Viral encephalopathy and retinopathy
Summary from the online OIE Diagnostic Manual
Viral encephalopathy and retinopathy ( VER ), or viral nervous necrosis ( VNN ) has been reported as a serious disease of larval and juvenile and sometimes older marine fish that occurs almost world-wide except for Africa ( 23 ). To date, the disease has been reported in at least 30 fish species, with the greatest impact being in sea bass (Lates calcarifer [ 10 ] and Dicentrarchus labrax [ 2 ]), groupers (Epinephelus akaara [ 22 ], E. fuscogutatus [ 3 ], E. malabaricus [ 6 ], E. moara [ 25 ], E. septemfasciatus [ 9 ], E. tauvina [ 4 ], E. coioides [ 19 ] and Cromileptes altivelis [ 34 ]), jack (Pseudocaranx dentex [ 21 ]), parrotfish (Oplegnathus fasciatus [ 33 ]), puffer (Takifugu rubripes [ 25 ]), and flatfish (Verasper moseri [ 32 ], Hippoglossus hippoglossus [ 11 ], Paralichthys olivaceus [ 26 ], Scophthalmus maximus [ 1 ]).
Virus particles of about 25-30 nm in diameter have been visualised in affected fish, and the agents in striped jack, barramundi, and European sea bass have been characterised and placed in the genus Betanodavirus, family Nodaviridae ( 5, 21 ) . Immunological studies have shown relationships between striped jack nervous necrosis virus ( SJNNV, the type species of the genus Betanodavirus ) and the other betanodaviruses. Genomic classification of betanodaviruses has shown close relationships, with major groupings being SJNNV-type, tiger puffer nervous necrosis virus ( TPNNV ) -type, barfin flounder nervous necrosis virus ( BFNNV ) -type and red-spotted grouper nervous necrosis virus ( RGNNV ) -type ( 27 ) . Complete nucleotide sequences of RNA1 and RNA2 of SJNNV and GGNNV ( a grouper betanodavirus ) have been reported ( 15, 29 ) .
All diseases are characterised by a variety of neurological abnormalities, such as erratic swimming behaviour ( spiral, whirling or belly-up at rest ) and vacuolation of the central nervous tissues. Usually there is also vacuolation of the nuclear layers of the retina. In general, younger fish have more severe lesions; older fish have less extensive lesions and these may show a predilection for the retina ( 23 ) . Intracytoplasmic inclusions have been described in brain cells of European sea bass, barramundi, Japanese parrotfish, and brownspotted grouper. Neuronal necrosis has been described in most species.
Interesting differences with regard to the occurrence and severity of the diseases are shown in Table 1. There are considerable variations in the age at which disease is first noted and the period over which mortality occurs. In general, the earlier the signs of disease occur, the greater is the rate of mortality. Although disease occurrence at the juvenile stages in some species is very rare, mass mortalities often occur at juvenile to young stages in the other fish species, but usually do not reach 100%, indicating the age-dependence of susceptibility ( 23 ) . Mortalities have been reported in production-size European sea bass ( 18 ) and grouper ( 9 ) , but even in these cases mortalities were greatest in younger fish.
It has been demonstrated that vertical transmission of the causative agent occurs in Pseudocaranx dentex and this fact is reflected by the early occurrence of clinical disease. This finding led to the successful control of VNN of larval striped jack, where elimination of virus-carrying broodstock by reverse-transcription polymerase chain reaction and disinfection of fertilised eggs by ozone were applied ( 20, 24 ) . Paradoxically, ovarian infection has also been reported in Dicentrarchus labrax in which disease is usually not seen until about 30 days post-hatch. The mode of transmission/introduction of the viruses, other than in gametes and by cohabitation, has not been demonstrated, but the possibilities include influent water, juvenile fish held on the same site, and carriage on utensils, vehicles, etc. It is possible that these small viruses are quite resistant to environmental conditions ( 8 ) and therefore readily translocated by commercial activities. Vaccination using a recombinant capsid protein is at the experimental stage ( 14, 30 ) .