Orbivirus
Encyclopedia
The genus
Orbivirus is a member of the Reoviridae
family
. This genus
currently contains 22 species
and at least 130 different serotype
s. Orbiviruses can infect and replicate within a wide range of arthropod and vertebrate hosts. Orbiviruses are named after their characteristic doughnut
shaped capsomers (Orbi in Latin
means ring
).
The type species is Bluetongue virus.
The three economically most important orbiviruses are Bluetongue virus, African horse sickness virus and Epizootic hemorrhagic disease virus all of which are transmitted by Culicoides
species.
particles are spherical in appearance and have icosahedral symmetry. An outer and an inner capsid
layer surround the genome
. The viron is constructed of two concentric protein shells, the sub-core layer which contain 120 copies/particle of the VP3 and the core-surface layer composed of 780 copies/particle of the VP7. VP1, VP4 and VP6 are minor enzymatic proteins that are packaged along with the ten genome segments within the central space of the virus core. The orbivirus outer-capsid layer is composed of two additional structural proteins (VP2 and VP5) which mediate cell-attachment and penetration during initiation of infection. The outer-capsid proteins are more variable than the core proteins and most of the non-structural proteins and the specificity of their reactions with neutralising antibodies determines the virus serotype.
genomes and therefore are classified as Class III viruses. Their genome is linear and is segmented into 10 segments of various lengths. One copy of each gene segment is packaged per virion. In most cases each gene segment encodes a single open reading frame (ORF). The genome encodes 7 major structural proteins (VP1-VP7) and 3 major non structural proteins (NS1-NS3). Exceptions to the one gene one protein rule are segment 9 (Seg-9) and segment 10 (Seg-10) both of which encode two nearly identical proteins initiated from in-phase AUG codons close together near the upstream termini (VP6 and VP6a encoded by Seg-9: NS3 and NS3a encoded by Seg-10)
An open reading frame spans almost the entire length of genome segment-9 and encodes VP6 (the viral helicase). A second open reading frame (OrfX) is also present on this segment and encodes a fourth non structural protein (NS4).
NS1 is the most abundant protein in BTV infected cells. It forms tubules that may be involved in translocation of progeny virus particles to the cell membrane. NS2 is phosphorylated by cellular kinases and is an important matrix protein of the granular viral inclusion bodies that form within the cytoplasm of infected cells. These viral inclusion bodies act as the centres of viral replication. The membrane glycoproteins NS3 and NS3a are expressed in large amounts in insect cells but not in mammalian cells. They are involved in the release of progeny virus particles from infected cells and may be involved in determination of both vector competence and virulence.
the host cell by endocytosis
and the outer capsid is subsequently removed. The whole cycle of viral replication
takes place within the cytoplasm of the host cell. Transcription of the viral genome into mRNA occurs within the core particle and mRNA is translated into proteins using the host cell ribosomes. Viral proteins are synthesized 2–14 days after initial infection. New virons self-assemble within the cytoplasm and are then released from the host cell by budding
. During the budding process they transiently acquire a lipid envelope which can be detected for a short period of time following their release but this is subsequently lost.
in sheep, cattle, goats and wild ungulates. BTV has been in the forefront of molecular studies for last three decades and now represents one of the best understood viruses at the molecular and structural levels. Other species of Orbiviruses are responsible for other diseases of animals such as African Horse sickness
and Equine encephalosis virus .
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Orbivirus is a member of the Reoviridae
Reoviridae
Reoviridae is a family of viruses that can affect the gastrointestinal system and respiratory tract. Viruses in the family Reoviridae have genomes consisting of segmented, double-stranded RNA...
family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
. This genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
currently contains 22 species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
and at least 130 different serotype
Serotype
Serotype or serovar refers to distinct variations within a subspecies of bacteria or viruses. These microorganisms, viruses, or cells are classified together based on their cell surface antigens...
s. Orbiviruses can infect and replicate within a wide range of arthropod and vertebrate hosts. Orbiviruses are named after their characteristic doughnut
Doughnut
A doughnut or donut is a fried dough food and is popular in many countries and prepared in various forms as a sweet snack that can be homemade or purchased in bakeries, supermarkets, food stalls, and franchised specialty outlets...
shaped capsomers (Orbi in Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
means ring
Ring
Ring may refer to:*Ring , a decorative ornament worn on fingers, toes, or around the arm or neck-Computing:* Ring , a layer of protection in computer systems...
).
The type species is Bluetongue virus.
The three economically most important orbiviruses are Bluetongue virus, African horse sickness virus and Epizootic hemorrhagic disease virus all of which are transmitted by Culicoides
Culicoides
Culicoides is a genus of biting midges in the subfamily Ceratopogonidae. Around 500 species of Ceratopogonidae are at present placed in the genus and this is split into many subgenera. Several species are known to be vectors of various diseases and parasites which can affect animals.-Notable...
species.
Virus Particles
The virons are non-enveloped particles that are between 70-80 nm in diameter. The virusVirus
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...
particles are spherical in appearance and have icosahedral symmetry. An outer and an inner 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...
layer surround 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....
. The viron is constructed of two concentric protein shells, the sub-core layer which contain 120 copies/particle of the VP3 and the core-surface layer composed of 780 copies/particle of the VP7. VP1, VP4 and VP6 are minor enzymatic proteins that are packaged along with the ten genome segments within the central space of the virus core. The orbivirus outer-capsid layer is composed of two additional structural proteins (VP2 and VP5) which mediate cell-attachment and penetration during initiation of infection. The outer-capsid proteins are more variable than the core proteins and most of the non-structural proteins and the specificity of their reactions with neutralising antibodies determines the virus serotype.
Genome
These viruses have double stranded RNARNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
genomes and therefore are classified as Class III viruses. Their genome is linear and is segmented into 10 segments of various lengths. One copy of each gene segment is packaged per virion. In most cases each gene segment encodes a single open reading frame (ORF). The genome encodes 7 major structural proteins (VP1-VP7) and 3 major non structural proteins (NS1-NS3). Exceptions to the one gene one protein rule are segment 9 (Seg-9) and segment 10 (Seg-10) both of which encode two nearly identical proteins initiated from in-phase AUG codons close together near the upstream termini (VP6 and VP6a encoded by Seg-9: NS3 and NS3a encoded by Seg-10)
An open reading frame spans almost the entire length of genome segment-9 and encodes VP6 (the viral helicase). A second open reading frame (OrfX) is also present on this segment and encodes a fourth non structural protein (NS4).
NS1 is the most abundant protein in BTV infected cells. It forms tubules that may be involved in translocation of progeny virus particles to the cell membrane. NS2 is phosphorylated by cellular kinases and is an important matrix protein of the granular viral inclusion bodies that form within the cytoplasm of infected cells. These viral inclusion bodies act as the centres of viral replication. The membrane glycoproteins NS3 and NS3a are expressed in large amounts in insect cells but not in mammalian cells. They are involved in the release of progeny virus particles from infected cells and may be involved in determination of both vector competence and virulence.
Replication
Many Orbiviruses preferentially infect vascular endothelial cells. Orbiviruses enterViral entry
Viral entry is the earliest stage of infection in the viral life cycle, as the virus comes into contact with the host cell and introduces viral material into the cell. The major steps involved in viral entry are shown below. Despite the variation among viruses, the generalities are quite similar...
the host cell by endocytosis
Endocytosis
Endocytosis is a process by which cells absorb molecules by engulfing them. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane...
and the outer capsid is subsequently removed. The whole cycle of viral replication
Viral replication
Viral replication is the term used by virologists to describe the formation of biological viruses during the infection process in the target host cells. Viruses must first get into the cell before viral replication can occur. From the perspective of the virus, the purpose of viral replication is...
takes place within the cytoplasm of the host cell. Transcription of the viral genome into mRNA occurs within the core particle and mRNA is translated into proteins using the host cell ribosomes. Viral proteins are synthesized 2–14 days after initial infection. New virons self-assemble within the cytoplasm and are then released from the host cell by budding
Budding
Budding is a form of asexual reproduction in which a new organism grows on another one. The new organism remains attached as it grows, separating from the parent organism only when it is mature. Since the reproduction is asexual, the newly created organism is a clone and is genetically identical...
. During the budding process they transiently acquire a lipid envelope which can be detected for a short period of time following their release but this is subsequently lost.
Pathogenesis
Orbiviruses primarily cause diseases in animals. The different Orbivirus species have different host specificities. Orbiviruses are vector-borne pathogens transmitted between vertebrate hosts by vectors such as mosquitoes, midges, gnats, sandflies and ticks. Bluetongue virus (BTV) is an Orbivirus that causes bluetongue diseaseBluetongue disease
Bluetongue disease or catarrhal fever is a non-contagious, non-zoonotic, insect-borne, viral disease of ruminants, mainly sheep and less frequently cattle, goats, buffalo, deer, dromedaries and antelope...
in sheep, cattle, goats and wild ungulates. BTV has been in the forefront of molecular studies for last three decades and now represents one of the best understood viruses at the molecular and structural levels. Other species of Orbiviruses are responsible for other diseases of animals such as African Horse sickness
African Horse sickness
African horse sickness is a highly infectious, and deadly disease. It commonly affects horses, mules, and donkeys. It is caused by a virus of the genus Orbivirus belonging to the family Reoviridae. This disease can be caused by any of the nine serotypes of this virus...
and Equine encephalosis virus .