A. Structure

 Rabies virus is a rhabdovirus with morphologic and bio chemical properties in common with vesicular stomatitis virus of cattle and several animal, plant, and insect viruses (Table 1). The rhabdoviruses are rod- or bullet-shaped particles measuring 75 × 180 nm (Figure 1). The particles are surrounded by a membranous envelope with protruding spikes, 10 nm long. The peplomers (spikes) are composed of trimers of the viral glycoprotein. Inside the envelope is a ribonucleocapsid. The genome is single-stranded, negative-sense RNA (12 kb; molecular weight 4.6 × 10⁶). Virions contain an RNA-dependent RNA polymerase. The particles have a buoyant density in CsCl of about 1.19 g/cm3 and a molecular weight of 300–1000 × 10⁶.

Table1. Important Properties of Rhabdoviruses

Fig1. Structure of rhabdoviruses. A: Electron micrograph of bullet-shaped particle typical of the rhabdovirus family (100,000×). Shown here is vesicular stomatitis virus negatively stained with potassium phosphotungstate. (Courtesy of RM McCombs, M Benyesh-Melnick, and JP Brunschwig.) B: Schematic model of rabies virus showing the surface glycoprotein spikes extending from the lipid envelope that surrounds the internal nucleocapsid and the matrix protein lining the envelope. The nucleocapsid comprises the single RNA genome plus nucleoprotein and the polymerase proteins. (Reproduced with permission from Cowan MK, Talaro KP: Microbiology. A Systems Approach, 2nd ed. McGraw-Hill, 2009. © McGraw-Hill Education.)

B. Classification

The viruses are classified in the family Rhabdoviridae. Rabies viruses belong to the genus Lyssavirus, whereas the vesicular stomatitis-like viruses are members of the genus Vesiculovirus. The rhabdoviruses are very widely distributed in nature, infecting vertebrates, invertebrates, and plants. Rabies is the major medically important rhabdovirus. Many of the animal rhabdoviruses infect insects, but rabies virus does not.

C. Reactions to Physical and Chemical Agents

Rabies virus survives storage at 4°C for weeks and at −70°C for years. It is inactivated by CO2, so on dry ice it must be stored in glass-sealed vials. Rabies virus is killed rapidly by exposure to ultraviolet radiation or sunlight, by heat (1 hour at 50°C), by lipid solvents (ether, 0.1% sodium deoxycholate), by trypsin, by detergents, and by extremes of pH.

D. Virus Replication

The rhabdovirus replication cycle is shown in Figure 42-2. Rabies virus attaches to cells via its glycoprotein spikes; the nicotinic acetylcholine receptor may serve as a cellular receptor for rabies virus. The single-stranded RNA genome is transcribed by the virion-associated RNA polymerase to five mRNA species. The template for transcription is the genome RNA in the form of ribonucleoprotein (RNP) (encased in N protein and containing the viral transcriptase). The monocistronic mRNAs code for the five virion proteins: nucleocapsid (N), polymerase proteins (L, P), matrix (M), and glycoprotein (G). The genome RNP is a template for complementary positive-sense RNA, which is responsible for the generation of negative-sense progeny RNA. The same viral proteins serve as polymerase for viral RNA replication as well as for transcription. Ongoing translation is required for replication, particularly of viral N and P proteins. The newly replicated genomic RNA associates with the viral transcriptase and nucleoprotein to form RNP cores in the cytoplasm. The particles acquire an envelope by budding through the plasma membrane. The viral matrix protein forms a layer on the inner side of the envelope, whereas the viral glycoprotein is on the outer layer and forms the spikes.

E. Animal Susceptibility and Growth of Virus

Rabies virus has a wide host range. All warm-blooded animals, including humans, can be infected. Susceptibility varies among mammalian species, ranging from very high (foxes, coyotes, wolves) to low (opossums); those with inter mediate susceptibility include skunks, raccoons, and bats (Table 2). The virus is widely distributed in infected animals, especially in the nervous system, saliva, urine, lymph, milk, and blood. Recovery from infection is rare except in certain bats, where the virus has become peculiarly adapted to the salivary glands. Hematophagous vampire bats may transmit the virus for months without themselves ever showing any signs of disease.

Table2. Animal Susceptibility to Rabies

When freshly isolated in the laboratory, the strains are referred to as street virus. Such strains show long and variable incubation periods (usually 21–60 days in dogs) and regularly produce intracytoplasmic inclusion bodies. Serial brain-to-brain passage in rabbits yields a “fixed” virus that no longer multiplies in extraneural tissues. This fixed (or mutant) virus multiplies rapidly, and the incubation period is shortened to 4–6 days. Inclusion bodies are found only with difficulty.

F. Antigenic Properties

There is a single serotype of rabies virus. However, there are strain differences among viruses isolated from different species (raccoons, foxes, skunks, canines, bats) in different geographic areas. These viral strains can be distinguished by epitopes in the nucleoprotein and glycoprotein recognized by monoclonal antibodies as well as by specific nucleotide sequences. There are at least seven antigenic variants found in terrestrial animals and bats.

The G glycoprotein is a major factor in rabies virus neuroinvasiveness and pathogenicity. Avirulent mutants of rabies virus have been selected using certain monoclonal antibodies against the viral glycoprotein. A substitution at amino acid position 333 of the glycoprotein results in loss of virulence, indicating some essential role for that site of the protein in disease pathogenesis.

Purified spikes containing the viral glycoprotein elicit neutralizing antibody in animals. Antiserum prepared against the purified nucleocapsid is used in diagnostic immunofluorescence for rabies.

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مواضيع ذات صلة

Pathogenesis and Pathology of Rabies Virus

Coronavirus Infections in Humans

Properties of Coronaviruses

Congenital Rubella Syndrome

Postnatal Rubella

Hendra Virus and Nipah Virus Infections

Measles (Rubeola) Virus Infections

Rhabdoviruses

Retroviruses

Reoviruses

Poxviruses

Role of Biomarkers in COVID-19