Dietary aflatoxin B1 decreases growth performance and damages the structural integrity of immune organs in juvenile grass carp (Ctenopharyngodon idella)

For many years, feed-borne mycotoxin contamination has been a serious and unavoidable problem in the aquaculture industry. This problem is extensively aggravated due to the increasing replacement of fish meal by plant protein. Aflatoxin B1 (AFB1) is a mycotoxin that has received a great deal of attention, owing to its prevalence in plant feedstuffs and detrimental effects on animals. The objective of this study was to investigate the effect of AFB1 on growth and health. The growth performance and structural integrity of immune organs were evaluated after feeding grass carp diets with 0, 29, 59, 86, 110 and 147 μg AFB1/kg for 60 days. The results indicated that dietary AFB1 caused poor growth performance and deformities in juvenile grass carp. In addition, we are the first to observe that AFB1 was detectable in the head kidney and spleen of juvenile grass carp and damages the structural integrity of those organs. And we, for the first time, explore potential mechanisms of this destruction effect to immune organs of fish, which might be partly involved in (1) attenuating antioxidant ability through up-regulation of Keap1a (not Keap1b) to suppress Nrf2 signalling leading to decrease of mRNA levels and activities of antioxidant enzymes (except GSTP1 mRNA level); (2) aggravating apoptosis partly by activating p38 MAPK signalling to activate mitochondria pathway and death receptor pathway; and (3) weakening TJs structure by promoting MLCK signalling to down-regulate the mRNA of TJs protein (claudin-12 was up-regulated) in the head kidney and spleen. Finally, based on the oxidative injury-related indexes (contents of MDA in the head kidney and PC in the spleen), the safe upper doses of AFB1 for grass carp were first estimated to be 30 and 29 μg/kg diet, respectively.