Selection for improved resistance to Aeromonas hydrophila in Indian major carp Labeo rohita: survival and innate immune responses in first generation of resistant and susceptible lines

Selection for disease resistance in fish may be performed directly on basis of survival data obtained in controlled challenge trials, or indirectly using information from immunological or molecular markers linked to differential survival. In the present study, several key innate immune parameters were measured in aeromoniasis resistant and susceptible lines of rohu Labeo rohita to assess their suitability as immune markers for use in indirect selection for increased resistance. Experimental infection with Aeromonas hydrophila (9.55 × 10(6) cfu g(-1) fish) through the intraperitoneal route produced higher survival in the resistant line (73.33%) as compared to the susceptible line (16.67%). Blood and liver tissue samples from both lines were collected to study some of the innate immune parameters and immune-related gene expression. The respiratory burst activity of blood phagocytes, serum myeloperoxidase activity and ceruloplasmin level were significantly (p < 0.05) higher in the resistant line compared to the susceptible line. Lower level of blood glucose and serum natural haemolysin titre were marked in the resistant line as compared to the susceptible line. No significant difference was measured in total serum protein concentration, antiprotease activity and bacterial agglutinin level between two lines, while the expression of transferrin, complement factor C3 and TLR 22-like transcripts were significantly (P < 0.05) higher in liver samples of the susceptible line. However, no such difference was found in β(2)-microglobulin and lysozyme gene expression between lines. The study demonstrated the possibility of using some of the investigated innate immune parameters as indirect marker traits for selection for improved resistance to aeromoniasis in rohu.

Family association between immune parameters and resistance to Aeromonas hydrophila infection in the Indian major carp, Labeo rohita

Seven innate immune parameters were investigated in 64 full-sib families (the offspring of 64 sires and 45 dams) from two year-classes of farmed rohu carp (Labeo rohita). Survival rates were also available from Aeromonas hydrophila infection (aeromoniasis) recorded in controlled challenge tests on a different sample of individuals from the same families. Due to strong confounding between the animal additive genetic effect and the family effects (common environmental+non-additive genetic), reliable additive (co)variance components and hence heritabilities and genetic correlations could not be obtained for the investigated parameters. Therefore, estimates of the association of challenge test survival with the studied immune parameters were obtained as product moment correlations between family least square means. These correlations revealed statistically significant (p<0.05) negative correlations of survival with bacterial agglutination titre (-0.48), serum haemolysin titre (-0.29) and haemagglutination titre (-0.34); and significant positive correlation with ceruloplasmin level (0.51). The correlations of survival to aeromoniasis with myeloperoxidase activity, superoxide production and lysozyme activity were found to be not significantly different from zero (p>0.05). Assuming that the negatively correlated candidate traits are not favourable as indirect selection criteria, the results suggest that ceruloplasmin level could potentially be a marker for resistance to aeromoniasis in rohu. The use of this immune parameter as an indirect selection criterion for increased resistance to aeromoniasis in rohu will, however, require that the parameter shows significant additive genetic variation and a significant genetic correlation with survival. Further studies are therefore needed to obtain a reliable heritability estimate for ceruloplasmin and its genetic correlation with survival from aeromoniasis.