| Whispovirus | |
|---|---|
Virus classification | |
| (unranked): | Virus |
| Class: | Naldaviricetes |
| Order: | incertae sedis |
| Family: | Nimaviridae |
| Genus: | Whispovirus |
| Species | |
White spot syndrome virus | |
White spot syndrome (WSS) is aviral infection ofpenaeid shrimp. The disease is highly lethal and contagious, killing shrimp quickly. Outbreaks of this disease have wiped out the entire populations of manyshrimp farms within a few days, in places throughout the world.
White spot syndrome virus (WSSV) is the lonevirus of the genusWhispovirus (white spot), which is the only genus in the familyNimaviridae.[1] It is responsible for causing white spot syndrome in a wide range of crustacean hosts.[2][3]
The disease is caused by a family of related viruses subsumed as the white spot syndrome baculovirus complex[4] and the disease caused by them as white spot syndrome.[5]
The first reported epidemic due to this virus is fromTaiwan in 1992.[6] Reports of losses due to white spot disease came from China in 1993[7] where it led to a virtual collapse of theshrimp farming industry. This was followed by outbreaks in Japan andKorea in the same year,Thailand, India andMalaysia in 1994, and by 1996 it had severely affectedEast Asia andSouth Asia. In late 1995, it was reported in the United States, 1998 inCentral andSouth America, 1999 in Mexico, in 2000 in thePhilippines, and in 2011 inSaudi Arabia.[citation needed]
In 2007, a government study inQueensland, Australia, established that the disease was present in almost 90% of supermarket-sold prawns. In 2016, Australian Government investigations found over 85% of imported prawns had the disease and contemplated prosecution against several importers. It was suggested that some importers had evadedbiosecurity inspection and/or that lax inspection processes by Biosecurity Australia allowed the disease to go undetected. It was claimed that the Australian prawn industry was not advised about the white spot disease in case the publicity might jeopardise any prosecutions.[8] In November 2016, the virus was detected on aprawn farm on theLogan River in south-east Queensland,[9] for the first time in the wild in Australia. The source was not known, but at that pointaquaculture farmers were sure that the disease could be eradicated, and not spread to wild prawns.[10] However, by March 2021 it was apparent that not only had it spread to wild prawns and smallcrabs in the Logan River, south ofBrisbane, but it was also being detected inDeception Bay and was widespread inMoreton Bay, in the Brisbane area, far away from the original infection on the prawn farms. In 2021, theAustralian Government was reviewing its import requirements, and farmers and fishers were urgingAustralian biosecurity laws to include a requirement that imported prawns should be cooked.[11]
White spot syndrome virus is a species ofvirus in the genusWhispovirus, in the familyNimaviridae. It is the only species in this family.[3][2]
WSSV is a rod-shaped,double-stranded,DNA virus, and the size of the enveloped viral particles have been reported to be 240–380 nm long and 70–159 nm in diameter andnucleocapsid core is 120–205 nm long and 95–165 nm in diameter.[2] The virus has an outerlipid bilayer membrane envelope, sometimes with a tail-like appendage at one end of the virion. The nucleocapsid consists of 15 conspicuous verticalhelices located along the long axis; each helix has two parallel striations, composed of 14 globular capsomers, each of which is 8 nm in diameter.[12]
| Genus | Structure | Symmetry | Capsid | Genomic arrangement | Genomic segmentation |
|---|---|---|---|---|---|
| Whispovirus | Ovoid | Enveloped | Circular | Monopartite |
The complete DNA sequence of the WSSV genome has been assembled into a circular sequence of 292,967 bp.[13] It encodes 531 putative open reading frames.
One of the proteins, WSSV449, has some similarity to host protein Tube and can function like Tube by activating the NF-κB pathway.[14]
Viral replication is nuclear; DNA-templated transcription is its method. The virus infects an unusually wide host range of crustaceans.[2][3] Transmission of the virus is mainly through oral ingestion and water-borne routes in farms (horizontal transmission) and vertical transmission (from infected mother prawns) in the case of shrimp hatcheries. The virus is present in the wild stocks of shrimp, especially in the coastal waters adjacent to shrimp-farming regions in Asian countries, but mass mortalities of wild shrimp are yet to be observed.
| Genus | Host details | Tissue tropism | Entry details | Release details | Replication site | Assembly site | Transmission |
|---|---|---|---|---|---|---|---|
| Whispovirus | Crustaceans | Ectoderm; mesoderm | Unknown | Unknown | Nuclear | Nuclear | Contact |
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The virus has a wide host range. While shrimp can survive with the virus for extended periods of time, factors such as stress can cause the outbreak of WSS. The disease is highly virulent and leads tomortality rates of 100% within days in the case of cultured penaeid shrimps. Most of the cultured penaeid shrimps (Penaeus monodon,Marsupenaeus japonicus,Litopenaeus vannamei, andFenneropenaeus indicus) are natural hosts of the virus. Several nonpenaeid shrimp were also found to be severely infected during experimental challenges. Many crustaceans such ascrabs (Scylla spp.,Portunus spp.),spiny lobsters (Panulirus spp.),crayfish (Astacus spp.',Cherax spp.) and freshwater shrimp (Macrobrachium spp.) are reported to be infected with variable severities depending on the lifestage of the host and presence of external stressors (temperature,salinity, bacterial diseases, pollutants).
Clinical signs of WSS include a sudden reduction in food consumption, lethargy, loose cuticle and often reddish discolouration, and the presence of white spots of 0.5 to 2.0 mm in diameter on the inside surface of thecarapace,appendages, and cuticle over the abdominal segments.[2]
In the host, WSSV infects a wide variety ofcells from ectodermal and mesodermal origin. Histological changes are seen in thegill epithelium,antennal gland,haematopoeitic tissue,nervous tissue,connective tissue, and intestinalepithelial tissue. Infected cells have prominent intranuclear occlusions that initially staineosinophilic, but becomebasophilic with age; hypertrophied nuclei with chromatin margination; and cytoplasmic clearing.[15] Pathogenesis involves widespread tissue necrosis and disintegration.
White spots on the shell of infected shrimp underscanning electron microscope appear as large, dome-shaped spots on the carapace measuring 0.3 to 3.0 mm in diameter. Smaller white spots of 0.02 to 0.10 mm appear as linked spheres on the cuticle surface. Chemical composition of the spots is similar to the carapace, calcium forming 80–90% of the total material, and may have derived from abnormalities of the cuticular epidermis.[16]
Several biochemical changes have been reported after infection with this virus:[17]glucose consumption and plasmalactate concentration increase, glucose 6 phosphate dehydrogenase activity increases, andtriglyceride concentration decreases. The voltage-dependent anion channel of themitochondrion is also upregulated.
Infection with WSSV differs from other described penaeid infectionsyellowhead virus (YHV) andinfectious hypodermal and hematopoietic necrosis virus (IHHNV) in the described histological findings, as YHV has a reduced tissue specificity, infecting only the intestinal epithelial tissues and IHHNV causes intranuclear occlusions that stain eosinophilic, but do not change over the course of the infection.[15]
Rapid and specific diagnosis of the virus can be accomplished usingnested[15] orquantitative PCR.[18]
No treatments for WSS are available.
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A large number ofdisinfectants are widely used in shrimp farms and hatcheries to prevent an outbreak. Stocking of uninfected shrimp seeds and rearing them away from environmental stressors with extreme care to prevent contamination are useful management measures. Site selection may be one of the most crucial in preventing WSS. Shrimp farmed in areas with relatively low temperature fluctuations and at water temperatures greater than 29°C had increased resistance to WSSV.[19]
