Reoviruses: structure, classification, and microbiology

Origin of the name and family composition

The family name Reoviridae has an interesting history. It is an acronym composed of the first letters of three English words:

Respiratory;
Enteric;
Orphan.

The term "orphan" historically stuck to these viruses because at the time of their initial discovery, scientists could not link them to any specific diseases.

To date, the family includes 15 genera. Of these, the 5 main genera are of the greatest medical significance, being proven pathogens for humans and various animals: Orthoreovirus, Orbivirus, Rotavirus, Coltivirus, and Seadornavirus.

Morphology and internal organization of the virion

Reoviruses belong to the category of non-enveloped viruses, as they lack a supercapsid (outer lipid envelope). Virions are spherical in shape and have a diameter of 70 to 80 nm.

The virus capsid has a complex structure with an icosahedral type of symmetry. It consists of three layers:

Outer capsid: formed by proteins extending from the intermediate layer and has characteristic protruding spikes on its surface.
Virion core: a dense complex including the inner capsid, transcription system, and genome. In rotaviruses, the inner capsid is specifically formed by the VP-2 protein.

The genetic apparatus of reoviruses is unique. It is a linear double-stranded RNA. The genome is fragmented and includes 10 to 12 independent segments. To initiate the replication cycle, the virion contains its own enzyme — RNA-dependent RNA polymerase (transcriptase).

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Reproduction and activation mechanism

All key stages of the reovirus life cycle take place in the cytoplasm of the infected host cell. It is there that both the replication of viral components and the final assembly of new virions are carried out.

Some representatives (in particular, rotaviruses and orthoreoviruses) are characterized by a special activation mechanism. Under the influence of proteolytic enzymes, virions undergo structural changes. As a result of this proteolysis, so-called "infectious subviral particles" are formed. They differ from the original virions by a significantly increased invasive capacity, meaning they penetrate susceptible cells much more easily.

Classification and epidemiology of pathogenic genera

According to modern taxonomy, the family is divided into two major subfamilies.

1. Subfamily Sedoreovirinae:

Genus Orbivirus: the name comes from the Latin word orbis (ring), which is associated with the ring-like shape of capsomeres. These are arboviruses (transmitted by arthropods). Includes Kemerovo virus (causes Kemerovo fever, vectors are ticks), Tribec virus, and bluetongue virus of sheep (transmitted by biting midges).
Genus Rotavirus: includes serogroups from A to G. It is the leading cause of widespread diarrheal infections.
Genus Seadornavirus: includes Banna virus. This is an arboviral infection transmitted by mosquitoes. The virus was first isolated in China in 1967 from patients with severe neurological symptoms.

2. Subfamily Spinareovirinae:

Genus Coltivirus: the causative agent of Colorado tick fever. It belongs to arboviral infections, with ticks serving as vectors.
Genus Orthoreovirus: includes human and simian orthoreoviruses (5 serotypes). They are extremely widespread in nature. Viruses of this genus are isolated from humans and mammals both in an absolutely normal state and during pronounced pathology (gastrointestinal and respiratory tract infections).

Principles of microbiological diagnostics

Diagnosis of diseases caused by arboviral members of the Reoviridae family relies on classical methods for detecting viruses and antibodies against them.

Virological method: based on isolating the pathogen itself. To do this, cell cultures are infected. The method of infecting laboratory animals is also actively used, in particular, intracerebral inoculation of material into suckling mice.
Serological method: aimed at detecting a specific immune response. The main goal is to detect an increase in antibody titer in paired blood sera of the patient. For this, standardized reactions are used: CF test (complement fixation test), HI assay (hemagglutination inhibition assay), and NT (neutralization test).

Briefly: answers to topical questions

Which proteins form the outer capsid of rotaviruses?

Sources indicate that the non-structural protein NSP4 promotes the assembly of the outer capsid from the VP7 protein; a complex with the VP4 protein is also formed during assembly.

What is the pathogenesis of watery diarrhea in rotavirus infection?

The pathogenesis of watery diarrhea in rotavirus infection is mixed in nature and includes two main mechanisms:

Osmotic mechanism — intracellular viral replication causes the destruction and desquamation of mature enterocytes. Due to the loss of disaccharidases, unhydrolyzed carbohydrates accumulate in the intestinal lumen, increasing the osmotic pressure of chyme, which leads to fluid efflux into the lumen.
Secretory mechanism — the non-structural protein NSP4 acts as a viral enterotoxin. It activates the enteric nervous system, directly stimulating the secretion of water and electrolytes into the intestinal lumen.

What drugs are used for the specific prevention of rotavirus infection?

Live oral vaccines are used for the specific prevention of rotavirus infection.

«Rotarix» — a live monovalent vaccine for oral administration. Intended for the prevention of gastroenteritis associated with serotypes G1, G2, G3, G4, and G9.
«Rotateq» — a live oral pentavalent vaccine. It is a reassortant product containing five strains created based on a combination of the genetic material of human and bovine rotaviruses.

How to memorize

Remembering the origin of the name is easy via the acronym REO: R — Respiratory, E — Enteric, O — Orphan (initially had no link to a specific disease).

Class and exam questions

What is the main feature of the genome of representatives of the Reoviridae family?

The reovirus genome consists of double-stranded RNA, which is not whole, but linearly fragmented into 10–12 separate segments.

Where does the assembly process of new viral particles take place?

Unlike some other viruses, all stages of reovirus reproduction, including final assembly, occur exclusively in the cytoplasm of the host cell.

What are "infectious subviral particles" and how are they formed?

These are specific structures of rota- and orthoreoviruses that are formed as a result of proteolysis. Due to this process, they acquire enhanced invasive capacity for cell entry.

Why did the genus Orbivirus get its name?

The name comes from the Latin word orbis, which means "ring". This is associated with the characteristic ring-like shape of capsomeres on the virion surface.

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What else to study on this topic

Structural organization of the intermediate and inner capsid layers
Functions of the VP-2 protein in members of the genus Rotavirus
Mechanism of action of reovirus RNA-dependent RNA polymerase (transcriptase)
Circulation cycle of arboviruses in the bodies of ticks, mosquitoes, and biting midges
Clinical presentation and pathogenesis of Colorado tick fever

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Reoviruses