Occurrence and Prevention of White Spot Syndrome (WSSV) in Malaysia

Southeast Asia is a significant area for world shrimp culture. However, in recent years, the production of cultured shrimp has markedly decreased as a result of serious viral disease outbreaks. Especially, the increased severity of widespread White Spot Syndrome Virus (WSSV) infection is the most serious threat to stable aquacultural production. In the case of Malaysia, outbreak of this disease was almost the same as Thailand situation that was WSSV disease occurring as serious problem from 1996. A lot of farms have given up shrimp culture due to heavy loss incurred by WSSV until 1997. Field research was performed through 1998 and 1999 on the occurrences of WSSV at farms in Malaysia. Dark-field microscope observation and Japanese PCR methods for PRDV detection were used for this research, because both methods were confirmed to use for detection of Malaysian strain of WSSV. From this research, WSSV outbreaks were found in Penang, Keda and Sarawak State, and information of the occurrence of viral disease was obtained. Almost all prawns in the ponds were dead by this disease. These dead prawns were showing many clear White spots on their carapace. Pathogenic viruses were confirmed by PCR from the samples WSSV. In this etiological study, as for the infection routes, it could be supposed two ways. One was that the prawn fry had been infected with this virus, and another was from supplied seawater containing the virus particles. As one of the preventive countermeasures against viral diseases, inactivation methods for WSSV were studied. This viral inactivation was tested with formalin and halogenous disinfectants and sodium hypochlorite. These chemicals were mixed with the virus and injected to the healthy prawns. As a result of these experiments, no mortality was shown using0.25% effective chloride in sodium hypochlorite and 0.5 ppm formalin. From these results, sodium hypochlorite of halogenouse disinfectants showed the effective inactivation even under lower concentration. It was suggested that these disinfectants were extremely useful for the WSSV inactivation.