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Research Project: INTEGRATED APPROACHES FOR IMPROVING AQUATIC ANIMAL HEALTH IN COOL AND COLD WATER AQUACULTURE

Location: Cool and Cold Water Aquaculture Research

Title: Reduction of rainbow trout spleen size by splenectomy does not alter resistance against bacterial cold water disease

Author
item Wiens, Gregory - Greg
item Marancik, David
item Zwollo, Patty
item Kaattari, Stephen

Submitted to: Developmental and Comparative Immunology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/4/2014
Publication Date: 11/7/2014
Citation: Wiens, G.D., Marancik, D.P., Zwollo, P., Kaattari, S. 2014. Reduction of rainbow trout spleen size by splenectomy does not alter resistance against bacterial cold water disease. Developmental and Comparative Immunology. 49: 31-37.

Interpretive Summary: Infectious disease represents a considerable burden to the rainbow trout aquaculture industry. One strategy for reducing disease related loss is to breed fish having improved genetic resistance against specific pathogens. The NCCCWA has bred a line of rainbow trout with enhanced disease resistance to challenge the pathogen Flavobacterium psychrophilum, and ARS has released this line to cooperators. Researchers are now trying to understand the molecular and functional mechanisms of the enhanced disease resistance. Previously, we observed a phenotypic and genetic correlation between disease resistance and the size of the spleen of unexposed fish. In other animals the spleen functions to filter out pathogens and removal of the spleen enhances susceptibility to bacterial infection. ARS researchers tested the hypothesis that the larger spleen size directly contributes to pathogen clearance by surgically removing the spleen and then comparing the survival of recovered animals to sham operated animals after F. psychrophilum infection. The results of two experiments demonstrate complete or partial removal of the spleen did not alter disease resistance. Thus, the data suggest that spleen size is an indirect indicator of bacterial cold water disease resistance in rainbow trout.

Technical Abstract: In lower vertebrates, the contribution of the spleen to anti-bacterial immunity is poorly understood. Researchers have previously reported a phenotypic and genetic correlation between resistance to Flavobacterium psychrophilum, the causative agent of bacterial cold water disease (BCWD) and spleen somatic index (spleen weight normalized to body weight, SI). Fish families with larger pre-challenge SI values were found to have greater BCWD survival (resistance) following intraperitoneal injection of a lethal dose of F. psychrophilum. Since the mammalian spleen is known to be crucial for capture and destruction of encapsulated bacteria, researchers tested the hypothesis that reduction of spleen size, by surgical splenectomy, should reduce the survival advantage of the larger-spleen, disease-resistant fish. Experiments were performed 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 (high and low SI). Following 65 to 81 days post-surgical recovery, fish were challenged with F. psychrophilum and mortality monitored for a minimum of 21 days. No significant difference in the relative survival was detected between splenectomized or sham-operated groups, while SI of splenectomized fish was reduced to an average of 8-12% of control animals. A positive correlation was observed between the SI, measured at the time of splenectomy, and time-to-death post-challenge. In summary, these experiments argue that larger spleen size alone is not sufficient for greater BCWD resistance, but rather it is an indirect indicator of immunological status.