What is it?
The causative agent of white spot disease is white spot syndrome virus, a large DNA virus assigned as the only member of the genus Whispovirus (family Nimaviridae).
The virus infects only crustaceans and appears not to be related to any other known viruses. All decapod crustaceans (order Decapoda), including prawns, lobsters and crabs from marine, brackish or freshwater environments, are considered susceptible to infection. However, the disease has mainly been a problem in farmed prawns.
It is not involved in the parasitic disease, common in finfish, also known as
white spot.
Where and when might it occur?
The virus is known to occur in fresh, brackish and marine water.
Rapid mortalities have been reported in many countries of up to 80 percent or more within three to 10 days.
All life stages are potentially susceptible, from eggs to broodstock.
Vertical transmission occurs from infected broodstock and the horizontal transmission of disease is usually via cannibalism of sick or dying prawns, or directly through contaminated water. Although not required for transmission, vectors of the virus include rotifers, marine molluscs, polychaete worms and non-decapodal crustaceans including Artemia salina, the copepods, non-crustacean arthropods and insect larvae. Birds can also transmit the disease from pond to pond by releasing caught prawns over neighbouring ponds.
White spot syndrome virus can persist and retain infectivity in seawater at 30˚C for at least 30 days (under laboratory conditions) and for at least four days in ponds.
Viral multiplication and disease appears to be induced by environmental and handling stress such as eye-stalk ablation, spawning, moulting, changes in salinity, temperature and pH and during plankton blooms. Imposing such stressors on suspect populations can a useful method to increase the probability of detecting virus.
Diagnosis
The disease often leads to the rapid onset of mass mortality (80 percent or more) in farmed penaeid prawns during the grow-out period. Other signs of the disease include lethargy, cessation of feeding and aggregations of moribund prawns near the water surface at the edge of the pond or tank.
Gross pathological signs are:
- loose carapace
- high degrees of colour variation, with a predominance of darkened (red-brown or pink) body surface and appendages
- heavy fouling of the surface and gills by external parasites
- white midgut line through the abdomen of severely affected larvae and postlarvae
- white calcium deposits embedded in the shell, causing white spots 0.53.0 mm in diameter
- delayed (or sometimes completely absent) clotting reaction of the haemolymph of infected shrimp
Prawns with white spot disease may not show distinctive clinical signs. If present, shell lesions can range from minute spots to discs that are several millimetres in diameter and may coalesce into larger plates. They are most easily observed by removing the cuticle over the cephalothorax, scraping away any attached tissue with the thumbnail and holding the cuticle up to the light. White spots in the cuticle are unreliable, even for preliminary diagnosis of white spot disease because similar spots can be produced by some bacteria, high alkalinity and other infectious or environmental conditions.
Microscopic pathological signs are:
- hypertrophied nuclei in gills and/or cuticular epithelium
- viral aggregates (shown as small reflective spots) in unstained smear preparations of the haemolymph by dark-field microscopy
- pathognomonic inclusion bodies in histological sections in target tissues
Source: Australian Government, Department of Agriculture, Fisheries and Forestry