Author(s): Jeff Cowley
Mourilyan virus (MoV) is a newly identified virus that infects penaeid prawns. Spherical to ovoid enveloped particles (85 x 100 nm dia.) possess bunyavirus-like morphology. RT-nested PCR testing has indicated that natural MoV infections occur commonly in black tiger (Penaeus monodon) and Kuruma (Penaeus japonicus) prawns from the wild or farmed commercially in Queensland. In each species, low-level MoV infections can be detected by in situ hybridisation (ISH) in vacuolated 'spheroid bodies' within the lymphoid organ. In heavily infected prawns, MoV is detected throughout the lymphoid organ and in connective tissues of other organs. In some P. japonicus, MoV has been identified in midgut and nerve tissues displaying histopathology consistent with gut-and-nerve syndrome. However, MoV infection has not been consistently observed within these abnormal tissues and additional studies are required to determine the relevance of MoV to this syndrome. Preliminary data suggests that the MoV genome comprises 4 segments of (sense se single-stranded RNA. BLAST searches using open reading frames (ORFs) encoded in 3 segments identified distant relationships to the L (RNA-dependent RNA polymerase), M (G1/G2 glycoprotein) and S (N nucleoprotein) proteins of Uukuniemi virus and other viruses within the genus Phlebovirus of the Bunyaviridae. In phleboviruses, the S RNA segment also contains a small non-structural protein (NSs) gene that is encoded in ambisense. In the MoV S1 RNA there is no ambisense coding strategy but a somewhat larger protein not obviously related to NSs is encoded in the small (S2) fourth RNA segment. Evidence of elevated levels of viral infection associated with disease episodes suggests that MoV may be a significant pathogen of farmed prawns in Queensland and elsewhere in the Asian region.