Home › Microbiology › Reoviruses: Structure, Pathogenesis, and Classification

Reoviruses

Reoviridae

For medical students2 min readUpdated 2026-10-10

Reoviruses are a large family of non-enveloped viruses characterized by a unique fragmented double-stranded RNA genome. Members of this family infect the respiratory and gastrointestinal tracts and act as agents of severe arboviral infections affecting the nervous system.

Shape and sizeSpherical shape, 70–80 nm in diameter

Genome structureLinear double-stranded RNA (10–12 segments)

EnvelopeSupercapsid absent (non-enveloped viruses), triple-layered capsid

ReproductionOccurs exclusively in the cytoplasm of the host cell

Origin of the Name and Family Composition

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

Respiratory;
Enteric;
Orphan.

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

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

Morphology and Internal Organization of the Virion

Reoviruses are categorized as non-enveloped viruses because they lack a supercapsid (outer lipid envelope). Virions are spherical and range from 70 to 80 nm in diameter.

The viral capsid has a complex structure with icosahedral symmetry and consists of three layers:

Outer capsid: formed by proteins extending from the intermediate layer, featuring characteristic surface spikes.
Virion core: a dense complex comprising the inner capsid, transcription machinery, and the genome. In rotaviruses, the inner capsid is specifically formed by the VP-2 protein.

The genetic apparatus of reoviruses is unique. It consists of 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 — an RNA-dependent RNA polymerase (transcriptase).

Reproduction and Activation Mechanism

All key stages of the reovirus life cycle take place in the cytoplasm of the infected host cell. Both the replication of viral components and the final assembly of new virions occur there.

Some members (particularly rotaviruses and orthoreoviruses) feature a special activation mechanism. Upon exposure to proteolytic enzymes, virions undergo structural changes, producing so-called "infectious subviral particles". These differ from original virions by having a significantly increased invasive capacity, meaning they penetrate susceptible cells much more readily.

Classification and Epidemiology of Pathogenic Genera

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

1. Subfamily Sedoreovirinae:

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

2. Subfamily Spinareovirinae:

Genus Coltivirus: the causative agent of Colorado tick fever. An arboviral infection transmitted by ticks.
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 completely normal states and during manifest 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 of isolating viruses and detecting antibodies against them.

Virological method: based on isolating the pathogen itself by inoculating cell cultures. Laboratory animals are also used, specifically via intracerebral inoculation of suckling mice.
Serological method: aimed at detecting a specific immune response. The main goal is to demonstrate a rising antibody titer in paired patient sera using standardized assays: complement fixation test (CFT), indirect hemagglutination assay (IHA), and neutralization test (NT).

Mnemonic

The origin of the name is easy to remember using the acronym REO: R — Respiratory (affecting the respiratory tract), E — Enteric (affecting the gut), O — Orphan (initially not linked to a specific disease).

Frequently asked questions

Which proteins form the outer capsid of rotaviruses?

Non-structural protein NSP4 assists in assembling the outer capsid from the VP7 protein, which also forms a complex with the VP4 protein during assembly.

What is the pathogenesis of watery diarrhea in rotavirus infection?

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

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

Which preparations are used for specific prevention of rotavirus infection?

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

Rotarix — a live monovalent oral vaccine for the prevention of gastroenteritis associated with serotypes G1, G2, G3, G4, and G9.
RotaTeq — a live oral pentavalent reassortant vaccine containing five strains created from a combination of human and bovine rotavirus genetic material.

What is the main feature of the Reoviridae genome?

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

Where does the assembly 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 formed as a result of proteolysis. Through this process, they acquire enhanced invasive capacity for cell entry.

Why is the genus Orbivirus named as such?

The name comes from the Latin word orbis, meaning "ring," due to the characteristic ring-like shape of capsomeres on the virion surface.

Go deeper

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 ticks, mosquitoes, and midges
Clinical presentation and pathogenesis of Colorado tick fever