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U.S. Department of Agriculture-Agricultural Research Service, National Center for Cool and Cold Water Aquaculture, Kearneysville, WV 25430
Selective breeding of animals for increased innate resistance offers an attractive strategy to control disease in agriculture. However, this approach is limited by an incomplete knowledge of the heritability, duration, and mechanism(s) of resistance, as well as the impact of selection on the immune response to unrelated pathogens. Herein, as part of a rainbow trout broodstock improvement program, we evaluated factors involved in resistance against a bacterial disease agent, Flavobacterium psychrophilum. In 2005, 71 full-sibling crosses, weighing an average of 2.4 g, were screened, and resistant and susceptible crosses were identified. Naive cohorts were evaluated at 10 and 800 g in size, and most maintained their original relative resistant or susceptible phenotypes, indicating that these traits were stable as size increased >300-fold. During the course of these studies, we observed that the normalized spleen weights of the resistant fish crosses were greater than those of the susceptible fish crosses. To test for direct association, we determined the spleen-somatic index of 103 fish crosses; created high, medium, and low spleen-index groups; and determined survival following challenge with F. psychrophilum or Yersinia ruckeri. Consistent with our previous observations, trout with larger spleen indices were significantly more resistant to F. psychrophilum challenge; however, this result was pathogen-specific, as there was no correlation of spleen size with survival following Y. ruckeri challenge. To our knowledge, this is the first report of a positive association between spleen size and disease resistance in a teleost fish. Further evaluation of spleen index as an indirect measure of disease resistance is warranted.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
1 This work was supported by Agricultural Research Service CRIS Project 1930-32000-002 "Host-Pathogen and Environmental Interactions in Cool and Cold Water Aquaculture."
2 Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.
3 Current address: U.S. Fish and Wildlife Service, Lamar Fish Health Center, 400 Washington Avenue, P.O. Box 155, Lamar, PA 16848.
4 Address correspondence and reprint requests to Dr. Gregory D. Wiens, National Center for Cool and Cold Water Aquaculture, 11861 Leetown Road, Kearneysville, WV 25430. E-mail address: greg.wiens{at}ars.usda.gov
5 Abbreviations used in this paper: BCWD, bacterial cold-water disease; RTFS, rainbow trout fry syndrome; SI, spleen somatic index; TYES, tryptone yeast extract with salts.
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  J. T. Silverstein, R. L. Vallejo, Y. Palti, T. D. Leeds, C. E. Rexroad III, T. J. Welch, G. D. Wiens, and V. Ducrocq Rainbow trout resistance to bacterial cold-water disease is moderately heritable and is not adversely correlated with growth J Anim Sci, March 1, 2009; 87(3): 860 - 867. [Abstract] [Full Text] [PDF]
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