White spot syndrome
Encyclopedia
White spot syndrome is a viral
infection of penaeid shrimp
. The disease is highly lethal and contagious, killing shrimps quickly. Outbreaks of this disease have wiped out within a few days the entire populations of many shrimp farm
s throughout the world.
The disease is caused by a family of related viruses subsumed as the White Spot Syndrome Baculovirus Complex (WSSV) and the disease caused by them as white spot syndrome (WSS).
in 1992, however, reports of losses due to white spot disease came from China
in 1993, where it led to a virtual collapse of the shrimp farming industry. This was followed by outbreaks in Japan
and Korea
in the same year, Thailand
, India
and Malaysia in 1994 and by 1996 it had severely affected East Asia
and South Asia
. In late 1995, it was reported in the USA, 1998 in Central
and South America
, 1999 in Mexico
and in 2000 in the Philippines
. Currently, it is known to be present in all shrimp growing regions except Australia
.
, and the size of the enveloped viral particles have been reported to be 240–380 nm
long and 70–159 nm in diameter and nucleocapsid core is 120–205 nm long and 95–165 nm in diameter. The virus has an outer lipid
bilayer membrane envelope, sometimes with a tail like appendage at one end of the virion. The nucleocapsid consists of 15 conspicuous vertical helices
located along the long axis, each helix has two parallel striations, composed of 14 globular capsomers, each of which is 8 nm in diameter.
The complete DNA sequence of WSSV genome has been assembled into a circular sequence of 292,967 bp
. 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.
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 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 shrimps are yet to be observed.
s of 100% within days in the case of cultured penaeid shrimps. Most of the cultured penaeid shrimps (Penaeus monodon
, Marsupenaeus japonicus, Litopenaeus vannamei, Fenneropenaeus indicus) are natural hosts of the virus. Several non-penaeid shrimps were also found to be severely infected during experimental challenges. Many crustaceans like crab
s (Scylla
spp., Portunus
spp.), spiny lobster
s (Panulirus
spp.), crayfish
(Astacus
spp., Cherax
spp.) and freshwater shrimp (Macrobrachium
spp.) are reported to be infected with variable severities depending on the life stage of the host and presence of external stressors (temperature
, salinity
, bacterial diseases, pollutants).
Clinical signs of WSSV 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 the carapace
, appendage
s and cuticle over the abdominal segments.
s from ectodermal and mesodermal origin. Histological changes are seen in the gill
epithelium, antennal gland, haematopoeitic tissue, nervous tissue
, connective tissue
and intestinal epithelial tissue. Infected cells have prominent intranuclear occlusions that initially stain eosinophilic
but become basophilic
with age; hypertrophied nuclei with chromatin margination; and cytoplasmic clearing. Pathogenesis involves widespread tissue necrosis and disintegration.
White spots on the shell of infected shrimp under scanning electron microscope
appear as large dome shaped spots on the carapace measuring 0.3 to 3 mm in diameter. Smaller white spots of 0.02 to 0.1 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 it is suggested to have derived from abnormalities of the cuticular epidermis.
A number of biochemical changes have been reported after infection with this virus: glucose
consumption and plasma lactate
concentration increase, glucose 6 phosphate dehydrogenase activity increases and triglyceride
concentration decreases. The voltage dependent anion channel of the mitochondrion
is also up regulated.
Rapid and specific diagnosis of the virus can be accomplished using nested or real-time
polymerase chain reaction
(PCR).
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
infection of penaeid shrimp
Penaeidae
Penaeidae is a family of prawns, although they are often referred to as penaeid shrimp. It contains many species of economic importance, such as the tiger prawn , whiteleg shrimp, Atlantic white shrimp and Indian prawn. Many prawns are the subject of commercial fishery, and farming, both in marine...
. The disease is highly lethal and contagious, killing shrimps quickly. Outbreaks of this disease have wiped out within a few days the entire populations of many shrimp farm
Shrimp farm
A shrimp farm is an aquaculture business for the cultivation of marine shrimp or prawns for human consumption. Commercial shrimp farming began in the 1970s, and production grew steeply, particularly to match the market demands of the United States, Japan and Western Europe...
s throughout the world.
The disease is caused by a family of related viruses subsumed as the White Spot Syndrome Baculovirus Complex (WSSV) and the disease caused by them as white spot syndrome (WSS).
History
The first reported epidemic due to this virus is from TaiwanTaiwan
Taiwan , also known, especially in the past, as Formosa , is the largest island of the same-named island group of East Asia in the western Pacific Ocean and located off the southeastern coast of mainland China. The island forms over 99% of the current territory of the Republic of China following...
in 1992, however, reports of losses due to white spot disease came from China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
in 1993, where it led to a virtual collapse of the shrimp farming industry. This was followed by outbreaks in Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
and Korea
Korea
Korea ) is an East Asian geographic region that is currently divided into two separate sovereign states — North Korea and South Korea. Located on the Korean Peninsula, Korea is bordered by the People's Republic of China to the northwest, Russia to the northeast, and is separated from Japan to the...
in the same year, Thailand
Thailand
Thailand , officially the Kingdom of Thailand , formerly known as Siam , is a country located at the centre of the Indochina peninsula and Southeast Asia. It is bordered to the north by Burma and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the...
, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
and Malaysia in 1994 and by 1996 it had severely affected East Asia
East Asia
East Asia or Eastern Asia is a subregion of Asia that can be defined in either geographical or cultural terms...
and South Asia
South Asia
South Asia, also known as Southern Asia, is the southern region of the Asian continent, which comprises the sub-Himalayan countries and, for some authorities , also includes the adjoining countries to the west and the east...
. In late 1995, it was reported in the USA, 1998 in Central
Central America
Central America is the central geographic region of the Americas. It is the southernmost, isthmian portion of the North American continent, which connects with South America on the southeast. When considered part of the unified continental model, it is considered a subcontinent...
and South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
, 1999 in Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
and in 2000 in the Philippines
Philippines
The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...
. Currently, it is known to be present in all shrimp growing regions except Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
.
Virology
WSSV is a rod-shaped double-stranded DNA virusDNA virus
A DNA virus is a virus that has DNA as its genetic material and replicates using a DNA-dependent DNA polymerase. The nucleic acid is usually double-stranded DNA but may also be single-stranded DNA . DNA viruses belong to either Group I or Group II of the Baltimore classification system for viruses...
, and the size of the enveloped viral particles have been reported to be 240–380 nm
Nanometre
A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...
long and 70–159 nm in diameter and nucleocapsid core is 120–205 nm long and 95–165 nm in diameter. The virus has an outer lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
bilayer membrane envelope, sometimes with a tail like appendage at one end of the virion. The nucleocapsid consists of 15 conspicuous vertical helices
Helix
A helix is a type of smooth space curve, i.e. a curve in three-dimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. Examples of helixes are coil springs and the handrails of spiral staircases. A "filled-in" helix – for...
located along the long axis, each helix has two parallel striations, composed of 14 globular capsomers, each of which is 8 nm in diameter.
The complete DNA sequence of WSSV genome has been assembled into a circular sequence of 292,967 bp
Base pair
In molecular biology and genetics, the linking between two nitrogenous bases on opposite complementary DNA or certain types of RNA strands that are connected via hydrogen bonds is called a base pair...
. 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.
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 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 shrimps are yet to be observed.
Clinical
The virus has a wide host range and is highly virulent and leads to mortality rateMortality rate
Mortality rate is a measure of the number of deaths in a population, scaled to the size of that population, per unit time...
s of 100% within days in the case of cultured penaeid shrimps. Most of the cultured penaeid shrimps (Penaeus monodon
Penaeus monodon
Penaeus monodon, the giant tiger prawn , is a marine crustacean that is widely reared for food.-Distribution:...
, Marsupenaeus japonicus, Litopenaeus vannamei, Fenneropenaeus indicus) are natural hosts of the virus. Several non-penaeid shrimps were also found to be severely infected during experimental challenges. Many crustaceans like crab
Crab
True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...
s (Scylla
Scylla
In Greek mythology, Scylla was a monster that lived on one side of a narrow channel of water, opposite its counterpart Charybdis. The two sides of the strait were within an arrow's range of each other—so close that sailors attempting to avoid Charybdis would pass too close to Scylla and vice...
spp., Portunus
Portunus
Portunus is a genus of crab which includes several important species for fisheries, such as the blue swimming crab, Portunus pelagicus and the Gazami crab, P. trituberculatus . The genus Portunus contains more than 90 extant species and over 40 further species known only from fossils ....
spp.), spiny lobster
Spiny lobster
Spiny lobsters, also known as langouste or rock lobsters, are a family of about 45 species of achelate crustaceans, in the Decapoda Reptantia...
s (Panulirus
Panulirus
Panulirus is a genus of spiny lobsters in the family Palinuridae, including those species which have long flagella on their first antennae. It contains the following species:* Panulirus argus — Caribbean spiny lobster...
spp.), crayfish
Crayfish
Crayfish, crawfish, or crawdads – members of the superfamilies Astacoidea and Parastacoidea – are freshwater crustaceans resembling small lobsters, to which they are related...
(Astacus
Astacus
Astacus is a genus of crayfish found in Europe and western Asia, comprising three extant and four extinct species....
spp., Cherax
Cherax
Cherax is the largest and most widespread genus of fully and partially aquatic crayfish in the Southern Hemisphere. Its members may be found in lakes, rivers and streams across most of Australia and New Guinea. In Australia the many species of Cherax are commonly known as yabbies...
spp.) and freshwater shrimp (Macrobrachium
Macrobrachium
Macrobrachium is a genus of freshwater shrimp characterised by the extreme enlargement of the second pair of pereiopods, at least in the male...
spp.) are reported to be infected with variable severities depending on the life stage of the host and presence of external stressors (temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, salinity
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
, bacterial diseases, pollutants).
Clinical signs of WSSV 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
Millimetre
The millimetre is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length....
in diameter on the inside surface of the carapace
Carapace
A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...
, appendage
Appendage
In invertebrate biology, an appendage is an external body part, or natural prolongation, that protrudes from an organism's body . It is a general term that covers any of the homologous body parts that may extend from a body segment...
s and cuticle over the abdominal segments.
Pathology
In the host WSSV infects a wide variety of cellCell
-Science and technology:*Cell , the functional basic unit of life*Cell , a term used in an electronic circuit design schematics*Cell , a three-dimensional element, part of a higher-dimensional object*Cell , a scientific journal...
s from ectodermal and mesodermal origin. Histological changes are seen in the gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
epithelium, antennal gland, haematopoeitic tissue, nervous tissue
Nervous tissue
Nervous tissue is one of four major classes of vertebrate tissue.Nervous tissue is the main component of the nervous system - the brain, spinal cord, and nerves-which regulates and controls body functions...
, connective tissue
Connective tissue
"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...
and intestinal epithelial tissue. Infected cells have prominent intranuclear occlusions that initially stain eosinophilic
Eosinophilic
Eosinophilic refers to the staining of certain tissues, cells, or organelles after they have been washed with eosin, a dye.Eosin is an acidic dye; thus, the structure being stained is basic....
but become basophilic
Basophilic
Basophilic is a technical term used by histologists. It describes the microscopic appearance of cells and tissues, as seen down the microscope, after a histological section has been stained with a basic dye. The most common such dye is haematoxylin....
with age; hypertrophied nuclei with chromatin margination; and cytoplasmic clearing. Pathogenesis involves widespread tissue necrosis and disintegration.
White spots on the shell of infected shrimp under scanning electron microscope
Scanning electron microscope
A scanning electron microscope is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern...
appear as large dome shaped spots on the carapace measuring 0.3 to 3 mm in diameter. Smaller white spots of 0.02 to 0.1 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 it is suggested to have derived from abnormalities of the cuticular epidermis.
A number of biochemical changes have been reported after infection with this virus: glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
consumption and plasma lactate
Lactate
Lactate may refer to:*The act of lactation*The conjugate base of lactic acid...
concentration increase, glucose 6 phosphate dehydrogenase activity increases and triglyceride
Triglyceride
A triglyceride is an ester derived from glycerol and three fatty acids. There are many triglycerides, depending on the oil source, some are highly unsaturated, some less so....
concentration decreases. The voltage dependent anion channel of the mitochondrion
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
is also up regulated.
Diagnosis
Infection with WSSV differs from other described penaeid infections Yellowhead virus (YHV) and Infectious 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 eosinophillic but do not change over the course of the infection.Rapid and specific diagnosis of the virus can be accomplished using nested or real-time
Real-time polymerase chain reaction
In molecular biology, real-time polymerase chain reaction, also called quantitative real time polymerase chain reaction or kinetic polymerase chain reaction , is a laboratory technique based on the PCR, which is used to amplify and simultaneously quantify a targeted DNA molecule...
polymerase chain reaction
Polymerase chain reaction
The polymerase chain reaction is a scientific technique in molecular biology to amplify a single or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence....
(PCR).