Histopathologic Changes in Disease‐Resistant‐Line and Disease‐Susceptible‐Line Juvenile Rainbow Trout Experimentally Infected with Flavobacterium psychrophilum

Abstract

A challenge to selectively breeding fish populations for improved disease resistance in aquaculture is an incomplete understanding of how artificial selection affects innate immunity at the host-pathogen level. The objective of this study was to determine whether Rainbow Trout Oncorhynchus mykiss bred for differential susceptibility to bacterial cold-water disease exhibited altered tissue damage and cellular inflammatory response following experimental challenge with Flavobacterium psychrophilum. Fish from disease-resistant (ARS-Fp-R) and disease-susceptible (ARS-Fp-S) lines were experimentally challenged as juveniles, and mortalities, as well as survivors, were sampled for histopathology during the acute phase of the disease. Microscopic lesions were quantified or semiquantified and statistically compared for changes over time and between genetic lines. Significant progression in the degree of perisplenitis, splenic necrosis, splenic inflammatory infiltrates, average splenic ellipsoid area, total splenic ellipsoid area, and peritonitis was present over time in both genetic lines on at least one postinfection time point. No differences were found between renal inflammatory infiltrates and renal hematopoietic cell depletion over time. Perisplenitis was significantly lower in fish from the ARS-Fp-R line on day 9 postinfection than in fish from the ARS-Fp-S line. The ARS-Fp-R line demonstrated a trend towards reduced splenic necrosis compared with the ARS-Fp-S line that approached significance, and fish from the ARS-Fp-S line were 3.6 times more likely than fish from the ARS-Fp-R line to have a higher splenic necrosis lesion score after day 3 postinfection. These findings support the hypothesis that differential survival is a result of divergence in disease magnitude and not altered disease course between genetic lines. Characterization of histopathologic changes between genetic lines and over time helps elucidate mechanisms of disease resistance and contributes to our understanding of disease pathogenesis in fish infected with F. psychrophilum. Received January 7, 2014; accepted March 10, 2014.

Innate immune cell signatures in a BCWD-Resistant line of rainbow trout before and after in vivo challenge with Flavobacterium psychrophilum

“…2 cell abundance in the spleen. This observation of similarity in direction of response shared by both genetic lines is in agreement with a hypothesis based on a histopathologic study on the same genetic strains (Marancik et al, 2014b) that differential survival after Fp-challenge results from a divergence of disease magnitude, not altered disease course.…”

“…Both genetic lines were generated through a selective breeding program at the National Center for Cool and Cold Water Aquaculture (NCCCWA). A number of innate and acquired immune components reportedly differ between the two lines, with R-line fish having an enhanced capacity for pathogen control relative to the S-line and R-line fish having significantly larger spleens (Marancik et al, 2014b)(Wiens et al, 2013b)(Hadidi et al, 2008), although the latter does not appear to have a protective role (Wiens et al, 2015). Additionally, R-line fish have a higher abundance of Early B-cell Factor-expressing (EBF) cells compared to S-line trout (Zwollo et al, 2015), although the identity of EBF + cells is more complicated than previously assumed (this study).…”

Proinflammatory cytokine and cytokine receptor gene expression kinetics following challenge with Flavobacterium psychrophilum in resistant and susceptible lines of rainbow trout (Oncorhynchus mykiss)

“…We report gene expression differences occurring in the spleen, as part of a larger study examining gene expression differences occurring simultaneously within infiltrating peritoneal cavity cells and anterior kidney tissue, as well as, changes in plasma biochemistry and plasma proteins. We chose to initially investigate the spleen as it is a dynamic site of disease manifestation and substantial histological changes occur following Fp injection including an increased inflammatory infiltrate, perisplenitis, splenic necrosis, and increase in average and total splenic ellipsoid area [39]. The degree of perisplenitis and necrosis are higher in ARS-Fp-S line fish compared to ARS-Fp-R line fish, consistent with differential pathogen control [39].…”

“…All fish utilized in this study were from the 2014 year class. Single-sire × single-dam matings were made within genetic lines between 3-year-old females and 1-year-old neomales as previously described [39]. Water temperatures in the egg incubation jars were manipulated so that all families hatched within a one-week period [12].…”

Flavobacterium psychrophilum Infections in Salmonid Broodstock and Hatchery‐Propagated Stocks of the Great Lakes Basin

“…Furthermore, Purcell et al (2008) suggested that phenotypic divergence between source stocks and GLB stocks may be related to disease epizootics or other conditions faced by salmonids in the unique environment of the Great Lakes. Differential susceptibility based on genetic differences has been demonstrated by the development of F. psychrophilum-resistant and susceptible strains of RBT (Marancik et al 2014(Marancik et al , 2015.…”