A field transmission experiment for acute hepatopancreatic necrosis syndrome (AHPNS) in whiteleg shrimp (Penaeus vannamei) was set up in two AHPNS-infected ponds. Six hapas were placed in each pond; of these, three were stocked with healthy shrimp (to investigate transmission via water) and the other three were stocked with both healthy and AHPNS-infected shrimp (to investigate transmission via water and cohabitation). At 10 days post stocking, healthy shrimp in both treatments showed typical signs of AHPNS pathology as seen in naturally
infected shrimp in ponds. Histopathological analysis revealed rounding and sloughing of hepatopancreatic (HP) epithelial cells, reduction in epithelium height, loss of certain cell types (B-, F- and R-cells), and severe haemocytic infiltration around HP tubules. Mortalities were noted in all experimental hapas; however, mortality rates in hapas stocked with both healthy and AHPNS-infected shrimp were higher than in hapas stocked with healthy shrimp only. Isolates of Vibrio bacteria obtained from infected shrimp were identified as V. parahaemolyticus. Polymerase chain reaction (PCR) analysis detected the thl gene from isolates of V. parahaemolyticus but not the thd or trh genes.