THE OCCURRENCE, DEVELOPMENT AND HISTOPATHOLOGY OF MONODON BACULOVIRUS IN PENAEUS-MONODON IN SOUTHERN THAILAND

The occurrence, development and histopathology of monodon baculovirus (MBV) in larvae, post-larvae PL) and broodstock of Penaeus monodon was studied over a period of 1 year in southern Thailand. In histological samples of captured male and female broodstock obtained from the Andaman Sea, the incidence of individuals with MBV occlusion bodies (OB's) was approximately 5.7% during the period. Absence of MBV virions in electron micrographs of oocytes of MBV-infected females, mature eggs and nauplii indicated that the virus may not be transmitted transovarially. Together, these facts suggest that elimination of MBV from hatcheries by screening of quarantined wild broodstock would be feasible. In the hatchery, baculovirus OB's began to be clearly identifiable in the hepatopancreas of larvae at the third zoeal stage. During all subsequent larval and PL developmental stages, there was a high incidence of infected individuals, some with extremely large numbers of OB's. The average incidence of PL positive for OB's in histological specimens approached 100% during the period of the study in a survey that included eight commercial PL producers on both the Gulf of Thailand and the Andaman Sea. However, the incidence of OB-positive animals gradually fell as the shrimp grew older and OB's were often undetectable in healthy juveniles derived from infected PL within approximately 2 weeks of stocking in grow-out ponds. In some instances, it was possible to follow groups of PL from the nursery stage through to harvest of market-size shrimp. Average survival data for such shrimp followed in a large number of ponds over a period of 1 year indicated that MBV was well tolerated by Penaeus monodon if other rearing conditions were optimal. However, in the event of environmental or other stress, the incidence of animals showing large numbers of MBV particles increased dramatically.