Introduction
Production of white leg shrimp (Litopenaeus vannamei ) experienced decades of spectacular growth. Global production was estimated at around 5.2 million MT in 2021, with a forecasted volume of 6 million MT in 2023. White leg shrimp is an interesting species for the aquaculture industry, due to its high growth rate, with excellent yields. Additionally, shrimp market demand is high, making it interesting for fish farmers to produce and export globally. W hite leg shrimp are known to have a relatively low feed conversion ratio and environmental impact. Nevertheless, the industry is increasingly faced with challenges, such as increased pressure of diseases. One of the most severe diseases in shrimp industry is early mortality syndrome (EMS). EMS, also known as Acute Hepatopancreatic Necrosis Disease (AHPND) is characterized by high mortalities, in many cases reaching 100 percent within 30-35 days of stocking of grow-out ponds, leading to huge financial losses for farmers. EMS is caused by the toxins produced by Vibrio parahaemolyticus , a bacterium that colonizes the gastrointestinal tract and damages digestive organs such as the intestine and hepatopancreas. Since mortalities are high and bacteria can spread over different farms quite fast, the economic losses are huge. S hrimp-producing countries in South East Asia, such as Thailand, India and Vietnam have experienced significant drops in volumes and export. In Thailand, for example, it is estimated that in the period between 2010 and 2016, EMS has caused financial losses of just under 12 billion dollars and the associated loss of tens of thousands jobs. Recovery from this period is still ongoing, and volumes in some countries in South East Asia are still below that of before the EMS era. Since it is difficult to completely prevent bacterial presence in the water , the focus of the industry is on increasing shrimp resilience through nutrition. One possible solution is the right application of selenium (Se) in the diet. Se plays an important role in preventing oxidative stress in humans and animals, as it i s part of the selenoenzyme glutathione peroxidase. Deficiencies in Se are often followed by decreased growth performance, and an increased susceptibility to diseases. In the battle against diseases like EMS, Se addition can be an important strategy to increase shrimp resilience, with decreased mortalities as result. For the addition of Se to the diet, various forms can be used. In general, it is accepted that organic Se in the form of L-selenomethionine is the only form of Se that can be stored into animal protein. All other forms of Se, such as inorganic sodium selenite or selenocysteine present as part of the organic Se in selenized yeast, cannot be stored into animal protein. Therefore, L-selenomethionine seems the preferred source to provide a safe deposit of Se inside the animal, ensuring an optimal Se status also during disease challenge. The goal of this experiment was to compare the effects of adding organic Se, in the form of L-selenomethionine to the diet and inorganic selenium, in the form of sodium selenite, on the antioxidant capacity, immune status and resistance against a V ibrio parahaemolyticus challenge.
Material and Methods
The animal experiment was conducted at Kasetsart University, Bangkok, Thailand. To investigate the effect of different sources of Se on white leg shrimp, three diets were created and fed in 6 replicates with 25 shrimp per repetition: 1) Control diet, a basal diet without added selenium; 2) Diet SS, control diet + 0.5 ppm Se from sodium selenite; 3) diet SM, control diet with + 0.5 ppm Se from L-selenomethionine (Excential Selenium 4000, Orffa Additives B.V.). Shrimp were fed one of the three experimental diets for 8 weeks, after which 30 shrimp were randomly selected from each treatment and immune and antioxidant parameters were measured. After the feeding trial, a challenge with Vibrio parahaemolyticus was conducted. Over the following period of 15 days, mortality was measured and, after the challenge period, immune and antioxidant parameters were once again measured. All generated data were analyzed using one-way ANOVAs.
Results and Discussion
Immunity and antioxidant capacity at the end of the 8 weeks feeding trial, showed improved health status of the shrimp for both treatments where Se was added to the diet. L-selenomethionine, compared to sodium selenite addition , showed a higher improvement in immune and antioxidant status of the shrimp. Superoxide dismutase (SOD), lysozyme activity and hemolymph protein levels were all shown to be significantly highest in animals fed the diet with L-selenomethionine (P<0.05). Besides, phenoloxidase activity showed a numerical improvement after addition of L-selenomethionine. On the other hand, hemocyte count and glutathione peroxidase (GPx) were unaffected by addition of Se, regardless of the source. Bacterial count before the challenge was shown to be the lowest in the shrimp fed diet SM, indicating shrimp on this diet have a better defense against the bacteria in an unchallenged environment (Table 1; P<0.05). Consequently, increased survival in unchallenged shrimp was observed for diet SM, showing the highest survival after 8 weeks (85.33%), followed by diet SS (83.33%) and control (77.33%) (P<0.05). After the Vibrio parahaemolyticus challenge, changes were visible in the health status of the animals, lysozyme activity and SOD were significantly improved by addition of L-selenomethionine. Other immune parameters were numerically improved by the addition of L-selenomethionine. Se addition significantly decreased the Vibrio count in the shrimp, with L-selenomethionine being significantly more effective compared to sodium selenite (Table 1; P<0.05).
After 15 days the survival of shrimp fed the diet with L-selenomethionine was significantly higher compared to the shrimp fed the diet with sodium selenite or without added Se (Figure 1; P<0.05 ). The results of survival after challenge demonstrate the increased defense capability of shrimp by addition of L-selenomethionine.
Conclusion
Addition of Se in shrimp diets seems to be of vital importance to maintain productivity in a global sector with increased disease pressure . Se is seen to improve the overall health status of animals by improving immunity, antioxidant capacity and survival of challenged and unchallenged shrimp. L-selenomethionine was shown to be highly effective in increasing shrimp health and disease resilience, compared to sodium selenite , indicating that L-selenomethionine, is a better solution for the shrimp feed sector.