**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**
Effect of thiamine deficiency on disease resistance in lake trout
Dale Honeyfield1, Chris Ottinger2, Christine Densmore2, and Phil McAllister2.
1 US Geological Survey, Leetown Science Center, Wellsboro PA, 16901. Dale_Honeyfield@usgs.gov (570-724-3322)
2 US Geological Survey, Leetown Science Center, Kearneysville, WV 25430. Chris_Ottinger@usgs.gov. (304-724-4453), Christine_Densmore@usgs.gov (304-724-4437), Phil_McAllister@usgs.gov (304-724-4441)
Disease resistance and immune function in thiamine replete and deficient lake trout Salvelinus namaycush were evaluated. Lake trout fry hatched from thiamine replete and deficient adults were challenged with virulent and avirulent isolates of infectious pancreatic necrosis virus (IPNV) 43-47 days post hatch. During days 1-21 post IPNV challenge there was a tendency for lower median virus titer values for deficient versus replete cohorts with deficient cohorts showing 100- to1000-fold lower median virus burden than did replete cohorts. The second interval of virus challenge extended from 22-109 d post primary challenge and provided for observation of ongoing distribution of virus within cohorts via excretion/cohabitation transmission. Mortality and virus-associated mortality values were greater than primary challenge values for both deficient and replete cohorts and for both avirulent and virulent virus exposures indicating sustained virus transmission and/or sustained virus replication in previously infected individuals. For replete cohorts median virus titer values decreased about 10-fold compared to primary challenge values, while median virus titer values for deficient cohorts remained consistent. Although trends were evident the results do not suggest that IPNV infection is modulated by thiamine status. However it is unclear if the results were confounded by endemic bacteria (Aeromonas salmonicida) in lake trout from Wellsboro broodstock . For bacterial resistance studies, nine month old lake trout fingerlings were used. For both bacterial and immune function thiamine deficient fish were maintained on 0.4 mg/kg dietary thiamine to prevent death but at a thiamine status below known requirements for other physiological functions. Thiamine replete fish were fed 2 mg thiamine/kg feed which is two time the dietary requirement. Approximately four weeks after l ake trout fingerlings to be used in the bacterial study were transported from Wellsboro to Leetown Science Center, an epizootic disease outbreak occurred in the thiamine deficient group but not in the thiamine replete lake trout fingerlings. Both thiamine replete and deficient lake trout tested positive for Aeromonas salmonicida, causative agent of furunculosis but mortality was only observed in the deficient fish. This suggests that thiamine deficiency increases the risk of disease progression. To relate disease to physiological processes a series of tests were conducted to characterize the immune function in l ake trout (200-250 g). Macrophage bactericidal activity and cytotoxic cell activity did not appear to be impacted by the level of thiamine deficiency. However, lymphocyte activity was differentially impacted with T-cell populations exhibiting reduced proliferation following mitogen stimulation. B-cells do not appear to be impacted. T-cells play a critical role in immunity to intracellular pathogens such as Renibacterium salmoninarum the etiologic agent associated with bacterial kidney disease (BKD). Thiamine depleted lake trout may be more susceptible to diseases caused by intracellular pathogens as a result of T-cell dysfunction. The leucopenia (decreased levels of circulating leukocytes) and lymphopenia (decreased levels of circulating lymphocytes) observed in the thiamine deplete lake trout in this study are not unexpected. Although not significantly different, adherent leukocyte bactericidal activity was higher in the thiamine depleted lake trout than in the replete fish. Results from the non-specific cytotoxic cell assays were highly variable in both the replete and depleted lake trout. Our data suggests that thiamine deficiency has no impact on the T-helper function. However, the data does indicate an impact on another function performed by T-cells. The response to TNP-LPS was strongly down regulated in the thiamine replete lake trout but not in the depleted fish. This difference in down regulation suggests that the thiamine depleted lake trout have a diminished T-suppressor function. Over all the data from the epizootic bacterial outbreak and immune function studies indicate that sub-optimal thiamine status adversely affects lake trout disease resistance and physiological response when challenged by disease.