Progress in characterization and control of yellow-head virus of Penaeus monodon

Yellow-head virus was first reported in Thailand in 1992 when it caused pond side losses of approximately 30 million US dollars. This review of the virus includes results of unpublished work by a Thai national task-force on yellow-head. Viral extracts in water remain infective in excess of 72 hours. Infections can also be spread by ingestion of infected animals and planktonic carrier shrimp (Palaemon styliferus) fed with tissue from infected tiger prawns. Similar tests indicated that bird feces were no infectious. There is strong indication that infections are initiated either by fresh feed inputs or by contaminated inlet water drawn soon after discharge from a nearby infected pond. Incubation time is 7 to 10 days. Strategies for limiting spread of the virus include strict hygiene procedures, disinfection of ponds and inlet canals, use of only dry commercial feeds, and fine screening of inlet water to eliminate carrier shrimp. When infected ponds are discharged, neighbors are discouraged from exchanging water for at least three days. There are reports of protection by medicinal plants and of resistance in some animals. Incidence of the virus in captured broodstock is estimated at less than 4 percent. Two repid histological methods have beeen devised to help in diagnosis of the disease in time for an emergency harvest decision. Virus infections are systemic, and rod-shaped, enveloped virions are assembled in the cytoplasm of cells of ectodermal and mesodermal origin. Purified virus particles from hemolymph preparations had a distinctive fringe by negative staining, and nucleic acid extracts from these preparations gave single-stranded RNA. Nucleic acid content, cytoplasmic assembly, and morphology by negative staining indicate that the virus may belong to the family Rhabdoviridae, rather than the Baculoviridae, as suggested in previous reports.