Wednesday, June 15, 2016

Disease Prevention in Shrimp Farming

Penaeid shrimp production is under continuous threat by bacterial and particularly viral infections which have caused disastrous collapses of the industry in all major shrimp producing countries. This article reports on recent progress in the development of feed additives capable of reducing the impact of diseases on productivity and profitability in shrimp farming, write Peter Coutteau, PhD and Tim Goossens, PhD, Nutriad.

Disease problems in shrimp production are complex and often still poorly understood. Regulations, consumer demands and sustainable management strategies restrict the number of drugs available to treat pathogens. Vaccines are likely to be ineffective in crustaceans, which lack a specific immune system similar to that of vertebrates.

Therefore, shrimp producers must consider the seed stock quality, husbandry procedures and healthy nutrition as the major tools to control disease. 
Diseases are number one threat

The production of crustaceans has shown an average annual growth rate of 18% over the period 1970-2008, which by far exceeds growth for all other aquaculture species (FAO, 2010). World shrimp aquaculture is producing now well over 4 million MT of shrimp (Valderrama, 2011). This rapid increase in crustacean production largely reflects the dramatic increase in white leg shrimp culture in China, Thailand, Vietnam and Indonesia since 2000. Despite this apparent success story in terms of production expansion, shrimp production in many regions continues to suffer important economic losses due to the impact of a wide variety of diseases.

Recent events illustrate the impact of disease outbreaks on shrimp production in major producing countries. The white spot syndrome virus (WSSV), one of the main causes of the stagnating shrimp industry in the nineties, is significantly affecting shrimp production in recent years in Mexico and Brazil. Early Mortality Syndrome or Acute Hepatopancreatic Necrosis Disease (EMS/AHPND), is presently disrupting production in the three major shrimp producing countries China, Thailand and Vietnam. EMS was first reported in China in 2009, it has spread to Vietnam, Malaysia and Thailand, and now causes annual losses of billions of USD.

EMS outbreaks typically occur within the first 30 days after stocking a newly prepared shrimp pond, and mortality can exceed 70%. EMS is caused by a bacterial agent, which is transmitted orally, colonizes the shrimp gastrointestinal tract and produces a toxin that causes tissue destruction and dysfunction of the shrimp digestive organ known as the hepatopancreas. The EMS/AHPND pathogen consist of a number of unique strains of a relatively common bacterium, Vibrio parahaemolyticus, which have acquired the capabilities to release the potent toxin.

Traditional approaches to boost shrimp health through the feed

A traditional approach to reduce the impact of shrimp diseases consists of increasing the level of key nutrients affecting the health and immunology of shrimp, including vitamin C and E, phospholipids, essential fatty acids, trace minerals and carotenoids.

These “booster feeds” are often supplemented with immunostimulants, mostly derived from the cell envelope of micro-organisms, such as polysaccharides, lipoproteins, and lipopolysaccharides. The continuous use of immunostimulants is generally discouraged due to the risks for over-stimulation of the immune defense system.

Alternating on/off regimes for feed additives is often impractical in farm operations. Encouraging results to improve disease resistance have been obtained by the continuous use of health enhancing booster feeds based on the selection of the appropriate immunostimulants in combination with a balanced nutritional supply of key nutrients to support the enhancement of the immune system (Table 1).

However, the efficacy of various commercially available immunostimulants to improve stress and/or disease resistance of fish and shrimp strongly depends on the type of the product and on the supply of adjuvant nutrients that are essential to support the buildup of the immune system.

Table 1: Effect of booster feed on production parameters in a farm in NE Brazil during episode of increased disease incidence due to a combination of intensive rains and increased incidence of infectious myonecrosis virus (IMNV) and necrotising hepatopancreatitis (NHP). Booster feed based on enhanced nutritional specifications and supplementation of an immunomodulator (AQUASTIM S, Nutriad) versus standard feed.

Novel approach: boosting the nutritional status and lipid reserves of the hepatopancreas
Shrimps do not tolerate high levels of dietary fat very well. A number of studies show reduced growth at levels above 10% of dietary lipid. Nevertheless, quality and quantity of dietary lipids play a primordial role in growth and health of shrimp. Shrimp have no or very limited capacity to biosynthesize a number of lipid molecules which are essential for normal growth, including cholesterol, highly unsaturated fatty acids and phospholipids. Fishmeal and fish oil are often the most important sources of cholesterol and HUFA in the diet. Increasing cost of these marine ingredients has forced formulators to reduce dietary specifications for these essential lipids.

Although these nutrient levels may not show significant differences on growth performance in feeding trials under controlled conditions, they may become critical for maintaining health and immune defenses under disease challenges and fluctuating ambient conditions encountered in production.

Furthermore, the energy status of shrimp is largely determined by its lipid reserves deposited in the hepatopancreas which functions both as a digestive gland as well as a storage depot for energy.

Therefore, farmers routinely look at squash preparates to evaluate the nutritional status of the hepatopancreas, with ample lipid reserves being an indicator of better resistance to stress and disease challenges.

Fig. 2: The hepatopancreas is the main organ of the shrimp’s digestive system responsible for digestion, absorption and storage of nutrients. Esophagus (E), gastric mill (GM), hepatopancreas (HP), mid gut (MG), hind gut (HG), and anus (A).
Lipid digestion in shrimp occurs for a big proportion intracellular in the hepatopancreas epithelium from where it is transported to the target organs via the haemolymphe under the form of lipoproteins (Fig. 2). The formation and absorption of lipid micelles from the lumen of the hepatopancreas tubuli is therefore a limiting step in the lipid digestive process.

Digestibility enhancers based on natural emulsifying agents, selected for their compatibility with the shrimp’s digestive system, have shown to be capable of complementing the process of emulsification and absorption of dietary fats in the hepatopancreas (Coutteau et al., 2012). This in turn improves the efficiency of shrimp to use fats as essential components and as source of energy for growth and surviving episodes of stress or disease pressure.

The enhancement of the lipid reserves in the hepatopancreas of white shrimp Penaeus indicus as a result of the supplementation of a digestibility enhancing additive was demonstrated recently by van de Braak et al. (2012). Histological analyses showed a three-fold increase of the percentage of shrimp with a high degree of lipid vacuolization in the hepatopancreas after supplementing the feed additive during one month

The results of a parallel pond study indicated 2% higher average body weight (ABW), 4% higher survival, and 6% higher biomass for the treatment ponds. However, removal of outliers for survival from control and treatment set showed 8% higher ABW, 12% higher survival, and 23% higher biomass.

Source - The Fish Site

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