Orbivirus

Key takeaways

Orbivirus belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Orbivirus to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Orbivirus from memory before moving on to harder problems.

Reference excerpt

Orbivirus is a genus of double-stranded RNA viruses in the order Reovirales and the family Sedoreoviridae. Unlike other reoviruses, orbiviruses are arboviruses. They can infect and replicate within a wide range of arthropod and vertebrate hosts. Orbiviruses are named after their characteristic doughnut-shaped capsomers (orbis in Latin means ring). Many orbiviruses are transmitted by ticks or haematophagus insect vectors (Culicoides, mosquitoes and sand flies) and have a wide host range that includes cattle, goats and sheep, wild ruminants, equids, camelids, marsupials, sloths, bats, birds, large canine and feline carnivores, and humans. 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. The genus contains 22 species and at least 130 different serotypes.

History In 1719, African horse sickness virus (AHSV) caused the first major recorded orbivirus epidemic, killing 1,500 animals. The most historically significant outbreak of orbivirus occurred in 1854–1855, when AHSV infected 70,000 horses. AHSV was discovered to be a virus in 1900 and bluetongue disease followed shortly thereafter in 1905. Outbreaks have occurred sporadically in the 20th and 21st centuries.

Virology

Structure

The virons are nonenveloped particles that are 70–80 nm in diameter. The virus particles are spherical in appearance and have icosahedral symmetry. An outer and an inner capsid layer surround the genome, and have T=13 and T=2 symmetry, respectively. The viron is constructed of two concentric protein shells, the subcore 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 10 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 RNA genomes, so 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 seven major structural proteins (VP1–VP7) and three major nonstructural 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 ORF spans almost the entire length of genome segment 9 and encodes VP6 (the viral helicase). A second ORF (OrfX) is also present on this segment and encodes a fourth nonstructural protein (NS4), which was predicted from sequence analysis of various orbiviruses including segment 9 of Great Island virus which contained a long NS4 ORF (around 21kDa). The existence of NS4 was experimentally confirmed in both insect-borne and tick-borne orbiviruses in 2011. NS1 is the most abundant protein in bluetongue virus 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 numbers 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.

Life cycle Many orbiviruses preferentially infect vascular endothelial cells. Orbiviruses enter 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.

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 disease 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.

Taxonomy

Species The genus Orbivirus contains the following species, listed by scientific name and followed by the exemplar virus of the species:

Serogroups The genus is divided into several (at least 14) serogroups. The serogroups are divided in some cases into subgroups. A number of member viruses have yet to be assigned to a serogroup. The serogroups are differentiated on the basis of a fourfold or greater difference in antibody based tests. These tests include ELISAs and complement fixation tests.

Vector groups Member viruses are transmitted by midges (Culicoides), mosquitoes, and ticks. The viruses transmitted by a particular type of vector are generally related both genetically and serologically.

See also Above Maiden virus Animal viruses

References

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Orbivirus

Start with the simplest possible case. Write down what Orbivirus claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Orbivirus before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Orbivirus ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Orbivirus

In research: Orbivirus appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Orbivirus in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Orbivirus is common in secondary-school and first-year university syllabi. It links to neighbouring topics Animal viral diseases, Bird diseases, Mammal diseases, so understanding it makes those chapters shorter.
In everyday life: Look for Orbivirus outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Orbivirus in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Orbivirus means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Orbivirus out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Orbivirus in simple terms?

Orbivirus is a genus of double-stranded RNA viruses in the order Reovirales and the family Sedoreoviridae. Unlike other reoviruses, orbiviruses are arboviruses.

Why does Orbivirus matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Orbivirus?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Orbivirus.