In this article we will discuss about:- 1. Habitat of Hepatitis B Virus 2. Morphology of Hepatitis B Virus 3. Replication 4. Cultivation and Stability.
Habitat of Hepatitis B Virus:
Hepatitis B (serum hepatitis) is caused by hepatitis B virus (HBV) belonging to family Hepadnaviridae (heptotropic DNA viruses) and to genus Orthrohepadnavirus. Under the electron microscope, sera from hepatitis B patients shows three types of particles (Fig. 14.13).
The most abundant are spherical particles, each 22 nm in diameter. The second type of particle is filamentous or tubular with a diameter of 22 nm and of varying length.
These two particles are antigenically identical and are surface components of HBV. The third type of particle, far fever in number, is a double-walled spherical structure being 42 nm in diameter. It was first described by Dane in 1970 and so is known as the Dane particle.
The latter represents the complete hepatitis B virus. However, the small spherical and filamentous or tubular particles are the unassembled components of the Dane particle, which is the infective form of the virus.
Morphology of Hepatitis B Virus:
Hepatitis B virus or Dane particle is a 42 nm DNA virus with an outer envelope and an inner core (27 nm in diameter) enclosing a 3.2 kilobase circular double-stranded DNA and a DNA polymerase (Fig. 14.14).
The envelop is made up of lipid, protein and carbohydrate and contains hepatitis B surface antigen (HBsAg). It encloses an inner icosahedral 27 nm nucleocapsid (core), which contains hepatitis B core antigen (HBcAg) and hepatitis Be antigen (HBeAg).
HBeAg is a soluble protein and is detected in the serum of persons with high virus titers and indicates high infectivity. The DNA is of most unusual structure. Its plus (+) strand is incomplete leaving 15-50% of the molecule single-stranded, whereas its minus (-) strand is complete and contains four overlapping open reading frames (genes) coding for multiple proteins (Fig. 14.15).
Replication of Hepatitis B Virus:
Hepatitis B virus replicates within hepatocytes. Viral DNA exists in the hepatocyte nucleus in the free extra chromosomal state or integrated with the cell chromosome. DNA is synthesized from an RNA template by reverse transcription.
Cultivation and Stability of Hepatitis B Virus:
HBV has not been cultivated in any conventional culture system in the laboratory. However, limited production of the virus and its proteins can be obtained from several cell lines transfected with viral genome. HBV proteins have been cloned in bacteria and yeast. The chimpanzee is susceptible to experimental infection and can be used as laboratory model.
Hepatitis B virus is a relatively heat stable virus. It maintains its viability at room temperature for long periods. Experiments on chimpanzee inoculations reveal that heat at 60°C for 10 minutes inactivates the virus by hundred to thousand fold, and treatment with hypochlorite (10,000 ppm available chlorine) and 2% gluteraldehyde reduces infectivity in 10 minutes. However, HBsAg may not be destroyed by such treatment.