Aquaculture Biotechnology Division, C. Abdul Hakeem College, Melvisharam-632509, India.
Abstracts of the papers presented at the International Conference of Indian Virological Society on “Emerging and Re-emerging viral Diseases of the Tropics and Subtropics” at Indian Agricultural Research Institute, New Delhi, India, December 11–14, 2007.
Macrobrachium rosenbergii is the most important cultured palaemonid in the world and it is now farmed on a large scale in many countries including India. A new disease similar to white tail disease (WTD) reported by Arcier et al. (1999) has been observed with 100% mortality within two or three days in the hatcheries and nursery ponds located in Andhra Pradesh and Tamil Nadu. Two viruses, Macrobrachium rosenbergii nodavirus (MrNV) and extra small virus-like particle (XSV) have been identified as causative agents of WTD. MrNV is a small icosahedral non-enveloped particle, 26–27 nm in diameter, identified in the cytoplasm of connective cells. XSV is also an icosahedral virus and 15 nm in diameter. Clinical signs observed in the infected animals include lethargy, opaqueness of the abdominal muscle, degeneration of the telson and uropods, and 100% within 4 days. The available diagnostic methods to detect WTD include RT-PCR, dot-blot hybridization, in situ hybridization and ELISA. In experimental infection, these viruses caused 100% mortality in post-larvae but failed to cause mortality in adult prawns. The RT-PCR assay revealed the presence of both viruses in moribund post-larvae and in gill tissue, head muscle, stomach, intestine, heart, hemolymph, pleopods, ovaries and tail muscle. Our research work indicates the possibility of marine shrimp acting as reservoir for MrNV and XSV and maintaining their virulence in tissue system of marine shrimp. Experiments were carried out to determine the possibility of vertical transmission of MrNV and XSV in M. rosenbergii. The results indicate that WTD may be transferred from infected brooders to their offspring during spawning. Artemia is a live feed organism to larvae and post-larvae of prawn and experiments were carried out to investigate the possibility that it might transmit these viruses to PL of freshwater prawns. The results showed that Artemia act as a career for these viruses. Replication of MrNV and XSV was investigated in apparently healthy C6/36 subclone of Aedes albopictus cell line. The results revealed that C6/36 cells were susceptible to these viruses. The replication of these viruses in C6/36 cells was confirmed by RT-PCR, acridine orange staining, infectivity study and electron microscopy. The gene encoding CP-17 of XSV was cloned and its sequence analyzed with sequences of other isolates such as French, Chinese, Taiwanese and Thai available in the GenBank. The phylogenetic analysis revealed that the Indian isolate was closely related to Taiwanese and Chinese isolates than French and Thai. This shows that the possible origin of the disease in India was from Taiwan and China through the import of prawn seed.
