White spot disease (WSD) is the most serious threat faced by the shrimp farming industry worldwide. WSD was first reported in farmed P. japonicus from Japan in 1992-93, but was thought to have been imported with live infected post-larvae from mainland China. The disease is transmitted vertically from infected brood stock to larvae and horizontally either by ingestion of infected organisms or through carrier organisms. Most crustaceans including all penaeid shrimps (monodon, vannamei, indicus etc.) and crabs can be affected by WSD. All the life stages of shrimp may get infected by this virus.

Causes

White spot disease is caused by a virus called White Spot Syndrome Virus (WSSV). It is a rod-shaped double-stranded DNA virus of 120-150 x 270-290 nm size, assigned to a new virus family, whispoviridae.

Symptoms

• Affected shrimp exhibit anorexia, lethargy, reduced appetite and reddish discoloration
• Sudden reduction in food consumption, loose cuticle with white spots
• Presence of white spots of 0.5 to 2.0 mm in diameter on the inside surface of the carapace, appendages, and cuticle over the abdominal segments.
• In Pacific white shrimp or vannamei shrimp, white spots may not be clearly visible. Mortality of shrimp may start 2-3 days after infection and reach 80-90 per cent within 5-7 days of onset of first mortalities, necessitating emergency harvest.

Diagnosis

WSD may be diagnosed based on gross signs such as the presence of the characteristic white spots, and rapid mortalities. White spots may not be always seen in the early stages of infection in shrimp. WSSV can be detected using polymerase chain reaction (PCR), or with molecular tools such as dot-blot and in situ hybridisation (ISH) tests.

Prevention

• Vertical transmission through infected brooders is also possible, wherein, the virus is transmitted to larvae. Hence, it is always advisable that PCR tested seeds only are stocked in the ponds. There is no treatment for WSSV. Prevention is the only way to avoid the disease.
• Rapid changes in water conditions have to be prevented.
• Avoiding shrimp stress, fresh feeds(crustacean), frequent water exchange is advisable.
• Once infected, treating the pond with chlorine would be of great help to kill infected shrimps and carriers.