| Summary: | Clinical signs and lesions of Taura syndrome virus (TSV) infection in Penaeus monodon were investigated by histological and in situ hybridization (ISH) analyses. Mortality among P. monodon inoculated with 2 genotypic variants of TSV (Th04Pm and Th04Lv) appeared on Day 3, with 2 out of 10 shrimp dying. Severe necrosis of cuticular epithelial cells and lymphoid organ spheroids, indicative of acute and chronic phase lesions of TSV infection, respectively, were detected in the samples. Both Th04Pm and Th04Lv belonged to a phylogenetic family of Asian TSV isolates. The results demonstrate that both mortality and histological lesions are associated with TSV infection in P. monodon.Infection with 4 genotypic variants of TSV (Bz01, Th04, UsHi94, and Ve05) in TSV-resistant (TSR) and TSV-susceptible (Kona) Litopenaeus vannamei was investigated. Survival probabilities of TSR shrimp were higher than those for Kona shrimp with all 4 variants. Th04, UsHi94, and Ve05 gave no Taura syndrome lesions with TSR shrimp. In contrast, TSR shrimp challenged with Bz01 and Kona shrimp with all 4 TSV variants exhibited severe necrosis of cuticular epithelial cells and lymphoid organ spheroids. Real-time reverse transcription polymerase chain reaction (RT-PCR) revealed that mean TSV copy numbers in TSR shrimp infected with Bz01, Th04, and UsHi94 were significantly (p < 0.0005) lower than those in Kona shrimp. In contrast, mean TSV copy numbers in TSR and Kona shrimp infected with Ve05 were not significantly different (p > 0.4). The results show that TSR L. vannamei are susceptible to infection but give high survival rates following challenge by all 4 variants of TSV.To identify the viral replication site within shrimp infected cells, the viral RNA was located in association with virus-induced membrane rearrangement by electron microscopic ISH. Ultrastructure in the infected cells, analyzed by transmission electron microscopy, included the induction and proliferation of intracellular vesicle-like membranes, while the intracytoplasmic inclusion bodies and pyknotic nuclei were frequently seen. TSV RNA and TSV particles were found to be associated with the membranous structures. The results suggest that the proliferating membranes carry the RNA replication complex and that they are the site of nascent viral RNA synthesis.
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