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IntroductiontoVirology
VIROLOGY
The virology is a branch of pathology, which deals with the diagnosis of viral
diseases.
Certain viruses are specific to the mankind. The field of medical virology deals
strictly with those viruses, which cause disease in the mankind.
Properties of Viruses
1. The classical Features which make viruses different from other
microorganisms are their small size,The viruses are very small particles
and are not visible by light microscopes.The electron microscope is
needed to see them.
2. Non-cellular structure,
3. Genome containing either DNA or ribonucleic acid (RNA) but not both,
4. Their inability to replicate outside of living cells. Because viruses
replicate inside cells, fewer drugs are available to treat virus infections
compared with bacterial infections, although vaccines are available against
virus diseases such as influenza, yellow fever, poliomyelitis, measles,
mumps, rubella, hepatitis A and B, and rabies.
5. They having no any metabolic activity and are not living organisms.These
are taken up by the cells&inside the cell, they invest their genetic material
(DNA or RNA).The viral genetic material utilizes metabolic machinery of
the host cell for the propagation of their genes as well as proteins.
6. The viruses may be plant viruses or animal viruses. The animal viruses
affect the animals including man.Certain viruses like that of Rabies, Yellow
fever,Tick born encephalitis,and Congo Crimean Haemorrhagic Fever
(CCHF) are transmitted to human beings from the infected animals. These
viral conditions are called as zoonosis.
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IntroductiontoVirology
Structure of Virus:
Virus particles, or virions are too small to be seen by the light microscope,
Size: Measuring only 20–300 nm.
They can however be seen by the electron microscope.
All viruses consist of a mass (core) of nucleic acid (DNA or RNA) surrounded by
a protective protein coat called a capsid. For RNA viruses, the genome can be
single stranded, double stranded or fragmented.
The genome of most DNA viruses is double stranded. The nucleic acid together
with the capsid form the nucleocapsid. The capsid is antigenic and also contains
the receptors which enable a virus to attach to the surface of its specific host
cell.
The capsid consists of a number of identical units called capsomeres. The
symmetry, or pattern, of capsides is one of the features used to classify viruses.
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IntroductiontoVirology
Capsid symmetry
Capsid symmetry is described as being:
● Icosahedral in which the capsomeres are arranged to form a symmetrical
icosahedran surrounding the nucleic acid. Small size (below 50 nm) icosahedral
viruses appear spherical.
● Helical in which the capsomers are arranged around a spiral of nucleic acid.
Helical viruses can appear spherical, elongated, or filamentous.
● Complex, meaning the capsid symmetry is neither icosahedral nor helical.
Virus envelope
Many helical viruses and a few icosahedral viruses are surrounded by an
envelope. This is a lipoprotein membrane composed of lipid and virus-specific
glycoprotein’s (antigens) that project as spikes from the surface of the envelope.
Compared with non enveloped (naked) viruses, enveloped viruses are more
sensitive to heat, detergents, and lipid solvents.
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Transmission routes for human viruses
1. By direct contact, e.g. sexually transmitted viruses such as HIV, herpes
simplex virus 2, and hepatitis B virus.
2. By ingesting viruses in food or water contaminated with faeces, e.g.
enteroviruses, rotaviruses, and hepatitis A virus.
3. By inhaling viruses in airborne droplets, e.g. influenza viruses, measles
virus, adenoviruses, respiratory syncytial virus, and rhinoviruses.
Overcrowding greatly assists in the spread of droplet infections.
4. By contact with an article, such as a floor mat contaminated with
papilloma virus (wart producing virus) or a towel contaminated with a
virus that causes eye infection.
5. By a mother infecting her child during pregnancy, e.g. cytomegalovirus or
rubella virus. Such infections may cause abortion, stillbirth, congenital
abnormalities, or illhealth of the newborn. Hepatitis B virus and HIV can
be transmitted from mother to baby during birth.
6. By transfusion of virus infected blood, e.g. HIV 1 and 2, hepatitis B virus,
and hepatitus C virus.
Effects of physical and chemical agents on viruses
1. Heat: Most viruses are inactivated at 56C0
for 30minutes or at 100 _C for
a few minutes.
2. Cold: Viruses are stable at low temperatures. Most can be satisfactorily
stored frozen although some viruses are partially inactivated by freezing
and thawing.
3. Ultraviolet (UV) irradiation: Inactivates viruses.
4. Chloroform, ether and other organic solvents: Viruses surrounded by an
envelope are inactivated. Un enveloped viruses are resistant.
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5. Oxidizing and reducing agents: Chlorine, iodine, hydrogen peroxide, and
formaldehyde, all inactivate viruses.
6. Phenols: Most viruses are relatively resistant.
7. Virus disinfectants: Include hypochlorite solutions and glutaraldehyde.
Tissues, however, may protect viruses by quenching disinfectant.
