Submitted to: Journal of Immunology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/11/2008
Publication Date: 3/15/2008
Citation: Hadidi, S., Glenney, G., Welch, T.J., Silverstein, J., Wiens, G.D. 2008. Spleen Size Predicts Resistance of Rainbow Trout to Flavobacterium psychrophilum Challenge. Journal of Immunology. 180:4156-4165. Interpretive Summary: Infectious disease is a substantial source of loss in US rainbow trout aquaculture and improved methods are needed to diminish this problem. One approach is to selectively breed animals for increased disease resistance. A current limitation is that we do not understand fish disease resistance mechanisms or how selection for resistance to one pathogen might affect resistance to unrelated pathogens. At the NCCCWA, we are selectively breeding fish to increase resistance of rainbow trout to the bacterial cold-water disease agent Flavobacterium psychrophilum. Herein, we demonstrate that most fish selected at a small size (2 g) for increased resistance remained resistant throughout their life cycle. We also found that fish with larger spleens were less likely to succumb to bacterial cold-water disease. Although fish with larger spleens were more resistant to F. psychrophilum, spleen size had no effect on susceptibility to an unrelated bacterial pathogen Yersinia ruckeri. To our knowledge, this is the first report of an association between spleen size and disease resistance for any fish species. Since spleen size is easy to measure, we suggest that this may be a useful selection parameter for disease resistance that should be evaluated in other fish populations.
Technical Abstract: 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.