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United States Department of Agriculture

Agricultural Research Service

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Location: Cool and Cold Water Aquaculture Research

2013 Annual Report

1a. Objectives (from AD-416):
Selective breeding for increased disease resistance is a powerful tool to complement animal health control strategies in the prevention of devastating disease. Through selective breeding over 3 generations, we (NCCCWA) have increased resistance to the bacterial cold water disease agent, Flavobacterium psychrophilum in RBT. Preliminary field testing indicates superior survival and on-farm performance with this selected RBT stock at sites experiencing endemic disease. Interestingly, this selection has also resulted in a significant increase in spleen size and alterations of the in vitro immune response profiles among the resistant fish. Thus, the overall impact of this selective breeding may have also significantly affected both innate and adaptive immune functions.

1b. Approach (from AD-416):
Our approach utilizes pedigreed rainbow trout lines, bred for differences in survival following laboratory challenge with the Fp strain, CSF259-93. This selection process has also led to increased spleen size in resistant fish and unique leukocyte response profiles to in vitro challenge with Fp LPS. The first objective will employ a panel of genome-sequenced Fp strains and other Gram-negative salmonid pathogens to probe the specificity of the induced resistance. Our extensive genomic analysis of Fp virulence variants will facilitate characterization of potential broad-based resistance. The second objective will dissect the cellular and molecular bases of this genetic resistance. Differential effects on cytokine and antibody production in critical immune tissues, including the spleen, will be determined. The third objective will address the impact of this selective breeding on immunoprophylaxis elicited by vaccination with homologous and heterologous Fp strains, as well as other bacterial pathogens.

3. Progress Report:
This is the second year of a project assessing whether selective breeding of rainbow trout for increased resistance against a specific agent (F. psychrophilum) has broadly enhanced immune mechanisms. Initially, efforts were focused on developing and validating an assay to accurately measure F. psychrophilum loads in infected fish. The assay detected 100% F. psychrophilum isolates (n=210) and did not amplify 23 non-target bacteria species isolated from fish farms. The assay was sensitive, with a lower detection limit of 3 genomic equivalents per reaction. This assay will be applied to future studies to characterize bacterial cold water disease pathogenesis. This year, we have made progress examining the specificity of genetic resistance by measuring post-challenge survival following exposure to common rainbow trout pathogens which included Yersinia ruckeri, Flavobacterium columnare and Weissella sp NC10. The survival of the BCWD resistant line (ARS-Fp-R), control line (ARS-Fp-C) and susceptible line (ARS-Fp-S) fish were compared following either immersion or injection challenge with Yersinia ruckeri CSF007-82. ARS-Fp-R line survival was significantly higher than ARS-Fp-S line survival following injection or waterborne challenge. Survival of the ARS-Fp-C line was not different from the ARS-Fp-S line. The results were repeated three times using different pools and year-classes of fish. Following immersion challenge with Flavobacterium columnare CSF298-10, ARS-Fp-R line survival was significantly higher than ARS-Fp-S line fish while the survival of the ARS-Fp-C line was intermediate. These studies provide evidence that selective breeding has produced measurable broad-based resistance against unrelated gram-negative pathogens. There was no survival difference between trout lines following challenge with either a gram-positive pathogen, Weissella sp. NC10, or the infectious hematopoietic necrosis virus. Studies examining the effects of breeding on the antibody response and cell populations were completed by collaborators in 2013.

4. Accomplishments

Last Modified: 06/25/2017
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