Introduction to the Viruses:

General properties of viruses:

1-They are very small in size, from 20-300 m.

2-They contain one kind of nucleic acid (RNA or DNA) as their

genome.

3-They are metabolically inert because they do not possess

enzyme systems necessary for the synthesis of new viral

material (ribosomes).

4-They are obligate intracellular parasites as they replicate

inside living cells.

5-They are only seen by electron microscope.

Viral Structure:

Each virus particle or virion is composed of :

A protein coat capsid

A nucleic acid core

Many viruses are naked but some are enveloped.

Viral capsid:

It is the protein coat surrounding the nucleic acid.

It is composed of small protein subunits arranged symmetrically around the nucleic acid called capsomeres.

Viral nucleic acid and viral Envelope:

Viruses contain either DNA or RNA but not both .

Most DNA viruses are double stranded, some are single stranded.

Most RNA viruses are single stranded, some are double stranded.

Virus envelope :

Many viruses are surrounded by a lipid or lipoprotein envelopes

which may be covered by spikes (glycoproteins).

Virus Symmetry

Viruses have three types of symmetry:Cubical symmetry: These viruses resemble a crystal and are called

icosahedral virus. Example: adenoviruses.

Helical symmetry: In which the particle is elongated. Most helical

viruses are enveloped . Example: influenza virus.

Complex symmetry: In which the viruses are complicated in

structure. Example: poxviruses and

bacteriophage.

Classification of Viruses:

Classical virus classification schemes have been based on the consideration of major properties of viruses:

1 -The type of nucleic acid which is found in the virion (RNA

or DNA, single stranded or double stranded(

2-The symmetry and shape of the capsid (Cubic, helical ,

complex.(

3 -The presence or absence of an envelope (enveloped, naked)

4 -The size of the virus particle.

5 -Antigenic properties .

6 -Biologic properties, including natural host range, mode of

transmission, vector relationship, pathogenicity, and tissue

tropism.

Classification of Viruses:

DNA

Icosahedral Complex

Pox virusesNaked Enveloped

Papilloma virus Herpes viruses

Hepatitis B

Classification of Viruses: a

Viral replication: Viruses multiply only in living cells.Steps of viral replication:1- Attachment (Adsorption):

Virus attaches to specific receptors on the cell surface.

2- Penetration (Entry or Engulfment):Non-enveloped viruses: penetrate host cell by process of

pinocytosis or phagocytosis forming cytoplasmic vacuole that dissolves

and then release nucleocapsid into the cytoplasm.

Enveloped viruses: penetrate host cell by process of endocytosis (fusion of viral envelope with the cell membrane), thus releasing nucleocapsid

directly into the cytoplasm.

3- Uncoating:Uncoating is the physical separation of viral nucleic acid from the

outer structural components (capsid). Done in cytoplasm by cell enzymes.

Viral replication: 4- Synthesis of viral components:

The essential step in virus replication is transcription of mRNA from the viral nucleic acid.

Once it is accomplished, viruses use cell components (ribosomes and tRNA) to translate the mRNA into different viral components.

This involves the synthesis of viral proteins (enzymes, capsid) and viral genomes.

5- Assembly:New virus particles are assembled by packaging of the

genome into capsid.

6- Release:Virus may be released to the extracellular environment due to

cell lysis, or, if enveloped, may bud from the cell leaving the host cell intact.

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Viral replication:

Effect of viruses on cells:

Many viruses inhibit host RNA, DNA or protein synthesis (or any combination of these).

In general, viruses could induce the following pathogenic effects:

1- Cytopathic effect (CPE): 

The presence of the virus often gives rise to morphological changes in the host cell.

These may include:

1-Membrane blebbing

2-Formation of multinucleated giant cells, or fused

cells, known as syncytia

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3-Production of inclusion bodies (accumulations of

viral proteins or virions) in the nucleus or

cytoplasm.

4-Rounding up and detachment of cells from the

culture dish.

5 -Cell lysis .

2-Latency:

Virus genome persists within the infected cells without

replication and the survival of infected cells.

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3- Chromosome damage:

As breakage, fragmentation, rearrangement, translocation.

4- Malignant transformation (oncogenesis):

Viruses are known to be the etiologic factors in the development

of human tumors including cervical and liver cancer.

Examples of oncogenic viruses include:

- Human papillomavirus (HPV) : Cervix Cancer

- Ebstein Barr virus (EB) : Nasopharyngeal carcinoma

- Hepatitis B and Hepatitis C viruses (HBV, HCV) : liver Cancer

- Human T cell lymphotorpic virus (HTLV): Adult T cell leukemia.

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Cytopathic effects: Left to Right :

Cytopathic effect of HSV, enterovirus 71, and RSV in cell culture .

Note the ballooning of cells in the case of HSV .

Note syncytia formation in the case of RSV.

Clinical examples on viral infection:

Skin infection: One of the most common viral infection of skin:Herpes virus infection.

Respiratory tracts infection: The most common viral infection of respiratory tracts: 1-Influenza virus infection. 2-Common cold Rhinovirus.

Digestive tracts infection:-Gastroenteritis: Rotavirus.-liver infection: Hepatitis A,B, and C.

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Sexually transmitted diseases :

The most common viruses are:

1-Genital Herpes.

2-Papilloma virus.

Nervous system Encephalitis and Meningitis:

1-Flaviviruses. (Arbovirus).

2-Rabies virus.

Diagnosis of viral infections: In general, diagnosis of viral infection can be performed by:1-Direct detection: The clinical specimen is examined directly for the presence of : A-Virus particles: Microscopy. B- Virus antigen: Microscopy. C- Viral nucleic acids: Molecular methods.

2- Virus isolation: For cultivation of viruses, the Clinical specimen is inoculated into: A- Cell culture. B- Chick embryo (fertilized eggs). C- Laboratory Animals.

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3- Serology:

-Elevated antibodies can be detected.

-The most common methods used in hospitals.

-Can be used to differentiate acute and latent infection.

-Different methods can be used in serology such as:

1-Agglutination methods.

2-ELISA.