Experimental transmission of white spot syndrome virus (WSSV) from black tiger shrimp Penaeus monodon to the sand crab Portunus pelagicus, mud crab Scylla setrata and krill Acetes sp.

Abstract

White spot syndrome virus (WSSV) is the cause of a widespread epizootic 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. However, detailed histological studies are needed to confirm that suspected carriers have active vital 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 WSSV. 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 did 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.