White spot baculovirus syndrome in the Indian shrimp Penaeus monodon and P. indicus

Sporadic occurrences of white spot baculovirus (WSBV) infections have been reported in shrimp farms throughout the maritime states of India. WSBV presents as a reddish discolouration with white spots on the exoskeleton and epidermis with muscle opacity. Onset of the disease is extremely rapid with mass mortalities. Infected juveniles and sub-adults of Penaeus indicus and P. monodon become lethargic surface frequently, exhibit loss of balance, with reduced feeding and preening activities. The nuclei of WSBV-infected epithelial (hypodermal), septal and secretory cells of the gill filaments exhibit basophilic hypertrophied nuclei with a reduced cytoplasmic volume. Massive tissue disintegration occurred in the ectodermal and mesodermal tissues. The electron-dense nucleoplasm of the gill epithelial cells is mostly replaced with virions. Electron microscopic examination revealed the presence of double-enveloped, non-occluded, rod-shaped virions with a tube-like or branched extension and empty capsids. The numbers of mitochondria, endoplasmic reticulum (ER) and Golgi were also reduced, as were the numbers of secretory or storage vesicles. WSBV is considered to be the main causative agent responsible for mass mortalities of juveniles and sub-adults in the cultured Indian penaeid shrimp, P. monodon and P. indicus. WSBV is highly pathogenic and readily transmitted from diseased shrimp to healthy susceptible shrimp via, contaminated water, faeces and by scavenging on dead infected shrimp. It may affect all stages of shrimp. The spread of the disease from cultured to natural systems and vice versa cannot be dismissed.