Prevalence of White Spot Disease in Penaeus Monodon In Relation To Environmental Changes and the Occurrence of Apoptosis

White s pot disease (WSD) is an important viral disease of cultured penaeid shrimp. Despite the amount of research that has been carried out on this disease, much still remains to be done. This study was undertaken to examine the effect of different environmental parameters on the occurrence and severity of outbreaks of WSD. The study also undertook to investigate the occurrence of apoptosis and presence or activation of related genes that might result from virus infection. Black tiger shrimp Penaeus monodon and pond water samples were collected from four shrimp farms located along the coast of Peninsular Malaysia, to investigate the relationship between environmental changes and the occurrence of WSD. In the study, it was found that occurrence of WSD was more widespread during the wet season, in association with sudden drops in water temperature and salinity. Laboratory experiments were also conducted to investigate the relationship between salinity, temperature and the occurrence of WSD. All the shrimp exposed to sudden increases in temperature and then returned to normal temperature in association with changed salinity were dead within four days of exposure. However, shrimp maintained at low salinity were less susceptible to disease. To study the development of white spot lesions during infection, shrimp were exposed to white spot syndrome virus (WSSV) via the water borne route. At one-hour post exposure, electron microscopy revealed the appearance of white spots, which were oval or ovate in shape. White spots were visible by the naked eye three days post infection. The study also showed that there were two types of white spots, one related to viral infection, and the other related to bacterial infection. An epicommensal Zoolhamnium sp. was able to penetrate the shrimp cuticle at these bacterial white spots. High numbers of apoptotic cell s were identified in moribund P. monodon infected with WSSV. Apoptotic cells showed DNA fragmentation by TUNEL fluorescence assay and gel electrophoresis of DNA extracts, while H&E stained sections revealed nuclear enlargement and chromatin condensation and margination in degenerated cells. The numbers of such cells present in tissues of WSSV infected shrimp increased with increasing severity of infection as determined by gross signs of white spots on the cuticle, number of inclusion bodies in histopathological sections and by single and double-step (nested) peR assay. The results suggested that apoptosis might be implicated in shrimp death caused by this virus.

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