Whlte spot syndrome virus (WSSV) is the cause of a widespread epizoot~c in cultured shrimp in Thailand and many other countries in Asia. A number of crustacean and other arthropod species have been proposed as reservoirs for the virus by feeding trials and PCR assays. Ho~vever, detailed histological studies are needed to confirm that suspected carriers have active viral infections. This study was carried out to determine whether 3 common crustacean residents of shrimp culture ponds (the sand crab Portunus pelagicus, the mud crab Scylla serrata, and krill Acetes sp.) could be experimentally infected with WSSV and, if so, what the effect of these infections would be. For krill, 3 routes of infection were tested (injection, ingestion and immersion) while for the crab species, only injection and ingestion were tested. WSSV preparations were made from naturally and experimentally infected Penaeus monodon as viral extracts from tissue homogenates (for injection and immersion) and as cut tissues (for ingestion). As determined by normal histology, electron microscopy and in situ DNA hybridization, all of the test species could be infected with M' SSV. For krill, injection was the most effective route (100% mortality in 3 d), followed by immersion (100% mortality in 5 d ) and ingestion (20% mortality in 9 d). For the crabs, injection was also the most effective route of infection. However, infection dld not necessarily lead to mortality. Mortality for injected sand crabs was 100% in 8 d but for mud crabs only 20% in 9 d . By ingestion exposure, there was no mortality for either species over the 9 d experimental period, even though infection was confirmed by histological examination. Based on the results of this study, the crab species and krill can be considered as viral reservoirs, since they are able to carry the infection and may persist for significant periods in the shrimp farming environment.
A new non-occluded baculo-like virus was detected from black tiger shrimp (Penaeus monodon) with yellow head disease in the southern part of Thailand. Virus particles observed in necrotic tissues of gills and lymphoid organ were bacilliform in shape, ranging from 150-200 nm in length and 40-50 nm in diameter, and were enveloped singly by a trilaminar unit membrane. From the experimental infection trials, it was assumed that this virus was the most virulent among the viruses which have been known in the black tiger shrimp, to date. Pathogenicity and some characters of the virus as well as pathological changes caused by this virus were described.
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