Hepatoprotective and stress - reducing effects of dietary Moringaoleifera extract against Aeromonashydrophila infections and transportation-induced stress in Nile tilapia, Oreochromisniloticus (Linnaeus 1757) fingerlings

In aquatic environment, fish are unavoidably exposed to wide ranges of stimuli associated with environmental stress and pathological challenges (Xie et al, 2008). Stress responses provide the animal with an ability to cope in the short-term during exposure to the encounter and increase its chance of survival under adverse conditions (Rapatsa and Moyo, 2014). Physiological stress biomarkers like glucose, cortisol and lysozyme biomarkers are the primary and secondary indicators of stress and cytological damage when fish in aquaculture are exposed to certain stressful conditions (Carlos et al, 2009). Different taxa, species and stages of fish have different tolerances to stress (Adebayo, 2006). This implies that for a particular stressor, severity may vary depending on the species to which it was applied (Barton, 2002).The liver is the main organ responsible for metabolism of both endogenous and exogenous compounds and therefore it is one of the first target organs for the toxic action of stressors (Cao et al, 2016). Environmental variables, particularly nutrition, are ultimately important in affecting fish in time of stress (Barton and Iwama, 2005). Most compounds absorbed by the intestine pass through the liver, which enables it to regulate the level of many metabolites in the blood (Good, 2004). Liver injury is often instigated by the bioactivation of complex reactions involving chemically reactive metabolites, which have the ability to interact with cellular macromolecules such as proteins, lipids and nucleic acids, leading to protein dysfunction, lipid peroxidation, DNA damage and oxidative stress (Larrey, 2000). 

Pathogenic organism can cause diseases in fish, for instance bacteria are unavoidable in fish because the fish body is made of pure protein with fatty materials which are good substrates for bacterial growth (Okaeme and Ibiwoye, 2001). Again, water in which fish develop is a favourable medium for bacterial growth (Subasinghe, 2005). In Nigeria, some of the common bacterial diseases of fish pond include Aeromonas hydrophila and bacterial septicemia (Okaeme, 2006). Aeromonas hydrophila in water-bodies and fish culture systems in Nigeria have been recognized as emerging disease problems in African catfish and tilapia production, and their effects include ulcerative dermatitis, fish egg blooming, spoiling and systemic mycoses (Awoniyi et al, 2007). Farmed fish frequently encounter and tolerate poor environmental conditions, which are well below the considered optimal (Adewolu and Adeoti, 2010). Fish undergoes physiological stress response consequent to handling and transportation procedures, such stress reduce the capacity of fish, hindering their ability to perform essential functions (Bly and Clem, 1991; Subashinge, 2005; Welker et al, 2007). It is generally accepted that oxidative stress plays a key role in the pathogenesis of drug induced hepatotoxicity; therefore plant extracts with antioxidant properties have received a lot of attention as possible preventive and therapeutic agents against stressors and subdued immunity response which can adversely affect the liver and health of cultured fish (Ojiako, 2014).

 Stressors in aquaculture are unavoidable and cause many harmful effects. Strategies to attenuate them should be considered. The use of plants extracts in aquaculture has increased rapidly for the prevention of diseases and also to avoid the indiscriminate use of antibiotics, which can lead to the development of resistant strains of pathogenic microbes (Chatterjee et al, 2006; Kaleeswaran et al, 2011). Phytogenic products and extracts are cheaper, non-toxic and biodegradable alternative to antibiotics. The anti-stress potential of plant extracts may be due to the presence of phenolic and polyphenolic compounds, and these bioactive compounds may render their effects through anti-oxidation of oxidative stress (Soosean et al, 2010). Moringa(drumstick, horse-radish) belongs to the moringaceae family, there are thirteen species of Moringa trees in the family moringaceae and Moringa oleifera is the most widely cultivated species (Ojiako, 2014). The M. oleifera tree is a single genus family of shrubs and trees cultivated across the whole of the tropical belt and used for a variety of purposes (Jahn, 1996; Becker, 2003). It can be recognized by the compound pinnate leaves, and the long narrow angular fruits containing large wind seed. Verdcourt (1993) stated that almost every part of the plant is of value for food and itis probably the most popular plant in ECHO's seedbank of underutilized tropical crops. Different parts of Moringa have shown great antioxidant activity (Anwar et al, 2007) as well as immunomodulatory function in animals (Ojiako, 2014). Moringa oleifera contains antioxidants which can inactivate damaging free radicals produced through normal cellular activity and from various stresses (Makanjuola et al, 2013; Rapatsa and Moyo, 2014). Traditionally, the leaves, fruits, flowers, and immature pods of this tree are edible(Ojiako, 2014). The leaves, in particular, have been found to contain phenolics and flavonoids which have various biological activities, including antioxidant, anticarcinogenic, immunomodulatory, antidiabetic and hepatoprotective functions and the regulation of thyroid status in human and animals (Hussain et al, 2014). 

Tilapia is considered as an excellent species for aquaculture in tropical and subtropical regions, as a result of the high tolerance to handling, stress situations and critical conditions, tolerance to sub optimal water quality, tolerance to high stocking densities, and its fast growth (El-Sayed, 1998). More than 22 tilapia species are cultured worldwide (FAO, 2014). However, the most commercially cultured species are Nile tilapia Oreochromis niloticus, blue tilapia Oreochromis aureus, and Tilapia rendalli(Lim and Webster, 2006). Nile tilapia is also acceptable in term of tastes and preferences because of the white flesh, neutral taste and firm texture this is why it is often called the ‘aquatic chicken’ (Fitzsimmons, 2008). Tilapia is the most commonly farmed fish after carp, salmonids and catfishes (FAO, 2014). Furthermore, the polyculture of O. niloticus in freshwater is associated with rice fields, vegetables or livestock (Madalla, 2008).The aim of the present study was to evaluate the hepatoprective and stress-reducing effects of dietary Moringa oleifera extract against A. hydrophila infections and transportation-induced stress in Nile tilapia, Oreochromis niloticus.