What is it?
The causative agent is gill-associated virus (GAV), genotype 2 of six distinguished genotypes in the yellowhead complex of viruses. GAV occurs commonly as a chronic infection in healthy broodstock and farmed black tiger prawns in eastern Australia. GAV has been associated with mortalities of up to 80 percent in black tiger prawn ponds in Australia, mainly affecting early to late juveniles.
The disease also affects Brown, Gulf Banana and Kuruma prawn species.
When and where might it occur?
Transmission can be horizontal, directly from the water column and through ingestion of infected material. Vertical transmission can also occur via surface contamination or infection of tissue surrounding the fertilised egg. Viral multiplication and disease appear to be induced by environmental stress.
Experimental infections with GAV indicate that larger (~20 g) kuruma prawns are less susceptible to disease than smaller (~613 g) prawns of the same species.
Diagnosis
The disease often results in high mortality (of up to 80 percent).
Affected prawns may cluster near the surface at the edge of the pond and may initially show a sudden increase in feeding, followed by a sudden decline.
Gross pathological signs are:
- reddening of body and appendages
- biofouling with exoparasites
- emaciation
- pink to yellow colouration of the gills.
Microscopic pathological signs are:
- lymphoid organ necrosis (hypertrophied nuclei, marginated chromatin and vacuolation).
Source: Australian Government, Department of Agriculture, Fisheries and Forestry