Aromatic Plants as Additives for Farmed Fish Dieteffects on the Immune System, Stress and Metabolism

Zusammenfassung

Aquaculture is the fastest growing animal food production sector worldwide, which has increased during the last decades and is expected to continue doing it contributing in the year 2030 almost two thirds of fish for food worldwide. However, this great activity that takes place in the fish farms also entails some problems among which are a large number of fish per cage, the worsening of the quality of the water, the handling of fish and the appearance of wounds, which implies the appearance of stress and negative effect on the immune system, provoking immunosuppression and the appearance of infections that can lead to the death of fish and therefore cause large economic losses. That is why fish are vaccinated and antibiotics have been used until a few years ago with the aim of improving the fish immune system to treat or prevent diseases or infections and their mortality, although both treatments may have negative effects in animals, humans and environment. In this sense, phytotherapy, that is the use of medicinal plants for the prevention or treatment of a wide variety of diseases and infections, or to maintain good health, has become a very good and interesting alternative to the use of antibiotics due to biological compounds they contain (phytochemicals), which can replace them as immuno-prophylactic agents to stimulate the immune system and also do not present negative impact on animals, humans or the environment. The aim of this Doctoral Thesis is to determine if some medicinal plants, or compounds present in them are possible for direct application in fish aquaculture, and in particular of gilthead seabream. To get our objective we divided this Doctoral Thesis in 3 parts or chapters. First part (Chapter 1): we studied the in vitro effect of different concentrations (0.001 - 1 mg/mL) of aqueous and ethanolic extracts from four different plants (oregano, date palm, purslane and moringa) on several fish cell types and against bacteria as well as their antioxidant activity. Toxicity and immunostimulatory capacity of plant extracts on gilthead seabream head-kidney (HK) leucocyte and a fibroblast cell line (SAF-1) was evaluated obtaining that adequate concentrations showed immunostimulatory capacity and promoted cell division. Afterwards, we demonstrate that plant extracts are able to inhibit the growth of tumor cells, as indicated by the toxicity against a fish hepato-carcinoma cell line (PLHC-1), as well as their strong antibacterial activity against three pathogenic fish bacteria (Vibrio harveyi, V. anguillarum and Photobacterium damselae subsp. piscicida). Finally, antioxidant activity of plant extracts was also studied and was very high in some of the studied samples. Second part (Chapter 2): we studied the dietary in vivo effect of some plants (oregano, date palm seeds and lemon peel) as feed additives on gilthead seabream. All the plant-supplemented diets failed to promote the seabream growth performance, and even one of them decreased it. After that, we evaluated the effect of the addition of plants on the innate and adaptive immune system of gilthead seabream, including humoral (immunoglobulin M [IgM] level, natural haemolytiyc complement, lysozyme, bactericidal, protease, antiprotease and peroxidase activities in serum and skin mucus) and cellular (HK leucocyte phagocytic, respiratory burst and peroxidase activities) parameters. In turn, the effect of plants was also studied on the activity of liver antioxidant enzymes (glutathione reductase, superoxide dismutase and catalase) and the total antioxidant activity present in serum and skin mucus. Finally, the expression of numerous HK immune-related genes and liver antioxidant genes was also studied. Positive results were obtained at both humoral and cellular immune responses and gene expression, as well as a tendency to improve the oxidative status of the fish.Finally, the effect of lemon peel on fish metabolism was also considered and it was observed that its addition had any negative or harmful effect in fish metabolism. Third part (Chapter 3): we studied the in vitro and in vivo effects of compounds identical to natural (NICs) ones ("artificial phytochemicals"). First, phytochemicals showed, in vitro, a very strong antibacterial activity against V. harveyi and V. anguillarum but no adverse effects on gilthead seabream HK leucocyte viability or immune responses. Secondly, we tested the dietary effects of various concentrations of a mixture of NICs on gilthead seabream growth performance and immune and oxidative status after challenging with a bacterial infection or not. The mixture of NICs improved the fish growth and the immune parameters of gilthead seabream.