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Pathogenesis of Viral Hemorrhagic Septicemia Virus (VHSV IVb) in Great Lakes Fish

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Title: Pathogenesis of Viral Hemorrhagic Septicemia Virus (VHSV IVb) in Great Lakes Fish
Department: Department of Pathobiology
Program: Pathobiology
Abstract: Viral hemorrhagic septicemia virus (VHSV IVb), an OIE listed virus, was isolated recently from the Great Lakes. Numerous wild freshwater species were obtained from mortality events in 2005/06 and were confirmed infected with VHSV IVb by virus isolation and polymerase chain reaction (nRT-PCR). The majority of fish examined had dramatic gross lesions including petechial hemorrhages on the body surfaces and visceral organs while some fish had no visible gross lesions. Most infected tissues had a pronounced vasculitis that was correlated with the presence of viral Ag using immunohistochemistry with a polyclonal antibody to VHSV IVb. Unusual histological lesions were also noted in some fish including a multifocal hepatic necrosis and these lesions were immunopositive for VHSV IVb. Although gonadal tissues had no significant lesions the cytoplasm of oocytes and spermatic cells contained viral antigen and the presence of viral RNA was confirmed using in situ hybridisation. Rainbow trout and fathead minnows were experimentally infected with VHSV IVb using two routes of infection; intraperitoneal injection (0.1 mL of 10e6) and immersion in three doses (10e4.5, 10e6.5, and 108.5 TCID50 mL-1). VHSV IVb was detected as early as 4 d and 7 d post-infection by nRT-PCR and viral isolation, and until 38 d post-infection. The cumulative mortality rates of infected rainbow trout recorded in this study were very low (2.6%) in comparison to fathead minnows (13%). Necrotic and intact branchial epithelial cells of rainbow trout contained viral antigen throughout the trial suggesting that they are more than merely a route of invasion. Rainbow trout gill cells (RTgill W-1) were therefore used to study VHSV IVB in vitro and to examine the ability of pathogen associated molecular patterns (PAMPs) to stimulate an antiviral state. Poly IC, LPS, FuGENE® HD and heat-killed VHSV IVb were used to pretreat RTgill-W1 cells while cytopathic effect and quantitative RT-PCR were used to evaluate viral infection. The most effective stimulants in inducing the antiviral state were heat-killed VHSV and poly IC.
Date: 2011-12
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