Early mortality syndrome on shrimp: what is it, causes, symptoms, prevention

Early Mortality Syndrome (EMS), also known as Acute Hepatopancreatic Necrosis Disease (AHPND), has emerged as a devastating threat to global aquaculture, particularly impacting shrimp farming. Since its initial detection in China in 2009, EMS has spread rapidly across Southeast Asia and beyond, causing substantial economic losses, estimated to exceed USD 1 billion annually. This insidious disease is characterized by high mortality rates, often reaching 100% within the first 30 days of stocking. Understanding its complexities and implementing robust prevention strategies are paramount for the sustainability of the industry.

The bacterial culprit and its mechanisms

At the heart of EMS lies the bacterium Vibrio parahaemolyticus, specifically strains that carry a plasmid encoding for PirA and PirB toxins. These toxins are ingested by the shrimp and then target the hepatopancreas, a vital digestive organ. The toxins cause severe tissue destruction and dysfunction, leading to the characteristic clinical signs of EMS. While V. parahaemolyticus is the primary identified causative agent, research also suggests the involvement of other Vibrio species like V. campbellii, V. harveyi, V. owensii, and V. punensis in expressing similar PirAB-like toxins.

Clinical manifestations of EMS in affected shrimp include:

• Pale and atrophied hepatopancreas: the digestive gland appears shrunken and discolored, often granular when pressed.
• Empty stomach and gut: indicating a cessation of feeding and digestive function.
• Soft exoskeleton: a sign of impaired molting and overall weakness.
• Erratic swimming or sinking to the bottom: demonstrating lethargy and loss of normal behavior.
• Reduced growth and overall vitality: shrimp fail to thrive and exhibit stunted development.

Risk factors and transmission pathways

Several environmental and management factors contribute to the incidence and severity of EMS outbreaks. These factors create an environment conducive to the proliferation of pathogenic Vibrio strains:

• Poor water quality: high organic matter, fluctuating pH (especially increases to 8.5-8.8), high temperatures, and high salinity can all exacerbate the problem.
• High stocking density: overcrowding increases stress on shrimp and facilitates rapid disease transmission.
• Low-quality postlarvae (PL): stocking PL already infected with Vibrio or those with compromised health significantly raises the risk.
• Insufficient aeration: leads to poor pond bottom conditions and accumulation of harmful substances.
• Feed mismanagement: overfeeding contributes to organic matter buildup and can stress shrimp.
• Lack of biosecurity measures: inadequate disinfection of water sources, equipment, and insufficient quarantine practices allow for the introduction and spread of the pathogen.

EMS can be transmitted horizontally, through cannibalism of infected shrimp, and vertically, from infected broodstock to offspring. The bacteria can also be transported by various aquatic organisms and remain viable in the environment, posing a constant threat.

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Diagnosis and prevention: a multifaceted approach

Early and accurate diagnosis is crucial for mitigating the impact of EMS. Histopathology of the hepatopancreas reveals characteristic tissue damage, while polymerase chain reaction (PCR) tests can detect the presence of the PirA and PirB toxin genes, often even before clinical signs appear. This allows for timely intervention and containment.

Prevention of EMS requires a comprehensive, integrated farm management approach:

• Biosecurity: strict biosecurity protocols are the first line of defense. This includes using sterile water sources, thorough disinfection of ponds and equipment, and meticulous handling of postlarvae.
• Quality postlarvae: sourcing Specific Pathogen-Free (SPF) and EMS-negative postlarvae from reputable hatcheries is paramount.
• Water quality management: maintaining optimal water parameters, including pH, salinity, and dissolved oxygen, and controlling organic matter accumulation, are essential. Mature microbial communities in ponds, often achieved through “greenwater” technology or microbially matured water systems, can outcompete fast-growing pathogenic bacteria.
• Nutritional support: a well-balanced diet that enhances shrimp immunity and gut health can significantly improve their resilience to disease. Feed additives, such as probiotics, prebiotics, immunostimulants, and certain botanicals or organic acids, play a crucial role in strengthening the shrimp’s natural defenses.
• Nursery phase: employing a nursery phase for young postlarvae allows them to grow larger and more robust before stocking into grow-out ponds, while also enabling early detection of any health issues.
• Integrated health management: continuous monitoring of shrimp health, water quality parameters, and Vibrio counts is vital for early detection and prompt response.

Innovations and the future of aquaculture health

The aquaculture industry is actively pursuing innovative solutions to combat EMS. Research focuses on understanding the complex interactions between diet, gut microbiota, and the immune system of aquatic animals. Advanced molecular biology and genomics techniques are being utilized to identify beneficial microbes and genes that enhance disease resistance. The development of functional feed additives is a key area of innovation, aiming to promote gut health and immune modulation without relying on antibiotics.

Vetagro, for instance, offers advanced nutritional solutions like AviPlus®Aqua, a unique blend of botanicals and organic acids. This innovative product is designed to optimize gut health and enhance growth performance in fish and shrimp. Utilizing patented microencapsulation technology, AviPlus®Aqua ensures that active ingredients are protected from leaching in water and thermal stress during feed manufacturing, allowing them to reach the intestinal level where they can exert their beneficial effects. This targeted delivery maximizes performance at lower inclusion rates, offering a crucial component in supporting fish health and resilience against pathogens.