General characteristics and classification
The family Rhabdoviridae represents a massively diverse group of infectious agents. To date, the scientific and medical community has identified about one hundred and eighty species belonging to this family. A unique feature of this group is the exceptionally wide range of potential hosts: various species of these viruses are capable of infecting both organisms of numerous animal species and plant cultures.
In medical and veterinary microbiology, it is customary to distinguish several key genera of the greatest epidemiological and economic importance:
- Genus Lyssavirus — of critical medical importance, as this genus includes the rabies virus, which causes a fatal disease.
- Genus Vesiculovirus — a vivid and typical representative of this group is the vesicular stomatitis virus.
- Genus Ephemerovirus — includes the specific pathogen causing bovine ephemeral fever.
Morphology and etymology
When studying family members under an electron microscope, their highly specific appearance stands out. Virions have a characteristic bullet-like shape. The structure of the viral particle is asymmetric and resembles a cylinder with one smoothly rounded pole, while the opposite end appears completely flat (as if truncated).
This unique visual feature served as the basis for the nomenclature of the entire family. The name itself originates from the Ancient Greek word rhabdos, which literally translates to «rod» or «staff». The physical parameters (dimensions) of virions vary over a fairly wide range: their length can reach from 120 to 300 nanometers, and their width is usually from 60 to 80 nanometers.
Ultrastructure and viral envelope
The complex structure of the virion is ensured by the presence of several important structural components. The outer protective barrier of the viral particle is the supercapsid — a specialized envelope based on a bilayer lipoprotein membrane. This membrane structure has a clear and strictly ordered spatial organization:
- Inner layer: On the inner side (facing the center of the virion), the lipoprotein membrane is densely lined with matrix protein, referred to in microbiology as the M protein.
- Outer layer: The outer surface of the supercapsid is densely covered with specialized spikes. These protruding structures are formed by glycoprotein G. The dimensions of each such spike are approximately 5 to 10 nanometers in length and about 3 nanometers across.
Ribonucleocapsid and genome organization
Beneath the protective layer of the lipoprotein supercapsid lies the internal component of the virus — the ribonucleocapsid. It is characterized by a helical symmetry type. The core of the capsid consists of genetic material represented by a single-stranded, strictly linear, unsegmented ribonucleic acid molecule. It is fundamentally important to note that this is specifically a minus-RNA (negative-sense molecule).
The viral genome functions in close association with specific nucleocapsid proteins, each performing its unique task:
- N protein (from the English term nucleocapsid) — performs a structural function, forming a dense protective protein coat for the genomic RNA molecule.
- L protein (large) and P protein (phosphoprotein) — combine to form a vital polymerase complex. This complex acts as a transcriptase, which is absolutely essential for the successful initiation and progression of viral particle reproduction inside the infected cell.
Briefly: answers to topical questions
Which target cell receptors does the rabies virus glycoprotein G bind to during infection?
Rabies virus glycoprotein G binds to specific nicotinic acetylcholine receptors of nervous tissue (nAChR), ensuring virus adsorption. The p75NTR receptor is involved in transporting the virus to the neuron cell body.
What are the main stages of the rhabdovirus replication cycle inside an infected cell?
The rhabdovirus replication cycle inside an infected cell includes five main stages.
- Adsorption and penetration — binding of viral glycoproteins to receptors and entry via clathrin-mediated endocytosis.
- Uncoating — release of the ribonucleocapsid from the endosome directly into the cytoplasm.
- Biosynthesis — transcription of incomplete plus-strands (mRNA) and replication of full-length plus-strand RNA involving viral RNA-dependent RNA polymerase.
- Translation and processing — synthesis of viral proteins on ribosomes and modification of glycoprotein G in the endoplasmic reticulum and Golgi apparatus.
- Assembly and release — formation of the ribonucleocapsid and release of new virions by budding through the cell membrane.
What is the mechanism of rabies virus spread through the human nervous system after a bite?
The spread of rabies virus (Rabies virus) through the nervous system occurs strictly via a neurogenic pathway.
- Centripetal transport — the virus moves retrogradely along motor neuron axons from the site of inoculation to the CNS at a speed of 50–100 mm/day. Movement is carried out using minus-end dynein motor proteins.
- Centrifugal spread — after replication in neuron cell bodies of the brain and spinal cord, the virus disseminates via efferent fibers of peripheral nerves to internal organs, primarily the salivary glands.
What laboratory diagnostic methods are used to confirm a rabies diagnosis?
The following methods are used to diagnose rabies:
- Direct fluorescent antibody test (DFA) — examination of corneal imprints and skin biopsy specimens.
- Enzyme-linked immunosorbent assay (ELISA) and complement fixation test (CFT) — serological detection of antibodies.
- Neutralization test (NT) — in mice using rabies immunoglobulin.
- Biological assay — isolation of the virus from saliva, tear fluid, or cerebrospinal fluid by intracerebral inoculation of suckling mice.
- Indirect fluorescent antibody test (IFAT) also belongs to the utilized specific diagnostic methods.
What medications are used for specific post-exposure prophylaxis of rabies?
Active and passive immunization preparations are used for specific post-exposure prophylaxis of rabies.
- Rabies vaccine (COCAV) — concentrated purified cell culture inactivated vaccine administered intramuscularly in a course of 6 injections for emergency immunity induction.
- Rabies immunoglobulin (RIG) — used to provide immediate passive immunity for severe injuries or bites in dangerous locations. Administered at a dose of 20 IU/kg, with part of the dose used to infiltrate the wound area and the remainder administered intramuscularly.
To remember the composition of the rhabdovirus polymerase complex, use the mnemonic: to start reproduction, the virus needs a Leader (L protein) and a Helper (P protein).
Class and exam questions
What shape do members of the Rhabdoviridae family have?
They have a bullet-like shape, representing a cylindrical structure with one rounded and one flat (truncated) end.
What type does the rhabdovirus genome belong to?
The genetic material of these viruses is represented by a single-stranded, linear, unsegmented minus-RNA molecule.
What components form the supercapsid?
The supercapsid consists of a bilayer lipoprotein membrane, lined on the inside with M protein (matrix) and covered on the outside with spikes consisting of glycoprotein G.
What is the function of the L-protein and P-protein in the virus structure?
L protein and P protein together form a specific polymerase complex (transcriptase), which is critically necessary for viral reproduction in the host cell.
What else to study on this topic
- Pathogenesis of diseases caused by rabies virus (genus Lyssavirus)
- Life cycle of negative-sense RNA viruses
- Epidemiological significance of vesicular stomatitis virus (genus Vesiculovirus)
- Mechanisms of formation and functions of glycoprotein G during cell interaction
- Features of the helical symmetry of the ribonucleoprotein
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