White Spot Syndrome in Shrimp
White spot disease (WSD) or White Spot Syndrome (WSS) is a serious disease of farmed shrimp globally. All penaeid shrimps (monodon, vannamei, indicus etc.) can be affected by WSD. All the life stages of shrimp may get infected by this virus.
Causative organism :- White spot disease is caused by a virus called as White Spot Syndrome Virus (WSSV). WSSV is a rod shaped double stranded DNA virus.
Mode of Transmission :- WSD can be transmitted horizontally as well as vertically. Disease transmission is mainly horizontal through oral ingestion and water/ soil/carrier animals. Many crustaceans such as crabs, squilla, copepods from marine and brackishwater are either hosts or carriers of WSSV, and thereby can transmit disease. Crawfish and freshwater prawn, scampi can also be affected by WSSV. Non-arthropod crustaceans such as Balanus sp and annelid such as polycheate worms can also act as carriers. This virus can survive in water without any host for several days and in soil for longer period. When larvae are stocked in ponds, they can get infected by WSSV through water or carriers if treated water is not used for farming. WSSV infection is most likely during early days of culture (DOC) if the ponds are inadequately dried. Mortality in a pond may start at any time and even as early as within a month of stocking. This should not be confused with early mortality syndrome (EMS) or acute hepatopancreatic necrosis disease (AHPND). 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.
o Affected shrimp exhibit anorexia, lethargy, reddish discoloration
o presence of circular white spots on the carapace and other exoskeletal parts.
o In Pacific white shrimp, or vannamei shrimp, white spots may not be clearly visible.
o 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.
There is no treatment for WSSV. Prevention is the only way to avoid the disease.
• increase acclimation times before stocking
• use non-specific immune stimulants (NSIS) and fortified mineral and vitamin diets to increase stress tolerance
• Consider stocking during times of the year that you know there will not be experiencing severe stresses from sudden changes in temperature and salinity. It has been reported that these types of stresses can precipitate an epizootic in a population that carries the virus.
• use good quality diets and continue the use of NSIS through out the life cycle
• stock at lower densities
• Monitor for the presence of vectors carrying WSSV in the ponds and control them.
• Sample the phyto and zooplankton in the pond before stocking and test by PCR for WSSV. Positive ponds should be avoided.
• Sample ponds frequently. Assure that sick and dying animals and any unusual patterns of mortality are sampled as a routine by PCR and/or histopathology. At the first sign of a problem, harvest the shrimp if you can.