Effects of GH on immune and endocrine responses of channel catfish challenged with Edwardsiella ictaluri

The effects of GH on immune and endocrine responses to channel catfish challenged with the bacterium Edwardsiella ictaluri were examined. Catfish (11.7 ± 1.0 g) treated with recombinant bovine growth hormone (rbGH) and challenged with E. ictaluri experienced similar mortality as control-exposed fish. Plasma activity of lysozyme was higher (P < 0.01) in rbGH-exposed fish. Compared to day 0 controls (non-exposed fish), IGF-I levels decreased (P < 0.05) in challenged fish while levels were similar (P > 0.10) between treatments. Abundance of GH receptor (GHR) mRNA tended to decrease (P = 0.055) in liver of challenged fish while toll like receptor 5 (TLR5) mRNA increased (P < 0.05) in liver compared to d 0 controls. An increase in lysozyme may suggest GH enhances a nonspecific immune response. A decrease in GHR mRNA and plasma IGF-I suggests a downregulation of the somatotropic axis in response to disease. The increase in TLR5 mRNA suggests that TLR5 may play a role in host response to bacterial challenge. While exogenous rbGH may play a stimulatory role to increase lysozyme levels, there was no apparent effect of rbGH on mortality to E. ictaluri.