Virus-associated white spot syndrome of shrimp in Taiwan: A review

White spot syndrome (WSS) is a viral disease which affects most of the commercially cultivated marine shrimp species, not just in Taiwan but globally. The principal clinical sign of WSS is the presence of white spots on the proximal surface of the cuticle of the diseased shrimp. The causative agent is a dsDNA virus which consists of an enveloped, rod-shaped nucleocapsid. White spot syndrome virus (WSSV) infection is characterized by rapid disease onset and high mortality. The mortality typically reaches 90% within 2 to 7 days after the onset of the disease. Recently, WSSV was also found in the form of a latent infection in freshly caught wild shrimp and crabs. Under stressful conditions, WSSV was triggered to replicate rapidly and subsequently caused the death of these animals. Using polymerase chain reaction (PCR), we successfully detected WSSV in cultured and captured shrimp and crabs. Detection of WSSV in non-cultured arthropods collected from WSSV-affected shrimp farms revealed that copepods, the pest crab Helice tridens, small pest palaemonid prawns, and possibly the larvae of an ephydrian insect were reservoir hosts of WSSV. The tissues targeted by WSSV originate from both the ectoderm and the mesoderm. In situ hybridization and transmission electron microscopy (TEM) showed evidence of WSSV in the reproductive organs of black tiger shrimp Penaeus monodon. Since infected oocytes either undergo necrosis or fail to develop to maturity, transovarial transmission is an unlikely pathway. Transovum transmission is a very real possibility, however, and perhaps one of the most effective ways of dealing with this is simply to rinse or disinfect the nauplii.