Hepatopancreatic Necrosis Disease (HPN) is a bacterial disease that poses a significant threat to farmed shrimp populations. It is now more formally known as Acute Hepatopancreatic Necrosis Disease (AHPND) and was previously called Early Mortality Syndrome (EMS). The disease affects commercially important species such as the whiteleg shrimp (Penaeus vannamei) and the giant tiger prawn (Penaeus monodon).
The Pathogen Behind HPN Disease
The cause of AHPND is specific strains of the bacterium Vibrio parahaemolyticus. While many Vibrio bacteria are naturally present in marine environments, the strains causing this disease are unique because they carry a specific plasmid. This plasmid contains the genes that produce a binary toxin comprised of two proteins, PirA and PirB, which are the agents of disease.
Once ingested by the shrimp, the bacteria colonize the hepatopancreas, the primary digestive and metabolic organ. The PirA and PirB toxins are then released, causing rapid destruction of hepatopancreas cells. This leads to dysfunction of the organ and the death of the shrimp, and is characterized as a toxicosis because the damage is caused by the toxin itself.
Transmission occurs through horizontal pathways within the farming environment, including the ingestion of contaminated water, feed, or pond sediment. A significant mode of transmission is the cannibalism of sick or dead shrimp, which releases large amounts of bacteria into the pond. Vertical transmission, where bacteria are passed from infected broodstock to their offspring, is another pathway that can introduce the pathogen into a farm system.
Identifying HPN Disease in Shrimp
Farmers first recognize AHPND through distinct behavioral and physical signs. The most apparent sign is the rapid onset of mass mortality, which can reach 100% within the first 30 to 40 days after stocking post-larvae. Affected shrimp often appear lethargic, stop feeding, and may exhibit erratic swimming patterns before dying.
Upon closer inspection, diseased shrimp show clear internal symptoms, including an empty stomach and midgut from their refusal to eat. The appearance of the hepatopancreas is another sign. In healthy shrimp, this organ is large and dark, but in shrimp with AHPND, it becomes pale, shrunken, and atrophied.
While these signs are strong indicators, a conclusive diagnosis requires laboratory analysis. A reliable method is polymerase chain reaction (PCR), a molecular technique used to detect the PirA and PirB toxin genes in samples from the shrimp or pond. Another diagnostic tool is histopathology, which involves the microscopic examination of hepatopancreas tissue to reveal the characteristic sloughing and destruction of the tubule epithelial cells.
Prevention and Control Measures
Preventing the introduction and spread of the pathogen requires strict biosecurity protocols. These measures include:
- Complete disinfection of ponds, equipment, and water systems between production cycles.
- Installing bird netting to prevent introduction from external sources.
- Securing farm perimeters.
- Controlling the movement of personnel and equipment.
The health and origin of the shrimp post-larvae are another point of control. Farmers are advised to source animals from hatcheries that certify their stock as specific pathogen-free (SPF) or specific pathogen-resistant (SPR). SPF stock is raised in biosecure facilities and tested to be free from AHPND and other known pathogens, while SPR shrimp have been selectively bred for enhanced survival when exposed to the disease.
Maintaining optimal conditions within the pond is another preventative measure. This involves water quality management, including stable levels of dissolved oxygen, pH, and alkalinity to minimize shrimp stress. Proper feed management is also necessary, as overfeeding can lead to excess waste that degrades water quality and allows harmful bacteria to proliferate in the pond sediment.
Proactive health strategies can enhance shrimp resilience. Functional feeds supplemented with additives like organic acids and plant extracts can improve digestive health. The application of probiotics (beneficial bacteria) and prebiotics (compounds that nourish them) helps establish a healthy gut microbiome that can outcompete pathogenic Vibrio species in the pond.
Industry Impact of HPN Disease
The emergence of AHPND has caused substantial economic losses for the global shrimp farming industry. Since it was first identified in 2009, the disease has spread through major production hubs in Asia and the Americas, including China, Vietnam, Thailand, Malaysia, and Mexico. The financial toll from lost production, reduced exports, and associated business closures is estimated to be in the tens of billions of dollars.
This threat has forced an evolution in aquaculture practices toward enhanced biosecurity and more controlled farming systems. Innovations such as indoor, biofloc, and recirculating aquaculture systems (RAS) have gained popularity as they offer greater control over the production environment and reduce pathogen entry. Research in genetics has also led to the commercial availability of shrimp lines with documented resistance to AHPND, providing farmers with a tool to mitigate risk.