaculture industry, and due to its pathogenicity, disease course, mortality rates, and wide host range, remains one of the most pathogenic viral diseases of finfish worldwide. A new freshwater strain of viral hemorrhagic septicemia virus IVb (VHSV-IVb) in the Great Lakes has been found capable of infecting a wide number of naive species, and has been associated with large fish kills in the Midwestern United States since its discovery in 2005. In this study, the yellow perch, Perca flavescence, one such species documented in several fish kills affiliated with VHSV, was used as a research model to elucidate host-virus interactions to better understand the mechanisms of viral infection. A direct comparison of viral infection kinetics and net mortality among yellow perch stocks derived from distinct genetic and geographic regions found large variation in susceptibility to the disease, suggesting that genetic variance within a population can play a significant role in survival after infection with VHSV-IVb. Upon investigation of the early acute innate immune response after exposure to the VHS virus, a significant up-regulation of Mx expression in the liver, as well as the inflammatory response genes IL-1β and SAA, in all three tissues sampled, head kidney, spleen, and liver, was directly correlated to viral load indicating the role of these genes in the initial stages of infection. Viral load increased most rapidly in the head kidney and spleen, suggesting that potential down-regulation of Mx in these tissues may represent a viral strategy to increase replication. Finally, when the pathology and distribution of the virus were monitored in different tissues of adult fish exhibiting a low level of VHSV infection over time, the finding of significant viral load in the gills and blood suggest that the sampling of these tissues may offer a more accurate, non-lethal alternative to viral screening from the currently, more traditional and lethal cell culture analysis of the head kidney/spleen. A significant viral load in the brain, however, particularly in the later stages of infection, also suggests that latent virus may remain in the brain neurons undetected in asymptomatic carriers of the virus, classifying the VHS virus as neurotropic, as well as a hemorrhagic.