Risk assessment and its control of synthetic antioxidant, Ethoxyquin, on immunity of farmed fish

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Ethoxyquin (EQ) has been used as an antioxidant in livestock, aquaculture and pet foods. Animal food safety law has established the upper limit of EQ in animal feed at 150 mg kg^(-1). However, the risk of EQ at the approved level for aquaculture feed (150 mg kg^(-1)) to fish health is unknown. Here, we examine the effect of EQ on the immunity of tilapia (Oreochromis niloticus). EQ concentration in the blood reached 0.16 mg kg^(-1) in fish fed EQ at the approved level. This level of EQ inhibited phagocytic activity of leucocytes in vitro and antibacterial activity of whole blood in vivo. Furthermore, pyknosis in the liver was observed throughout the duration of feeding. However, after 30 days of experimental challenge with feed containing 150 mg kg^(-1) EQ, no significant difference was observed in mortality. Although EQ at the approved level in feed causes immunity depression in fish, the severity of immune depression does not lead to a lowering of disease resistance for short feeding periods.