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White Spot Syndrome
Aetiology
Classification of the Causative Agent
White spot virus (WSV) or white spot syndrome virus (WSSV) is a presently unclassified rod-shaped to obovate, enveloped double-stranded DNA virus with a single filamentous appendage. The cellular location is nuclear and the genome is large at approximately 290 kbp.
Resistance to Physical and Chemical Action
| Temperature: |
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Inactivated at 50C/20 minutes, 70C for 5 minutes, drying at 30C.
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| pH: |
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Inactivated by pH 1 for 10 minutes, pH 3 for 1 hour, pH 12 for 10 minutes at 25C. |
| Chemicals: |
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Inactivated by 15% NaCl 28C for 24 hours, 0.5 g/ml residual ozone for 10 minutes at room temperature, oxidising reagents and ethyl ether. |
| Disinfectants: |
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Inactivated by sodium hypochlorite and povidone iodine at 100 ppm for 10 minutes and 10 ppm for 30 minutes, benzalkonium chloride 75 ppm for 10 minutes. |
| Ultraviolet: |
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Inactivated by 9 105 w s/cm2 for 60 minutes. |
| Survival: |
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At low concentration, remains viable in sterile sea water 5 days at 28C, longer at lower temperatures. Viable in water from outbreak ponds approximately 4 days at 25-28C. |
Epidemiology
Hosts
- WSV is highly infectious for most known species of cultivated penaeid shrimp.
- Natural infections have been recorded from black tiger shrimp (Penaeus monodon), Kuruma shrimp (P. japonicus), Chinese white shrimp (P. chinensis), banana prawns (P. merguiensis and P. inducus), white shrimp (P. vannamei) and other penaeids.
- Until proven otherwise, it is reasonable to assume that all cultured penaeids would be susceptible to infection.
- Natural infections also occur in many species of decapods (crabs, crayfish, lobsters and shrimp) or other crustaceans, although often not lethal.
Transmission
- Horizontal transmission may be direct or vectorial, water being the major abiotic vector.
- Rapid transmission occurs from infected shrimp through the water and by cannibalism of weak or moribund shrimp.
- Animate vectors include over 40 known species of crustaceans.
- The major source of infection for rearing ponds is grossly healthy carrier fry that have acquired the virus from spawners in shrimp hatcheries.
- Transovarial transmission has been confirmed and intra-ovum transmission has not been ruled out. However, if it exists, its frequency is very low.
Sources of Virus
The mode of virus shedding from infected shrimp has not been established, but cohabitants with infected carriers acquire active infections within 36-48 hours.
Infected transport water, intake water, nets and other equipment.
Occurrence
White spot disease (WSD) has been recorded from most Asian countries were penaeid shrimp are pond reared. Original outbreaks were reported from the People's Republic of China in 1993 and they spread rapidly thereafter to Japan, Taipei China and the rest of Asia, but not Australia. Since early 1999, it has been widely reported from shrimp farms in the southern United States of America, Central America and northern South America. Disease outbreaks may occur at all seasons and at all phases of pond rearing, but they seem to be favoured by widely fluctuating environmental conditions. Other precipitating factors may include stress induced by chemical and insecticide residues.
For detailed information on occurrence, see recent issues of World Animal Health and the OIE Web site.
Diagnosis
Gross Signs
- Decreased feed consumption or cessation of feeding and increased mortality in the population.
- Lethargic or moribund shrimp accumulate at the pond surface and edges with slow to erratic swimming behaviour.
- Overall body colour often reddish.
- Minute to large (several mm diameter) white inclusions embedded in the cuticle, especially in the removed carapace held to the light after scraping off attached tissue (not always seen).
Lesions
- There are no pathognomonic gross lesions.
- White inclusions in the cuticle are not always seen, especially in very acute infections. In some cases they may be very small and difficult to resolve.
- Red body coloration not always seen.
- Histopathology a minimum requirement for diagnostic confirmation.
Differential Diagnosis
- High alkalinity may sometimes induce formation of calcium deposits in the cuticle in the absence of WSD.
- Poor water quality in rearing ponds (high ammonia, low oxygen, high and low pH).
Laboratory Diagnosis
Procedures
Confirmation of the agent
- Histopathology by haematoxylin-eosin (H&E)-stained sections shows moderate to large numbers of hypertrophied nuclei with basophilic central inclusions surrounded by marginated chromatin in tissues of ectodermal and mesodermal origin (particularly subcuticular tissues of the stomach, cephalothorax and gills). Early-stage hypertrophied nuclei show Cowdry type-A inclusions.
- Polymerase chain reaction (PCR) of tissues and haemolymph.
- Western blot assay.
- In situ DNA hybridisation.
- Transmission electron microscopy.
Serological tests
- None currently available.
Samples
Identification of the agent
- Whole shrimp fixed for histopathological examination.
- Haemolymph, gill tissue or pleopods homogenised in lysis buffer.
Disease Prevention and Management
Sanitary Prophlaxis
- PCR Screening and elimination of infected broodstock from the hatchery.
- Stocking of ponds with post larvae (PL) of known health status or screened as negative for WSSV by PCR.
- Proper cleaning and disinfection of ponds before stocking, including removal of potential carrier crustaceans.
Elimination or screening of potential carriers from exchange water.
- Avoidance of exchange of equipment amongst ponds.
- Avoidance of the use of fresh aquatic feeds.
- Continuous removal and destruction of moribund and dead shrimp whenever they appear.
In outbreaks
Strict isolation of outbreak ponds with movement controls and control of human traffic.
Destruction of all infected and exposed shrimp by incineration or burial.
Thorough cleaning and disinfection of outbreak ponds.
Treatment
References
CHANG P.S., CHEN L.J. & WANG Y.C. (1998). The effect of ultraviolet irradiation, heat, pH, ozone, salinity and chemical disinfectants on the infectivity of white spot syndrome baculovirus. Aquaculture, 166, 1-17.
LIGHTNER D.V. (1996). Handbook of Pathology and Diagnostic Procedures for Diseases of Penaeid Shrimp. World Aquaculture Society, Baton Rouge, USA.
LO C.F., LEU J.H., HO C.H., CHEN C.H., PENG S.E., CHEN Y.T., CHOU C.M., YEH P.Y., HUANG C.J., CHOU H.Y., WANG C.H. & KOU G.H. (1996b). Detection of baculovirus associated with white spot syndrome (WSBV) in penaeid shrimps using polymerase chain reaction. Dis. Aquat. Org., 25, 133-141.
NADALA E.C.B., TAPAY L.M., CAO S. & LOH P.C. (1997). Detection of yellow-head virus and Chinese baculovirus in penaeid shrimp by the Western blot technique. J. Virol. Methods, 69, 39-44.
Chapter 4.1.2. in the OIE Diagnostic Manual for Aquatic Animal Diseases, OIE (World Organisation for Animal Health), Paris, France.
Chapter 4.1.2. in the OIE International Aquatic Animal Health Code, OIE (World Organisation for Animal Health), Paris, France.
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