Child level

Same level

Genera

A picorna-like virus is the cause of mortalities in larval barramundi (Lates calcarifer) in Tahiti (Renault et al., 1991).

Possibly a nodavirus?

Epidemiology

Massive losses of juvenile barramundi have been reported by several Australian hatcheries. Larvae are usually first affected about 15-18 days after hatching when the fry are 5-8 mm in length. Earliest signs may be seen as early as 9 days after hatching when the fry are approximately 4 mm in length. Observation reveals a few thin and lethargic fish. This is followed by a rapidly increasing prevalence of pale coloured anorectic, thin fish which display abnormal swimming behaviour, darting or corkscrew. Corneal clouding may be evident. Death is rapid, 70% overnight with a secondary peak 20-30% a few hours later. Many of the survivors are lethargic.

Pathology

Microscopic lesions of vacuolation (often massive) in the brain, spinal cord and retina are practically pathognomonic for this disease.

Associated with these lesions are aggregations of non-enveloped spherical viral particles measuring 25-27 nM in diameter.

Treatment

Control has been achieved by thorough and complete disinfection using iodophors and 1% caustic soda. This is time consuming and costly and it is therefore best to isolate the virus before it enters the hatchery. The virus is not transmitted in the egg or atemia used as food. The logical source of the virus is the water.

References

Glazebrook, J.S., Heasman, M.P. and de Beer, S.W. (1990). Picorna-like viral particles associated with mass mortalities in larval barramundi, Lates calcarifer Bloch. Journal of Fish Diseases 13: 245-249.

Munday, B.L., Nakai, T. and Nguyen, H.D. (1994). Antigenic relationship of the picorna-like virus of larval barramundi, Lates calcarifer Bloch to the nodavirus of larval striped jack, Pseudocaranx dentex (Bloch & Schneider). Aust. Vet. J. 71: 384-385.

Renault, T., Haffner, Ph., Baudin Laurencin, F., Brevil, G. and Bonami, J.R. (1991). Mass mortality in hatchery reared sea bass (Lates calcarifer) larvae associated with the presence in the brain and retina of virus-like particles. Bulletin of the European Association of Fish Pathologists 11: 68-73.