Acute Hepatopancreatic Necrosis Disease (AHPND / EMS) in Shrimp

AHPND (early mortality syndrome) is caused by Vibrio parahaemolyticus carrying the PirAB toxin. It destroys the shrimp hepatopancreas, causing mass early mortality.

Overview

Acute hepatopancreatic necrosis disease (AHPND), initially called early mortality syndrome (EMS), is a bacterial disease that has caused major losses in shrimp aquaculture since around 2009. The cause is specific strains of Vibrio parahaemolyticus carrying a virulence plasmid (pVA1, roughly 69 kb) that encodes the binary toxin PirAvp and PirBvp. The disease emerged in southern China and spread to Vietnam, Malaysia, and Thailand by 2012, later reaching the Americas including Mexico. It is recognized as a notifiable disease by the World Organisation for Animal Health (WOAH).

Affected species

The main affected hosts are the principal farmed penaeid shrimp: Pacific white shrimp (Penaeus vannamei, also written Litopenaeus vannamei) and black tiger shrimp (Penaeus monodon). Because these species dominate global shrimp production, AHPND has had severe economic consequences across major producing regions.

Clinical signs

• Mass mortality early in the production cycle, often within the first 20 to 35 days after stocking
• Pale to white hepatopancreas with marked atrophy, reduced in size by half or more
• Empty stomach and empty midgut
• Lethargy and reduced or stopped feeding
• Soft shell and slow growth
• Black streaks or spots in the hepatopancreas in the terminal stage

Pathology and transmission

The bacteria colonize the shrimp stomach and secrete the PirAB toxin, which destroys the hepatopancreas: tubule epithelial cells degenerate, round up, detach, and slough into the tubule lumen, followed by secondary bacterial colonization. Transmission is oral and waterborne, occurring through ingestion of infected tissue, cohabitation with infected shrimp, and contaminated water.

Diagnosis

Diagnosis combines histopathology of the hepatopancreas, which shows characteristic tubule epithelial cell sloughing, with PCR assays that detect the pirA and pirB toxin genes on the virulence plasmid. Toxin-gene PCR distinguishes AHPND-causing strains from non-pathogenic Vibrio parahaemolyticus, which is essential because the species itself is common in the environment.

Treatment, control and prevention

There is no recommended antibiotic cure; routine antibiotic use is discouraged because of resistance and food-safety concerns. AHPND is managed as a microbial-management and biosecurity problem.

• Stock specific-pathogen-free (SPF) and PCR-screened postlarvae from certified hatcheries
• Screen and remove sources of the pathogen from broodstock, water, and live feeds
• Treat incoming and waste water and disinfect ponds and equipment
• Manage pond conditions and stocking density to limit Vibrio blooms
• Use beneficial microbial management, such as probiotics, to suppress opportunistic Vibrio