Submitted to: Journal of Aquatic Animal Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/5/2014
Publication Date: 8/11/2014
Citation: Marancik, D.P., Leeds, T.D., Wiens, G.D. 2014. Histopathologic changes in disease-resistant-line and sdisease-susceptible-line juvenile rainbow trout experimentally infected with Flavobacterium psychrophilum. Journal of Aquatic Animal Health. 26:181-189.
Interpretive Summary: Selective breeding for disease resistance offers new opportunities for improving fish welfare in aquaculture. At our location, we have produced a line of rainbow trout that has increased resistance against bacterial cold water disease (BCWD). The mechanisms governing this resistance are unknown. We used histopathology to compare tissue damage and inflammation in the BCWD resistant line and a BCWD susceptible line following challenge with Flavobacterium psychrophilum, the bacteria that causes BCWD. In our experiment, we infected rainbow trout with F. psychrophilum and sampled fish on days 0, 1, 3, 6, and 9 post-infection. Tissue damage and the amount of inflammation present in tissue was observed with a microscope and quantified either on a scale of 1-3 or by using image analysis software. Both the resistant and susceptible lines showed significant signs of tissue damage in the spleen, kidney, abdominal cavity and muscle. The resistant line showed significantly less change in a couple of disease perameters in the spleen, suggesting they have less damage in tissue, secondary to bacterial infection, than the susceptible line. These findings demonstrate that increased survival of the resistant line is associated with less tissue damage. This study is important because it describes a mechanism associated with increased survival of the resistant line and demonstrates tissue changes associated with the early stages of BCWD, which have not yet been described.
Technical Abstract: A challenge to genetically selecting fish for disease resistance is an incomplete understanding of how phenotypic selection affects innate immunity at the host-pathogen level. The objective of this study was to explore if tissue damage and cellular inflammatory response was associated with survival in pedigree lines of rainbow trout Oncorhynchus mykiss bred for differential susceptibility to Flavobacterium psychrophilum. Experimentally-infected juvenile, disease resistant (ARS-Fp-R) and disease susceptible (ARS-Fp-S) line fish were examined by histopathology during the acute-phase of disease and in mortalities. Quantifiable and semi-quantifiable lesions demonstrated significant disease progression in the level of peritonitis, splenic capsule width, splenic inflammatory infiltrates, average splenic ellipsoid area, total splenic ellipsoid area, and splenic necrosis. No differences were found for renal inflammatory infiltrates and renal hematopoietic cell depletion over time. ARS-Fp-R line fish had significantly lower splenic capsule width on day 9 post-infection than ARS-Fp-S line fish, suggesting a reduced level of perisplenitis. The ARS-Fp-R line also demonstrated a lower odds ratio of splenic necrosis and splenic inflammatory infiltrates between days 6 and 9 and a trend towards reduced splenic necrosis that approached significance, compared to ARS-Fp-S line fish. These findings support the hypothesis that differential survival is a result of divergence in disease magnitude between genetic lines and contribute to our understanding of disease pathogenesis in F. psychrophilum infected fish.