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Title: Spleen size is an indirect indicator of rainbow trout bacterial cold water disease resistance

item Wiens, Gregory - Greg
item DHARMASRI, POORNA - College Of William & Mary
item STEPHENS, LAURA - College Of William & Mary
item ZWOLLO, PATTY - College Of William & Mary
item KAATTARI, STEPHEN - Virginia Institute Of Marine Science

Submitted to: Aquaculture Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 11/25/2012
Publication Date: 2/25/2013
Citation: Wiens, G.D., Dharmasri, P., Stephens, L., Zwollo, P., Kaattari, S.L. 2013. Spleen size is an indirect indicator of rainbow trout bacterial cold water disease resistance. Aquaculture Conference Proceedings. P0214.

Interpretive Summary:

Technical Abstract: The contribution of the spleen to anti-bacterial immunity in lower vertebrates is poorly understood. The spleen first appears as a recognizable organ in shark and bony fish lineages while factors influencing its size and functions in lower vertebrates have received little attention. We have previously reported a phenotypic and genetic correlation between innate resistance to Flavobacterium psychrophilum, the causative agent of bacterial cold water disease (BCWD), and spleen index (spleen weight normalized to body weight, SI). Fish families with larger pre-challenge SI values were found to have greater BCWD resistance (survival) following intraperitoneal injection of a lethal dose of F. psychrophilum. We have also found that spleen size is a highly heritable trait between families of rainbow trout. Since the mammalian spleen is known to be crucial for capture and destruction of encapsulated bacteria, we tested the hypothesis that a larger spleen results in greater pathogen clearance and fish survival. A prediction of this hypothesis is that removal of the spleen, by surgical splenectomy, should reduce the survival advantage of the larger spleen, resistant fish. Experiments were performed to test this hypothesis using two separate lines of fish that had previously been selected either based on BCWD survival (resistant and susceptible), or selected based on spleen size phenotype (high and low SI). Surgical splenectomy or sham operation were randomly performed on 21 resistant/susceptible line fish and 127 hi/low SI fish. Following 65 to 81 days post-surgery, fish were challenged with F. psychrophilum and mortality monitored over 21 days. No significant difference in relative survival phenotype was detected between splenectomized or sham operated fish. These results indicate that larger spleen size does not have a direct role in pathogen control; rather larger spleen size may be an indirect indicator of immunological status or difference in cellular populations throughout the animal. Ongoing studies are now utilizing flow cytometery to quantify variation in spleen, anterior kidney, PBL and peritoneal cavity cell populations of naive resistant and susceptible lines of fish as well as the cellular response post-infection with F. psychrophilum. In summary, these experiments argue that the spleen provides neither a necessary or sufficient role in generating BCWD resistance; rather spleen size may be an indirect indicator of immunological status. The pedigree lines of rainbow trout may serve as a model for future study of the genetic and cellular basis of disease resistance in lower vertebrates.