Current status of research on yellow-head virus and white-spot virus in Thailand.

Flegel, T.W., Boonyaratpalin, S. & Wuthyachumnarnkul, B.
In: Diseases in Asian aquaculture III. (Eds. Flegel, T.W. & MacRae, I.H.), Fish Health Section, Asian Fisheries Society, Manila, Philippines.
In southern Thailand, yellow-head virus (YHV) first caused extensive losses in cultivation of the black tiger prawn, Penaeus monodon, in late 1992. The same area was subsequently affected by a widespread epizootic from white spot virus (WSV), also called systemic ectodermal and mesodermal baculovirus (SEMBV), white spot baculovirus (WSBV), PmNOBII and PmNOBIII, beginning in later 1994. The pond side losses caused by these two viral diseases may account in substantial part for the 25% drop in shrimp production for the first half of 1995 when compared to the first half of 1994. Intensive research activity has been focused on these two viruses in Thailand since their discovery here in 1992 (YHV) and 1994 (WSV), and this review summarizes the results of that research. Since the work on YHV has been reviewed once already in 1995, only subsequent developments with that virus will be described here. Losses from YHV have continued, although the severity and frequency of outbreaks decreased sharply after the beginning of 1994. However, work has shown that the occurrence of the virus in cultivating ponds is widespread in shrimp that show no gross signs of disease. This constitutes a very rapid change in the consequence of viral exposure and it has led to a hypothesis fpr the existence of a currently unknown mechanisms for rapid tolerance or resistance to viral disease in penaeid shrimp. Another major development with YHV was the discovery that it is an RNA virus. In addition to its characterization, the nucleic acid of YHV was used as the template for PCR amplification with primers designed from conserved sequences of RNA-dependent RNA-polymerase genes of the Rhabdoviridae, and the predicted 450 bp product was obtained. From YHV-RNA, cDNA has been prepared and cloned. Progress in the development of diagnostic techniques using selected probes derived from these clones is described. This includes dot blot hybridization tests, in situ hybridization tests and PCR amplification tests. Pond side losses to WSV in Thailand and other countries in Asia continue to be severe and most concern is now focused on this agent rather than YHV. Seven species of penaeid shrimp and a number of other crustaceans are now believed to be susceptible to this virus. Substantial progress has been made in understanding the nature of this virus and how to control it. The nucleic acid has been purified and characterized as high molecular weight double stranded DNA (168 kbp). As with YHV above, the nucleic acid has been cloned and selected probes from these clones have been prepared and tested as diagnostic tools by dot blot hybridization, in situ hybridization and PCR amplification. The results of this work are described. These tools are being used to test broodstock animals, post larvae and other crustaceans as potential sources of the virus in the shrimp farming system. In terms of prevention and control of YHV and SEMBV disease outbreaks, many of the recommended procedures are similar. Since both viruses can be spread by infected water and by non-cultivated crustacean carriers, current preventative measures emphasize water management and exclusion of carrier species. Finally, there have been several cases of mixed infections of YHV and WSV in Thiland and elsewhere. The impact of these dual infections has not yet been assessed.
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