**The title, authors, and abstract for this completion report are provided below. For a copy of the completion report, please contact the GLFC via e-mail or via telephone at 734-662-3209**
Predicting the effect of viral hemorrhagic septicemia virus IVb on walleye recruitment.
John S. Lumsden, Jessica Grice, Alex Reid
Fish Pathology Laboratory, Dept. Pathobiology, Ontario
Veterinary College, University of Guelph, Guelph ON N1G 2W1
The purpose of the present study was to provide data to predict the effect of viral hemorrhagic septicemia virus (VHSV) genotype IVb on walleye in the Great Lakes. We hypothesized that VHSV IVb will reduce the survivability of stocked fry but that the impact can be blunted by the choice of available strains or stocking during water temperatures that are less permissible for viral replication. Three objectives were put forward to test the hypothesis. The first was to predict morbidity/mortality in naive walleye using a biologically relevant waterborne infection and re-infection model at temperatures permissible for VHSV replication (~10-12oC). The reference strain of walleye (White Lake, OMNR) was not very susceptible to experimental infection via waterborne exposure (1x107 virions; <5% mortality) and was only moderately impacted by a large dose of virus given intraperitoneally (i.p.) (1x107 virions; 30-40% mortality). Walleye were more resistant that fathead minnows but more susceptible than rainbow trout using similar experimental conditions. Naïve walleye stocked to VHSV-contaminated waters should, given optimum health and nutrition, experience low mortality. Mortality could be increased should undue environmental stressors occur, particularly during the phase of increased morbidity, approximately 12 days after virus exposure. In addition, fish previously exposed via the water to VHSV were almost completely resistant to re-infection via i.p.- injection five months later. Fish stocked in the fall, and which likely experience limited mortality after exposure to VHSV, are likely also protected during the following spring mortality events. The second objective was to determine the relative susceptibility of several genetically distinct Great Lakes strains. In first experiment performed using four different walleye strains, one was significantly less susceptible to experimental i.p. infection. The utility of this strain, i.e. use for stocking into contaminated waters, requires confirmation in a separate set of experiments in 2011. The third and final objective was to perform infection trials (at optimal temperatures for VHSV, (~10-12oC) in fish that survived previous exposure at non-permissible temperatures for viral replication (>18oC). Preliminary experiments have demonstrated that there is no difference in morbidity or mortality in fish exposed to VHSV at a constant 12oC or 18oC, or in groups of fish exposed initially at 18oC with the water temperature falling over two weeks to 12oC. Virus titres in tissues from these fish are presently being analyzed using quantitative PCR to determine the course of action in the second round of experiments. To date therefore it appears that is likely that there is a low impact of infection with VHSV on stocked walleye, and that strain selection would further reduce any impact.