Download VIROLOGY

By
Prof. Mohamed I. Bassyouni
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Viruses are parasites at the genetic level. They are the
smallest infectious agents known. They can infect man,
animals, insects, plants and bacteria. The following
properties distinguish viruses from other organisms:
1- They are very small in size.
2- They contain one kind of nucleic acid (RNA or DNA).
3- They are metabolically inert, as they do not possess
ribosomes or protein synthesizing apparatus.
4- They are obligate intracellular parasites.
5- They cannot be grown on artificial culture media and are
grown in tissue culture, embryonated eggs or living
animals.
They vary in size from 20-450 nm.
a- They can pass through bacterial filters.
b- They require high speeds (ultracentifugation) for
their sedmentation 10,000-30,000 rpm ( bacteria
require 1,000-3,000 rpm).
c- They are only seen by electron microscopy (EM),
except poxviruses.
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Structures compared
From Medical Microbiology, 5th ed., Murray, Rosenthal & Pfaller, Mosby Inc., 2005, Fig. 6-4.
Each virus particle or virion is composed of a protein
coat or capsid and nucleic acid core. The capsid with
its enclosed nucleic acid is called the nucleocapsid.
Many viruses are naked but some viruses are
enveloped. Other internal organs found in some
viruses are enzymes, and matrix proteins which are
present in enveloped viruses and mediate the
interactions between the capsid and envelope.
It is composed of small protein subunits called
capsomers. The arrangement of the capsomers determines
virus symmetry. Functions of the capsids are:
1- It protects the viral genome (DNA or RNA) against
inactivation by nucleases.
2- It is responsible for the structural symmetry of virions
i.e. icosahedral or helical.
3- It participates in attachment of virions to susceptible
cells.
4- Capsid proteins are important antigens that induce
antibodies that neutralize virus infectivity and, activate
cytotoxic T cells to kill virus-infected cells.
5- Variation in capsid proteins is responsible for the
different viral serotypes in non-enveloped viruses.
Helical
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Icosahedral
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20-sided with 12 corners
Vary in the number of
capsomers
Each capsomer may be
made of one or several
proteins
Some are enveloped
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complex
Fig 6.9a,c
Viruses contain either DNA or RNA but not both. Most DNA
viruses are double stranded, while most RNA viruses are single
stranded. The nucleic acid may be linear or circular. Some RNA
viruses have segmented genome e.g. rotavirus and influenza
virus. The molecular weight and type of nucleic acid are specific
for each virus group. All viruses have one copy of either genome
(haploid) except retroviruses which have two copies (diploid).
Viral genomes are used as vectors in gene therapy and in
recombinant a virulent virus vector vaccines. Functions of the
nucleic acid are:
1- It is the infectious part of the virus; coreless particles are noninfections.
2- It carries the genetic information for (a) Virus replication. (b)
Virulence or ability to parasitize cells. (c) Antigenic specificity of
the protein coat.
Many viruses are surrounded by a lipid or lipoprotein,
which may be covered by glycoprotein spike-like
projections, which attach to host cell receptors during the
entry of the virus into the cell, e.g. haemagglutinin (HA)
and Neuraminidase (NA) spikes in influenza virus. Due to
their lipid content, such viruses are sensitive to ether. Loss
of lipids results disruption of virus and loss of infectivity.
The envelope may be partially or completely derived from
host membranes during release from the cell by budding.
The surface proteins, whether the virus capsid proteins or
the envelope glycoproteins, are the principal antigens
against which the host mounts its immune response. They
are also the determinants of type specificity.
Some viruses carry enzymes e.g. RNA polymerase, which is
present in negative sense RNA viruses to copy their mRNA
e.g. orthomyxoviruses and the reverse transcriptase
enzyme (RT) present in retroviruses to make a cDNA copy
of the viral RNA.
Virus Symmetry
The arrangement of the capsomers, in the capsid, gives the
virus its geometric symmetry. Viruses have three types of
symmetry:
1- Cubical symmetry; These viruses resemble a crystal
and are called icosahedral viruses e.g. herpsviruses and
adenoviruses.
2- Helical symmetry in which the particle is elongated. The capsomers
are arranged in a ribbon which is wound in the form of a helix or spiral
around the spiral nucleic acid. All human viruses that have helical
symmetry are enveloped e.g. influenza virus.
3- Complex symmetry in which the viruses are complicated in
structure e.g. poxviruses which are brick shaped with ridges on the external
surface. The bacteriophage is another example of complex symmetry.
Atypical virus-like agents:
1- Defective viruses are composed of viral nucleic acid and
proteins but cannot replicate without a helper virus, which
provides the missing function. These usually have a mutation or
a deletion of part of their genetic material. During the growth of
most human viruses, many more defective than infectious virus
particles are produced.
2- Pseudovirions contain host cell DNA instead of
viral DNA within the capsid. They are formed
during infection with certain viruses when the
host cell DNA is fragmented and pieces are
incorporated within the capsid. Pseudovirions can
infect cells, but they do not replicate.
3- Prions are infectious particles that are composed
solely of protein. They contain no detectable
nucleic acid. They cause slow diseases.
1.
2.
3.
4.
5.
6.
Adsorption – binding of virus to specific molecule on
host cell.
Penetration –genome enters host cell.
Replication – viral components produced.
Assembly - viral components assembled.
Maturation – completion of viral formation.
Release – viruses leave cell to infect other cells.
Fig 6.11
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 Not all bacteriophages lyse cells
 Temperate phages insert their viral DNA into the
host chromosome & viral replication stops at there
until some later time.
 Lysogeny- bacterial chromosome carries phage
DNA
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 Spectrum of cells a virus can infect
 cell has to have a specific structure (receptor) on its surface
for viral attachment
 cell has to contain all of the enzymes and materials needed
to produce new virions
 May be one species or many
 HIV (only humans) vs rabies (many animals)
 May be one tissue or many within a host
 Hepatitis (liver) vs polio (intestinal & nerve cells)
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1.
2.
3.
Animal virus replication is more complex than
phage replication because host cells are more
complex.
Animal viruses cannot inject their DNA.
Lysogeny for phage, latency for animal viruses
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1-Adsorption.
2-Penetration/uncoating of genome.
3-Eclipse.
4-Duplication/synthesis.
5-Assembly.
6-Release.
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 1- Attachment: Virus and cell are brought into contact by
random collision, but attachment is specific and occurs
only if the cell membrane contains specific receptors for
the virus, e.g. human immunodeficiency virus (HIV) binds
to CD4 receptors on immune cells i.e. helper T cells,
macrophages and dendritic cells.
 2- Penetration and uncoating: Non-enveloped virions are
taken into animal cells by endocytosis where they are
uncoated by lysosomal enzymes. Enveloped viruses
penetrate the membrane by fusion between the virus
envelope and the cell membrane releasing the
nucleocapsid into the cell; uncoating my occur at the cell
surface e.g. bacteriophages, in the cytoplasm e.g.
poliovirus, or in the nucleus e.g. herpesvirus. Uncoating
renders viral nucleic acid accessible for transcription.
 3- Eclipse: It is the period after penetration during
which no infectious virus can be detected inside the
host cell. During this phase, the cell is redirected, by
the viral nucleic acid (genome), toward synthesizing
viral components. The eclipse phase ends with the
appearance of virus particles.
 4- Intracellular viral synthesis: It includes synthesis of
both viral nucleic acid and proteins. The viral nucleic
acid (genome) replicates by using a strand of the
parental nucleic acid as a template for the production
of progeny DNA or RNA molecules.
 5- Assembly of viral nucleic acid and protein coats to
form mature virus particles occurs in the cytoplasm
e.g. poliovirus or in the nucleus e.g. herpes viruses.
 6- Release : Mature virus particles will accumulate in
the cell in enormous numbers and are released by
either of two processes. One is by rupturing the cell
i.e. cytolysis, which usually occurs with non-enveloped
viruses. The other is by a slow process of leaking or
budding through the cell membrane, which occurs
with enveloped viruses where they acquire lipoprotein
envelope during budding from the outer cell
membrane ( this is mediated by matrix proteins).
Herpes virus acquire their envelope from the nuclear
membrane.
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Penetration
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Release by budding
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1-Changes in size & shape.
2-Cytoplasmic inclusion bodies.
3-Nuclear inclusion bodies.
4-Nells fuse to form multinucleated cells.
5-Cell lyses.
6-Alter DNA.
7-Transform cells into cancerous cells.
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Obligate intracellular parasites require appropriate
cells to replicate.
1-Live laboratory animals
2-Bird embryos – chicken, duck; intact, selfsupporting unit, sterile, self-nourished
3-Cell culture
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Prions - misfolded proteins, contain no nucleic
acid
cause spongiform encephalopathies – holes in the brain
common in animals
scrapie in sheep & goats
bovine spongiform encephalopathies (BSE), aka mad cow disease
humans – Creutzfeldt-Jakob Disease
Viroids - short pieces of RNA, no protein coat
only been identified in plants, so far.
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- More difficult than other agents
- Consider overall clinical picture
-Take appropriate sample
- Infect cell culture- look for characteristic cytopathic
effects
- Screen for parts of the virus
- Screen for immune response to virus (antibodies)
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 1- Microscopy
 * Electron microscope is used to determine virus
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particles.
* Large viruses may be seen by light microscope e.g.
small pox virus.
* Immunoelectron microscope; addition of antibodies
causes aggregation of virus particles.
* Immunofluorescent.
* Inclusion bodies can be seen either intranuclear or
intracytoplasmic and are helpful in the diagnosis of
certain virus infections.
 2- Virus Culture
 Cell cultures, chicken embryos, or laboratory animals are used
for recovery of viruses from clinical samples to establish disease
etiology.
 The type of cell culture used for virus cultivation depends on the
sensitivity of the cell to a particular virus (i.e. viral tropism).
 3- Detection of Viral Antigens
The recognition of hepatitis B antigens in sera of patients, rota
virus antigen in feces, rabies virus antigens in brain, Herpes
simplex virus in vesicular lesions is achieved by:
 * Direct immunofluorescence using specific antibody labeled
with fluorescent substance, e.g. diagnosis of rabies in brain
tissue of animals.
 * Detection of viral antigens in sera of patients by
radioimmunoassay or enzyme immunoassay e.g. hepatitis
antigens.
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 4- Nucleic Acid Hybridization
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This rapid diagnostic method is based on using
fragments of DNA or RNA that are complementary to the
viral nucleic acid (molecular probe) labeled with an
enzyme or a radioactive substance. This method is highly
specific and sensitive.
 5- Polymerase Chain Reaction (PCR)
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This technique involves amplification of a short sequence
of a specific viral DNA or RNA leading to accumulation of
large amounts of that nucleic acid sequence which can be
detected easily.
 6- Serological Methods
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Antiviral antibodies appear 10 days after exposure to
infection. In the acute phase of the disease, they are of the
IgM class or detection of a 4-fold or greater rise of antibody
titer (IgG class). The procedures used are complement
fixation, virus neutralization, hemagglutination inhibition,
indirect immunofluorescence, radioimmunoassay and
enzyme immunoassay.
 Viruses should reach a susceptible cell before they can
produce disease. Therefore, they should have:
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1- Portal of entry; namely the respiratory tract,
gastrointestinal tract (GIT), skin, urogenital tract or
conjunctiva.
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2- A pathway through the body; namely the blood,
lymphatics or nerves.
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3- A target organ which may be CNS, skin, glands,
liver …etc.
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Many viral infections are subclinical. The same virus may
produce a variety of diseases. The same disease may be
produced by a variety of viruses. The outcome of virus
infections is determined by the interactions of virus and
the host and is influenced by the genetics of each. Viruses
may produce local or systemic infections, which may
reach the full-blown picture, or more commonly end in
subclinical form. Some viruses persist.
 1- Local Infections which occur at the portal of entry
with no viraemia e.g. viral influenza and common cold at the
mucous membrane of the respiratory tract. Rotavirus infection at
the GIT causing diarrhoea. It is characterized by short
incubation period, short lasting immunity that is mediated
by IgA and interferon.
 2- Systemic Infections: after primary replication at the
site of entry, the virus travels through the blood or lymphatics
causing viraemia, or through the nerves to reach a distant
target organ that has specific receptors for the virus e.g.
poliovirus, mumps and measles.
 Systemic infections are characterized by: long incubation
period, long lasting immunity that is mediated by IgG and
IgM. Infection can be stopped at the viraemic stage immune
mechanisms i.e. neutralizing antibodies. This leads to
subclinical or abortive infections. Gamma globulins given to
contacts of a case may abort infection if given during the
incubation period before viraemic stage.
 3- Persistent viral infections: Sometimes virus persist
for a long time in the host in one of the following forms :
a- Chronic infections in which the virus can be
continuously detected with no or mild symptoms e.g.
hepatitis B and C chronic carriers.
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b- Latent infections in which the virus persists
hidden most of the time, with periodic reactivation
and development of clinical lesions containing the
virus e.g. herpes viruses and HIV infections.
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c- Slow virus infections the have a very long
incubation period of months or years with no clinical
symptoms e.g. subacute sclerosing panencephalitis
with incubation period of 2-20 years and is caused by a
variant of measles virus and HIV. Other slow
infections caused by non-conventional agents called
prions e.g. Creutzfeldt-Jakob disease and kuru in
man.
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Mechanisms that help persistence of virus
include:
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1- Integration of a DNA provirus into host cell DNA e.g.
retroviruses.
2- Rapid antigenic variation.
3- Virus spread from cell to cell without an extracellular phase.
4- Decreased expression of MHC-1 by the action of viral genes
which leads to decreased recognition of virus infected cells by
CTLs.
5- Location within an immunologically sheltered sanctuary
e.g. the brain.
6- Formation of virus-antibody complexes which remain
infections.
7- Immunosuppression as in AIDS.
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