In this article we will discuss about:- 1. Morphology of Rabies Virus 2. Growth Characteristics of Rabies Virus 3. Resistance.
Morphology of Rabies Virus:
Rabies virus (Fig. 14.12) is bullet-shaped, 180 x 75 nm in size, with one end rounded or conical and the other plane or concave. The genome that occurs in the central core is un-segmented, linear, negative sense, 10-12 kilobase, single-stranded RNA enclosed within a helically wound nucleocapsid.
RNA- dependent RNA polymerase enzyme, which is essential for the initiation of replication of the virus, is enclosed within the virus in association with the ribonucleoprotein core. The latter is surrounded by viral membrane (matrix protein), which may be invaginated at the plane end.
The viral membrane in turn is surrounded by a lipoprotein envelop that carries knob-like spikes made up of glycoprotein G. The spikes do not cover the plane end of the virus and may be released from the envelop by treatment with lipid solvents or detergents.
Growth Characteristics of Rabies Virus:
Freshly isolated rabies virus from natural infections of humans or animals is called street virus. It results in fatal encephalitis in experimental animals in laboratory after long and variable inoculation period of 1-12 weeks. Negri bodies (intracytoplasmic inclusion bodies) can be observed in the brains of inoculated laboratory animals dying of street virus infection.
After several serial intracerebral passages in rabbit the virus undergoes certain alterations and becomes what is called fixed virus. The latter is more neurotropic, though it is much less infective by other route.
Following the intracerebral inoculation, the virus gives rise to fatal encephalitis after a short and fixed incubation period of about a week. Negri bodies are usually demonstrable in the brains of animals during of fixed virus infection. The fixed virus causes paralytic rather than furious symptoms and is used for vaccine production.
Rabies virus can be cultured in chick embryos. The virus is usually inoculated into the yolk sac of the embryo. Serial propagation in chick embryos has led to the development of attenuated vaccine strain like Flury and Kelev. Stains adapted to duck egg give high yield of virus and have been used for the preparation of inactivated vaccines.
Several primary and continuous cell cultures (e.g. chick embryo fibroblast, baby hamster kidney, mouse neuroblastoma, human diploid lung fibroblasts, Vero monkey kidney cells with minimal cytoplasmic effects) are used to grow rabies virus. The fixed virus strains grown in human diploid lung libroblasts, chick embryo, and Vero monkey kidney cell cultures are used for vaccine production.
Resistance of Rabies Virus:
Rabies virus is sensitive to alcohol and lipid solvents (e.g. ether, chloroform, acetone), iodine preparations, quaternary ammonium compounds, soap, and detergents.
It can be inactivated by phenol, formalin, beta-propiolactone, sunlight, ultraviolet irradiation, and heat at 50°C for one hour or 60°C for 5 minutes. The virus dies at room temperature but can survive for weeks when stabilized by 50% glycerol. It can remain stable for several days at 0-4°C, and can be preserved at -70°C or by lyophilisation (freeze-drying).