Taura syndrome
Encyclopedia
Taura syndrome is one of the more devastating diseases affecting the shrimp farming industry worldwide.
Taura syndrome was first described in Ecuador
during the summer of 1992. In March 1993, it returned as a major epidemic
and was the object of extensive media coverage. Retrospective studies have suggested that a case of Taura syndrome might have occurred on a shrimp farm
in Colombia
as early as 1990 and that the virus was already present in Ecuador in mid-1991. Between 1992 and 1997 the disease spread to all major regions of the Americas where Penaeus vannamei is cultured. It is estimated that the economic impact of TS in the Americas during that period might have exceeded 2 billion US dollars.
disease in banana plantations.
The outbreak of black leaf disease led to an increase in fungicide
usage within the Taura River
basin district near the city of Guayaquil
. It was thought that the fungicides Tilt (propiconazole
, Ciba-Geigy) and Calixin (tridemorph, BASF
) used to control black leaf, ran off into nearby ponds and were responsible for the disease. Analytical data demonstrated propiconazole
in water, sediments and hepatopancreas
tissues of shrimp harvested from affected farms in Ecuador. No other pesticides were discovered.
In January 1994, at the request of Ciba-Geigy, a workshop on Taura syndrome was held at the Aquaculture Pathology Laboratory of the University of Arizona
. Experts from several countries with expertise in shrimp and insect pathology, shrimp nutrition, toxicology, myocology, water quality and farm management participated in the workshop. Industry representatives also participated. The group developed recommendations as to the standardization of the research on Taura syndrome and suggested that studies be done to evaluate whether fungicide or an as-yet unrecognized agent was responsible for the syndrome.
Dr. Jim Brock, the aquatic disease specialist for the State of Hawaii during this period,first demonstrated the that the disease could be transmitted by feeding Taura victims to healthy shrimp in early 1994. The dying test shrimp were then fed to a new set of shrimp who was dying at the same rate. Rivers' postulates were fulfilled in 1994 by Dr. Ken Hasson and co-researchers at the University of Arizona. This proved the viral etiology of the syndrome. The virus was named Taura syndrome virus often referred to as TSV. The virus is referred by the name of Infectious cuticular epithelial necrosis virus (ICENV) by some authors in Latin America. Taura syndrome is a notifiable disease by the Office international des Épizooties (OIE), which reflects the serious nature and devastating impact of the disease.
family, genus Cripavirus
. It has since been reassigned to a second genus in the same family - the Aparavirus.
TSV is a 32 nm
non-enveloped particle with an icosahedral morphology and a buoyant density of 1.338g/ml. The genome is single-stranded positive-sense and has 10,205 nucleotide
s (excluding the 3' poly-A tail
). The capsid
consists of three major proteins: CP1 (40 kDa), CP2 (55 kDa) and CP3 (24 kDa) alongside a minor protein of 58 kDa.
Audelo-del-Valle in 2003 reported that certain primate cell lines could be used to culture TSV. Later studies demonstrated that their report was based on misinterpretated data. TSV does not appear to be a potential zoonosis. All virus amplifications require the use of live shrimp, as there is no continuous cell line that supports the growth of shrimp viruses.
and have resulted in the emergence of several genetic variants of the virus. As of May 2009, four genetic clusters are recognized: Belize
(TSV-BZ), America (TSV-HI), South-east Asia and Venezuela
. The Belize strain is considered the most virulent. Point mutations in TSV capsid
proteins might provide specific isolate with selective advantages such as host adaptability, increased virulence
or increased replication ability. Even small variations in the TSV genome
can result in substantial differences in virulence
.
All TSV variants are similar in shape and size with light variations. The average size of TSV-BZ virus particles is 32.693+/- 1.834 nm compared to TSV-HI with a size of 31.485 +/- 1.187 nm. The region of highest genetic difference is within the capsid protein CP2 with pairwise comparison
of nucleotide showing a 0 to 3.5% difference amongst isolates. The most variations in CP2 occur at the 3'-terminal sequence, this may be because it is less constrained by structural requirements and more exposed than other regions of the protein.
, Peru
, Brazil
, El Salvador
, Guatemala
, Honduras
, Belize, Mexico
, Nicaragua
, Panama
, Costa Rica
, Venezuela as well as from the States of Hawaii, Texas
, Florida
and South Carolina
. Until 1998, it was considered to be a Western Hemisphere virus. The first Asian outbreak occurred in Taiwan
. It has more recently been identified in Thailand
, Myanmar
, China
, Korea
and Indonesia
where it has been associated with severe epizootics in farmed Penaeus vannamei and Penaeus monodon.
The wide distribution of the disease has been attributed to the movement of infected host stocks for aquaculture purposes. This might have been helped by the highly stable nature of the virus. It is thought that importation of TSV-infected Penaeus vannamei from the Western Hemisphere was at the origin of the outbreak in Taiwan. This was further suggested by the genomic similarity of the Taiwan and Western Hemisphere isolates. TSV appeared in Thailand in 2003. Due to the similarities in deduced CP2 amino acids sequence and the chronology of the disease outbreaks in relation to imported stocks it is likely that at least some of the Thai isolates originated from Chinese stocks.
Taura syndrome can spread rapidly when introduced in news areas. A shrimp farmer described the 1995 outbreak in Texas as "This thing spread like a forest fire... There was no stopping it. I just sat there and watched it, and in a matter of three days, my shrimp were gone. Dead!" (reported in Rosenberry: Shrimp news international in 1995).
s.
There is variation within species and TSV-resistant strains of shrimp have been developed. Wild stocks are showing increased resistance, perhaps through intense natural selection. Reports of Taura syndrome in the wild are limited, but in February 1995, the Mexican Fisheries Ministry reported the presence of TSV in wild-type shrimp captured on the border of Mexico and Guatemala. As of 2007, there were no confirmed reports indicating that TSV is infectious to other groups of decapod or non-decapod crustaceans.
After the initial infection, the acute phase develops. Clinical signs can occur as early as 7 hours post-infection in some individuals and last for about 4–7 days. Infected shrimp display anorexia
, lethargy and erratic swimming behavior. They also present opacification of the tail musculature, soft cuticle and, in naturally occurring infection, a red tail due to the expansion of the red chromatophores. Mortality during this phase can be as high as 95%. The acute phase is characterized histologically by multifocal areas of nuclear pyknosis/karyorrhexis and numerous cytoplasmic inclusion bodies in the cuticular epithelium and the subcutis of the general body surface, all appendages, gills, hindgut, esophagus and stomach. The pyknosis and karyorrhexis give a “buckshot” appearance to the tissue and are considered pathognomonic
for the disease. In severe infections the antennal gland
tubule epithelium
, the hematopoietic tissues and the testis are also affected. This occurs mainly in severe infection following injection of viral particles and has not been reported from naturally infected Penaeus vannamei. Shrimp that survive the acute stage enter a transitional stage.
Shrimp in the transitional phase show randomly distributed melanized
(brownish/black) lesions within of the cuticle of the cepahlothorax and tail region. These foci are the sites of acute lesions which have progressed onto subsequent stages of hemocytic inflammation, cuticular epithelium regeneration and healing, and which might be secondarily infected with bacteria. These foci are negative for TSV by in situ hybridization
(ISH) using a TSV specific cDNA
probe. Histologically these shrimp present focal active acute lesions and the onset of lymphoid organ spheroids (LOS) development. By ISH with TSV specific probes a diffuse positive signal can be observed within the walls of the lymphoid organ of normal appearance with or without focal probe signals within developing LOS. These shrimp will be lethargic and anorexic, possibly because of the redirection of all their energy and metabolic resources toward wound repair and recovery. If the shrimp undergo another successful moult
following the transitional phase they will cast off the melanized lesions and enter the chronic phase.
The chronic phase is first seen 6 days post-infection and persist for at least 12 months under experimental conditions. This phase is characterized histologically by the absence of acute lesions and the presence of LOS of successive morphologies. These LOS are positive by ISH for TSV. A low prevalence of ectopic spheroids can also be observed in some cases. LOS are not by themselves characteristic of TSV infection and can be found in other viral diseases of shrimp such as lymphoid organ vacuolization virus (LOVV), lymphoid parvo-like virus (LPV), lymphoid organ virus (LOV), rhabdovirus of penaeid shrimp
(RPS) and yellow head virus
(YHV). Diagnosis of the disease during the chronic phase is problematic as shrimp do not display any outward signs of the disease and do not show mortality from the infection. Survivors may become carrier for life. Shrimp with chronic TSV infection are as vigorous as uninfected shrimp, as demonstrated by their inability to tolerate a salinity drop as well as uninfected shrimp.. A 2011 study by Laxminath Tumburu looked at the interrelationship between an environmental stressor (pesticide Endosulfan) and Taura Syndrome Virus (TSV), and their interactions on the susceptibility and molting of marine penaeid shrimp L. vannamei, and found that the interference of endosulfan-associated stress lead to increasingly higher susceptibility at postmolt stage during the acute phase of the TSV disease cycle..
of dead infected shrimp. The virus can be spread from one farm to another by seagulls and aquatic insects. Infectious TSV has been found in the feces of laughing gulls (Larus atricilla) that fed on infected shrimp during an epizootic in Texas. Controlled laboratory studies have documented that TSV remains infectious for up to one day after passage through the gut of white leghorns chicken (Gallus domesticus) and laughing gulls. Although, vertical transmission is suspected this has not been experimentally confirmed.
Shrimp surviving a TSV infection are life-long carriers of the viruse and are a significant source of virus for susceptible animals. It has been hypothesized that TSV was introduced to Southeast Asia with chronically infected shrimp imported from the Western Hemisphere. The ability of TSV to remain at least partly infectious after one or several freeze-thaw cycles might be a contributing factor facilitating its spread in the international commerce of frozen commodity products. Mechanisms by which infected frozen shrimp could spread the virus include: reprocessing of shrimp at processing plants with release of infectious liquid wastes, disposal of solid wastes in landfills where seagulls could acquire the virus and then spread it, the use of shrimp as bait by sport fishermen and the use of imported shrimp as fresh food for other aquatic species.
s used for routine evaluation. Predatory birds are attracted to diseased ponds and feed heavily on the dying shrimp. The unique signs of infection caused by TS such as the cuticular melanized spots can provide a strong presumptive diagnosis
, but care must be taken as these can be confused with other diseases, such as bacterial shell disease. In general pathognomonic
histopathological lesions are the first step in confirmatory diagnosis. Discrete foci of pyknotic and karyorhectic nuclei
and inflammation are seen within the cuticular tissues. The lymphoid organ might display spheroids, but is otherwise unremarkable.
The genome
of the virus
has been cloned and cDNA
probes are available for diagnosis. Reverse transcriptase polymerase chain reaction (RT-PCR) methods have been developed for detection of TSV and are very sensitive. Real-time techniques allow for quantification of the virus. The IQ2000TM TSV detection system, a RT-PCR method, is said to have a detection limit of 10 copies per reaction.
RNA-based methods are limited by the relative fragility of the viral RNA. Prolonged fixation in Davidsons' fixative might result in RNA degradation due to fixative-induced acid hydrolysis. An alternative for virus detection is the utilization of specific monoclonal antibodies
(MAbs) directed against the relatively stable proteins in the viral capsid. Rapid diagnostic tests using MAbs are now in common use for white spot syndrome virus and are being marketed under the commercial name of Shrimple. Similar tests for TSV, Yellowhead virus and Infectious Hypodermal and Haematopoietic Necrosis Virus are currently under development.
(SPF) or specific pathogen resistant (SPR) shrimp. Relatively simple laboratory challenges can be used to predict the performance of selected stocks in farm where TSV is enzootic. The resistant shrimp lines currently raised have reached nearly complete resistance to some TSV variants and further improvement due to breeding for TSV-resistance is expected be minor for these variants. Significant improvements in TSV survival were made through selective breeding despite low to moderate heritability for this trait.
A management strategy used to reduce the impact of TS has been the practice of stocking post larva shrimp at increased stocking density. Following this strategy, farms would experience mortality due to TS at an early stage of the production cycle, before substantial feeding had begun, and the surviving shrimp would be resistant to further TSV challenges. Other techniques used with limited efficacy have been the polyculture of shrimp with tilapia and maintenance of near optimal water quality conditions in the grow-out ponds with reduction of organic loading. Transgenic shrimp expressing an antisense TSV coat protein (TSV-CP) exhibited increased survival in TSV challenges. The public perception of transgenic animals as well as current technical limitations, limit the use of transgenic animals as a mean of disease control.
Taura syndrome was first described in Ecuador
Ecuador
Ecuador , officially the Republic of Ecuador is a representative democratic republic in South America, bordered by Colombia on the north, Peru on the east and south, and by the Pacific Ocean to the west. It is one of only two countries in South America, along with Chile, that do not have a border...
during the summer of 1992. In March 1993, it returned as a major epidemic
Epidemic
In epidemiology, an epidemic , occurs when new cases of a certain disease, in a given human population, and during a given period, substantially exceed what is expected based on recent experience...
and was the object of extensive media coverage. Retrospective studies have suggested that a case of Taura syndrome might have occurred on a 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...
in Colombia
Colombia
Colombia, officially the Republic of Colombia , is a unitary constitutional republic comprising thirty-two departments. The country is located in northwestern South America, bordered to the east by Venezuela and Brazil; to the south by Ecuador and Peru; to the north by the Caribbean Sea; to the...
as early as 1990 and that the virus was already present in Ecuador in mid-1991. Between 1992 and 1997 the disease spread to all major regions of the Americas where Penaeus vannamei is cultured. It is estimated that the economic impact of TS in the Americas during that period might have exceeded 2 billion US dollars.
Overview
The 1992 Ecuadorian TS epidemic occurred concurrently with an outbreak of black leaf wiltBlack leaf
Black Leaf is a physiological plant disorder that affects some grape varieties such as Concord. It is essentially a potassium deficiency that causes the leaves on a vine to turn purple and eventually black as chlorophyll is lost...
disease in banana plantations.
The outbreak of black leaf disease led to an increase in fungicide
Fungicide
Fungicides are chemical compounds or biological organisms used to kill or inhibit fungi or fungal spores. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality and profit. Fungicides are used both in agriculture and to fight fungal infections in animals...
usage within the Taura River
Taura River
The Taura River is a river of Ecuador. It flows into the Churute Estuary in the Gulf of Guayaquil. It gives its name to the shrimp disease Taura syndrome, which was first described in shrimp farms along the river....
basin district near the city of Guayaquil
Guayaquil
Guayaquil , officially Santiago de Guayaquil , is the largest and the most populous city in Ecuador,with about 2.3 million inhabitants in the city and nearly 3.1 million in the metropolitan area, as well as that nation's main port...
. It was thought that the fungicides Tilt (propiconazole
Propiconazole
Propiconazole is a triazole fungicide, also known as a DMI, or demethylation inhibiting fungicide due to its binding with and inhibiting the 14-alpha demethylase enzyme from demethylating a precursor to ergosterol. Without this demethylation step, the ergosterols are not incorporated into the...
, Ciba-Geigy) and Calixin (tridemorph, BASF
BASF
BASF SE is the largest chemical company in the world and is headquartered in Germany. BASF originally stood for Badische Anilin- und Soda-Fabrik . Today, the four letters are a registered trademark and the company is listed on the Frankfurt Stock Exchange, London Stock Exchange, and Zurich Stock...
) used to control black leaf, ran off into nearby ponds and were responsible for the disease. Analytical data demonstrated propiconazole
Propiconazole
Propiconazole is a triazole fungicide, also known as a DMI, or demethylation inhibiting fungicide due to its binding with and inhibiting the 14-alpha demethylase enzyme from demethylating a precursor to ergosterol. Without this demethylation step, the ergosterols are not incorporated into the...
in water, sediments and hepatopancreas
Hepatopancreas
The hepatopancreas, digestive gland or midgut gland is an organ of the digestive tract of arthropods, molluscs and fish. It provides the functions which in mammals are provided separately by the liver and pancreas, including the production of digestive enzymes, and absorption of digested...
tissues of shrimp harvested from affected farms in Ecuador. No other pesticides were discovered.
In January 1994, at the request of Ciba-Geigy, a workshop on Taura syndrome was held at the Aquaculture Pathology Laboratory of the University of Arizona
University of Arizona
The University of Arizona is a land-grant and space-grant public institution of higher education and research located in Tucson, Arizona, United States. The University of Arizona was the first university in the state of Arizona, founded in 1885...
. Experts from several countries with expertise in shrimp and insect pathology, shrimp nutrition, toxicology, myocology, water quality and farm management participated in the workshop. Industry representatives also participated. The group developed recommendations as to the standardization of the research on Taura syndrome and suggested that studies be done to evaluate whether fungicide or an as-yet unrecognized agent was responsible for the syndrome.
Dr. Jim Brock, the aquatic disease specialist for the State of Hawaii during this period,first demonstrated the that the disease could be transmitted by feeding Taura victims to healthy shrimp in early 1994. The dying test shrimp were then fed to a new set of shrimp who was dying at the same rate. Rivers' postulates were fulfilled in 1994 by Dr. Ken Hasson and co-researchers at the University of Arizona. This proved the viral etiology of the syndrome. The virus was named Taura syndrome virus often referred to as TSV. The virus is referred by the name of Infectious cuticular epithelial necrosis virus (ICENV) by some authors in Latin America. Taura syndrome is a notifiable disease by the Office international des Épizooties (OIE), which reflects the serious nature and devastating impact of the disease.
Identification and description of the virus
Taura syndrome virus was first classified as a possible member of the family Picornaviridae based on biological and physical characteristics. It was later reclassified in the DicistroviridaeDicistroviridae
The Dicistroviridae are a family of Group IV insect-infecting viruses. Some of the insects commonly infected by dicistroviruses include aphids, leafhoppers, flies, bees, ants, silkworms.-Taxonomy:...
family, genus Cripavirus
Cripavirus
Cripaviruses are a genus of + sense, single-stranded RNA viruses. If a ssRNA virus is +sense, it means that the viruses can produce proteins directly from their RNA genome upon entering a cell; and therefore, may not require an RNA polymerase packaged in with it, as this may be produced from the...
. It has since been reassigned to a second genus in the same family - the Aparavirus.
TSV is a 32 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...
non-enveloped particle with an icosahedral morphology and a buoyant density of 1.338g/ml. The genome is single-stranded positive-sense and has 10,205 nucleotide
Nucleotide
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...
s (excluding the 3' poly-A tail
Messenger RNA
Messenger RNA is a molecule of RNA encoding a chemical "blueprint" for a protein product. mRNA is transcribed from a DNA template, and carries coding information to the sites of protein synthesis: the ribosomes. Here, the nucleic acid polymer is translated into a polymer of amino acids: a protein...
). The capsid
Capsid
A capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic...
consists of three major proteins: CP1 (40 kDa), CP2 (55 kDa) and CP3 (24 kDa) alongside a minor protein of 58 kDa.
Audelo-del-Valle in 2003 reported that certain primate cell lines could be used to culture TSV. Later studies demonstrated that their report was based on misinterpretated data. TSV does not appear to be a potential zoonosis. All virus amplifications require the use of live shrimp, as there is no continuous cell line that supports the growth of shrimp viruses.
Variants of the virus
RNA viruses such as TSV have high rates of spontaneous mutation. These very high rates might be due to the lack of proofreading function of the RNA-dependent RNA polymeraseRNA-dependent RNA polymerase
RNA-dependent RNA polymerase , , or RNA replicase, is an enzyme that catalyzes the replication of RNA from an RNA template...
and have resulted in the emergence of several genetic variants of the virus. As of May 2009, four genetic clusters are recognized: Belize
Belize
Belize is a constitutional monarchy and the northernmost country in Central America. Belize has a diverse society, comprising many cultures and languages. Even though Kriol and Spanish are spoken among the population, Belize is the only country in Central America where English is the official...
(TSV-BZ), America (TSV-HI), South-east Asia and Venezuela
Venezuela
Venezuela , officially called the Bolivarian Republic of Venezuela , is a tropical country on the northern coast of South America. It borders Colombia to the west, Guyana to the east, and Brazil to the south...
. The Belize strain is considered the most virulent. Point mutations in TSV capsid
Capsid
A capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic...
proteins might provide specific isolate with selective advantages such as host adaptability, increased virulence
Virulence
Virulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...
or increased replication ability. Even small variations in the TSV genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
can result in substantial differences in virulence
Virulence
Virulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...
.
All TSV variants are similar in shape and size with light variations. The average size of TSV-BZ virus particles is 32.693+/- 1.834 nm compared to TSV-HI with a size of 31.485 +/- 1.187 nm. The region of highest genetic difference is within the capsid protein CP2 with pairwise comparison
Pairwise comparison
Pairwise comparison generally refers to any process of comparing entities in pairs to judge which of each entity is preferred, or has a greater amount of some quantitative property. The method of pairwise comparison is used in the scientific study of preferences, attitudes, voting systems, social...
of nucleotide showing a 0 to 3.5% difference amongst isolates. The most variations in CP2 occur at the 3'-terminal sequence, this may be because it is less constrained by structural requirements and more exposed than other regions of the protein.
Geographic distribution
TSV has been reported from virtually all shrimp-growing regions of the Americas including Ecuador, ColombiaColombia
Colombia, officially the Republic of Colombia , is a unitary constitutional republic comprising thirty-two departments. The country is located in northwestern South America, bordered to the east by Venezuela and Brazil; to the south by Ecuador and Peru; to the north by the Caribbean Sea; to the...
, Peru
Peru
Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....
, Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
, El Salvador
El Salvador
El Salvador or simply Salvador is the smallest and the most densely populated country in Central America. The country's capital city and largest city is San Salvador; Santa Ana and San Miguel are also important cultural and commercial centers in the country and in all of Central America...
, Guatemala
Guatemala
Guatemala is a country in Central America bordered by Mexico to the north and west, the Pacific Ocean to the southwest, Belize to the northeast, the Caribbean to the east, and Honduras and El Salvador to the southeast...
, Honduras
Honduras
Honduras is a republic in Central America. It was previously known as Spanish Honduras to differentiate it from British Honduras, which became the modern-day state of Belize...
, Belize, 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...
, Nicaragua
Nicaragua
Nicaragua is the largest country in the Central American American isthmus, bordered by Honduras to the north and Costa Rica to the south. The country is situated between 11 and 14 degrees north of the Equator in the Northern Hemisphere, which places it entirely within the tropics. The Pacific Ocean...
, Panama
Panama
Panama , officially the Republic of Panama , is the southernmost country of Central America. Situated on the isthmus connecting North and South America, it is bordered by Costa Rica to the northwest, Colombia to the southeast, the Caribbean Sea to the north and the Pacific Ocean to the south. The...
, Costa Rica
Costa Rica
Costa Rica , officially the Republic of Costa Rica is a multilingual, multiethnic and multicultural country in Central America, bordered by Nicaragua to the north, Panama to the southeast, the Pacific Ocean to the west and the Caribbean Sea to the east....
, Venezuela as well as from the States of Hawaii, Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
, Florida
Florida
Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...
and South Carolina
South Carolina
South Carolina is a state in the Deep South of the United States that borders Georgia to the south, North Carolina to the north, and the Atlantic Ocean to the east. Originally part of the Province of Carolina, the Province of South Carolina was one of the 13 colonies that declared independence...
. Until 1998, it was considered to be a Western Hemisphere virus. The first Asian outbreak occurred in Taiwan
Taiwan
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...
. It has more recently been identified in 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...
, Myanmar
Myanmar
Burma , officially the Republic of the Union of Myanmar , is a country in Southeast Asia. Burma is bordered by China on the northeast, Laos on the east, Thailand on the southeast, Bangladesh on the west, India on the northwest, the Bay of Bengal to the southwest, and the Andaman Sea on the south....
, 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...
, 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...
and Indonesia
Indonesia
Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...
where it has been associated with severe epizootics in farmed Penaeus vannamei and Penaeus monodon.
The wide distribution of the disease has been attributed to the movement of infected host stocks for aquaculture purposes. This might have been helped by the highly stable nature of the virus. It is thought that importation of TSV-infected Penaeus vannamei from the Western Hemisphere was at the origin of the outbreak in Taiwan. This was further suggested by the genomic similarity of the Taiwan and Western Hemisphere isolates. TSV appeared in Thailand in 2003. Due to the similarities in deduced CP2 amino acids sequence and the chronology of the disease outbreaks in relation to imported stocks it is likely that at least some of the Thai isolates originated from Chinese stocks.
Taura syndrome can spread rapidly when introduced in news areas. A shrimp farmer described the 1995 outbreak in Texas as "This thing spread like a forest fire... There was no stopping it. I just sat there and watched it, and in a matter of three days, my shrimp were gone. Dead!" (reported in Rosenberry: Shrimp news international in 1995).
Species of susceptible shrimp
TSV is known to affect many shrimp species. It causes serious diseases in the postlarval (PL), juvenile and adult stages of Penaeus vannamei. It also affects severely Penaeus setiferus, Penaeus stylirostris, Penaeus schmitti, Metapenaeus ensis. Penaeus chinensis are highly susceptible to the disease in experimental bioassayBioassay
Bioassay , or biological standardization is a type of scientific experiment. Bioassays are typically conducted to measure the effects of a substance on a living organism and are essential in the development of new drugs and in monitoring environmental pollutants...
s.
There is variation within species and TSV-resistant strains of shrimp have been developed. Wild stocks are showing increased resistance, perhaps through intense natural selection. Reports of Taura syndrome in the wild are limited, but in February 1995, the Mexican Fisheries Ministry reported the presence of TSV in wild-type shrimp captured on the border of Mexico and Guatemala. As of 2007, there were no confirmed reports indicating that TSV is infectious to other groups of decapod or non-decapod crustaceans.
Pathology and disease cycle
In farm situations, TS often causes high mortality during the first 15 to 40 days of stocking into shrimp ponds. The course of infection may be acute (5–20 days) to chronic (more than 120 days) at the pond and farm level. The disease has three distinct phases that sometimes overlap: acute, transition and chronic. The disease cycle has been characterized in detail in Penaeus vannamei.After the initial infection, the acute phase develops. Clinical signs can occur as early as 7 hours post-infection in some individuals and last for about 4–7 days. Infected shrimp display anorexia
Anorexia (symptom)
Anorexia is the decreased sensation of appetite...
, lethargy and erratic swimming behavior. They also present opacification of the tail musculature, soft cuticle and, in naturally occurring infection, a red tail due to the expansion of the red chromatophores. Mortality during this phase can be as high as 95%. The acute phase is characterized histologically by multifocal areas of nuclear pyknosis/karyorrhexis and numerous cytoplasmic inclusion bodies in the cuticular epithelium and the subcutis of the general body surface, all appendages, gills, hindgut, esophagus and stomach. The pyknosis and karyorrhexis give a “buckshot” appearance to the tissue and are considered pathognomonic
Pathognomonic
Pathognomonic is a term, often used in medicine, that means characteristic for a particular disease. A pathognomonic sign is a particular sign whose presence means that a particular disease is present beyond any doubt...
for the disease. In severe infections the antennal gland
Coxal gland
The coxal gland is a gland found in some arthropods, for collecting and excreting urine. They are found in all arachnids , and in other chelicerates, such as horseshoe crabs. The coxal gland is thought to be homologous with the antennal gland of crustaceans...
tubule epithelium
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
, the hematopoietic tissues and the testis are also affected. This occurs mainly in severe infection following injection of viral particles and has not been reported from naturally infected Penaeus vannamei. Shrimp that survive the acute stage enter a transitional stage.
Shrimp in the transitional phase show randomly distributed melanized
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
(brownish/black) lesions within of the cuticle of the cepahlothorax and tail region. These foci are the sites of acute lesions which have progressed onto subsequent stages of hemocytic inflammation, cuticular epithelium regeneration and healing, and which might be secondarily infected with bacteria. These foci are negative for TSV by in situ hybridization
In situ hybridization
In situ hybridization is a type of hybridization that uses a labeled complementary DNA or RNA strand to localize a specific DNA or RNA sequence in a portion or section of tissue , or, if the tissue is small enough , in the entire tissue...
(ISH) using a TSV specific cDNA
Complementary DNA
In genetics, complementary DNA is DNA synthesized from a messenger RNA template in a reaction catalyzed by the enzyme reverse transcriptase and the enzyme DNA polymerase. cDNA is often used to clone eukaryotic genes in prokaryotes...
probe. Histologically these shrimp present focal active acute lesions and the onset of lymphoid organ spheroids (LOS) development. By ISH with TSV specific probes a diffuse positive signal can be observed within the walls of the lymphoid organ of normal appearance with or without focal probe signals within developing LOS. These shrimp will be lethargic and anorexic, possibly because of the redirection of all their energy and metabolic resources toward wound repair and recovery. If the shrimp undergo another successful moult
Moult
In biology, moulting or molting , also known as sloughing, shedding, or for some species, ecdysis, is the manner in which an animal routinely casts off a part of its body , either at specific times of year, or at specific points in its life cycle.Moulting can involve the epidermis , pelage...
following the transitional phase they will cast off the melanized lesions and enter the chronic phase.
The chronic phase is first seen 6 days post-infection and persist for at least 12 months under experimental conditions. This phase is characterized histologically by the absence of acute lesions and the presence of LOS of successive morphologies. These LOS are positive by ISH for TSV. A low prevalence of ectopic spheroids can also be observed in some cases. LOS are not by themselves characteristic of TSV infection and can be found in other viral diseases of shrimp such as lymphoid organ vacuolization virus (LOVV), lymphoid parvo-like virus (LPV), lymphoid organ virus (LOV), rhabdovirus of penaeid shrimp
Snakehead rhabdovirus
Snakehead rhabdovirus is a Novirhabdovirus that affects various species warm water wild and pond-cultured fish in Southeast Asia, including snakehead species for which it is named.-Isolation:...
(RPS) and yellow head virus
Yellowhead disease
Yellowhead disease is a viral infection of shrimp and prawn, in particular of the giant tiger prawn , one of the two major species of farmed shrimp. The disease is highly lethal and contagious, killing shrimp quickly. Outbreaks of this disease have wiped out in a matter of days the entire...
(YHV). Diagnosis of the disease during the chronic phase is problematic as shrimp do not display any outward signs of the disease and do not show mortality from the infection. Survivors may become carrier for life. Shrimp with chronic TSV infection are as vigorous as uninfected shrimp, as demonstrated by their inability to tolerate a salinity drop as well as uninfected shrimp.. A 2011 study by Laxminath Tumburu looked at the interrelationship between an environmental stressor (pesticide Endosulfan) and Taura Syndrome Virus (TSV), and their interactions on the susceptibility and molting of marine penaeid shrimp L. vannamei, and found that the interference of endosulfan-associated stress lead to increasingly higher susceptibility at postmolt stage during the acute phase of the TSV disease cycle..
Routes of transmission
The most likely route for transmission of TSV is cannibalismCannibalism
Cannibalism is the act or practice of humans eating the flesh of other human beings. It is also called anthropophagy...
of dead infected shrimp. The virus can be spread from one farm to another by seagulls and aquatic insects. Infectious TSV has been found in the feces of laughing gulls (Larus atricilla) that fed on infected shrimp during an epizootic in Texas. Controlled laboratory studies have documented that TSV remains infectious for up to one day after passage through the gut of white leghorns chicken (Gallus domesticus) and laughing gulls. Although, vertical transmission is suspected this has not been experimentally confirmed.
Shrimp surviving a TSV infection are life-long carriers of the viruse and are a significant source of virus for susceptible animals. It has been hypothesized that TSV was introduced to Southeast Asia with chronically infected shrimp imported from the Western Hemisphere. The ability of TSV to remain at least partly infectious after one or several freeze-thaw cycles might be a contributing factor facilitating its spread in the international commerce of frozen commodity products. Mechanisms by which infected frozen shrimp could spread the virus include: reprocessing of shrimp at processing plants with release of infectious liquid wastes, disposal of solid wastes in landfills where seagulls could acquire the virus and then spread it, the use of shrimp as bait by sport fishermen and the use of imported shrimp as fresh food for other aquatic species.
Diagnostic methods
A presumptive diagnosis of acute TSV infection can be established by the presence of dead or dying shrimp in cast netCast net
A cast net, also called a throw net, is a net used for fishing. It is a circular net with small weights distributed around its edge.The net is cast or thrown by hand in such a manner that it spreads out on the water and sinks. This technique is called net casting or net throwing. Fish are caught as...
s used for routine evaluation. Predatory birds are attracted to diseased ponds and feed heavily on the dying shrimp. The unique signs of infection caused by TS such as the cuticular melanized spots can provide a strong presumptive diagnosis
Presumptive test
In medical and forensic science, a presumptive test is an analysis of a sample which establishes either:#The sample is definitely not a certain substance#The sample probably is the substance....
, but care must be taken as these can be confused with other diseases, such as bacterial shell disease. In general pathognomonic
Pathognomonic
Pathognomonic is a term, often used in medicine, that means characteristic for a particular disease. A pathognomonic sign is a particular sign whose presence means that a particular disease is present beyond any doubt...
histopathological lesions are the first step in confirmatory diagnosis. Discrete foci of pyknotic and karyorhectic nuclei
Karyorrhexis
Karyorrhexis is the destructive fragmentation of the nucleus of a dying cell whereby its chromatin is distributed irregularly throughout the cytoplasm. It is usually preceded by pyknosis and is followed by karyolysis and can occur as a result of either programmed cell death or necrosis....
and inflammation are seen within the cuticular tissues. The lymphoid organ might display spheroids, but is otherwise unremarkable.
The genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
of the virus
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...
has been cloned and cDNA
Complementary DNA
In genetics, complementary DNA is DNA synthesized from a messenger RNA template in a reaction catalyzed by the enzyme reverse transcriptase and the enzyme DNA polymerase. cDNA is often used to clone eukaryotic genes in prokaryotes...
probes are available for diagnosis. Reverse transcriptase polymerase chain reaction (RT-PCR) methods have been developed for detection of TSV and are very sensitive. Real-time techniques allow for quantification of the virus. The IQ2000TM TSV detection system, a RT-PCR method, is said to have a detection limit of 10 copies per reaction.
RNA-based methods are limited by the relative fragility of the viral RNA. Prolonged fixation in Davidsons' fixative might result in RNA degradation due to fixative-induced acid hydrolysis. An alternative for virus detection is the utilization of specific monoclonal antibodies
Monoclonal antibodies
Monoclonal antibodies are monospecific antibodies that are the same because they are made by identical immune cells that are all clones of a unique parent cell....
(MAbs) directed against the relatively stable proteins in the viral capsid. Rapid diagnostic tests using MAbs are now in common use for white spot syndrome virus and are being marketed under the commercial name of Shrimple. Similar tests for TSV, Yellowhead virus and Infectious Hypodermal and Haematopoietic Necrosis Virus are currently under development.
Methods of control
Management strategies for the disease have included raising more resistant species such as the Western blue shrimp (Penaeus stylirostris) and stocking of specific pathogen freeSpecific Pathogen Free
Specific Pathogen Free is a term used for laboratory animals that are guaranteed free of particular pathogens. It is always accompanied by a list of the absent pathogens.Use of SPF animals ensures that specified diseases do not interfere with an experiment...
(SPF) or specific pathogen resistant (SPR) shrimp. Relatively simple laboratory challenges can be used to predict the performance of selected stocks in farm where TSV is enzootic. The resistant shrimp lines currently raised have reached nearly complete resistance to some TSV variants and further improvement due to breeding for TSV-resistance is expected be minor for these variants. Significant improvements in TSV survival were made through selective breeding despite low to moderate heritability for this trait.
A management strategy used to reduce the impact of TS has been the practice of stocking post larva shrimp at increased stocking density. Following this strategy, farms would experience mortality due to TS at an early stage of the production cycle, before substantial feeding had begun, and the surviving shrimp would be resistant to further TSV challenges. Other techniques used with limited efficacy have been the polyculture of shrimp with tilapia and maintenance of near optimal water quality conditions in the grow-out ponds with reduction of organic loading. Transgenic shrimp expressing an antisense TSV coat protein (TSV-CP) exhibited increased survival in TSV challenges. The public perception of transgenic animals as well as current technical limitations, limit the use of transgenic animals as a mean of disease control.