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- Adsorption:

Virus attaches to the cell surface.

2- Penetration (Entry):

Enveloped viruses: receptor mediated endocytosis

3- Uncoating:

Uncoating is the physical separation of viral nucleic acid from the outer

structural components.

4- Transcription of mRNA.

5- Synthesis of viral components: This involves the synthesis of viral proteins and viral genomes.

6- Assembly:

New virus particles are assembled by packaging of the genome into capsid.

7- Release:

Virus may be released due to cell lysis, or, if enveloped, may bud from the cell.

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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.