Survey of Viruses

Chapter 24

Definition of a Virus

Sub microscopic entity consisting of a

single nucleic acid surrounded by a

protein coat and capable of replication

only within the living cells of bacteria,

animals or plants.

Definition of a Virus

Obligate

Intracellular

Parasite

Virion Structure

Nucleic Acid

Spike Projections

ProteinCapsid

Lipid Envelope

VirionAssociatedPolymerase

Virion Morphology• Simple Structure

• Repetitive Structure

• High Level of Redundancy

Virus Morphology

Helical Icosahedral

Virus Replication1 Virus attachment

and entry1 2 Uncoating of virion

2

3 Migration ofgenome nucleicacid to nucleus

3

4 Transcription5 Genome replication4

5

6 Translation of virusmRNAs

6

7 Virion assembly78 Release of new

virus particles

8

Transmission of Viruses• Respiratory transmission

– Influenza A virus• Faecal-oral transmission

– Enterovirus• Blood-borne transmission

– Hepatitis B virus• Sexual Transmission

– HIV• Animal or insect vectors

– Rabies virus

In vivo Disease Processes

• Cell destruction

• Virus-induced changes to gene expression

• Immunopathogenic disease

Acute Virus Infections

• Localised to specific site of body

• Development of viraemia with widespread infection of tissues

Poliovirus

Poliovirus

• Enterovirus.• Possesses a RNA

genome.• Transmitted by the faecal

oral route.• Cause of gastrointestinal

illness and poliomyelitis.

Properties of the virus

Influenza A virus

• Myxovirus• Enveloped virus with a

segmented RNA genome• Infects a wide range of

animals other than humans

• Undergoes extensive antigenic variation

• Major cause of respiratory infections

Properties of the virus

Influenza A virus Infection• Spread by respiratory route• Virus infects cells of the respiratory tract• Destruction of respiratory epithelium

– Secondary bacterial infections

• Altered cytokine expression leading to fever

e.g interleukin-1 and interferon

Spread of influenza virus

Respiratory Tract

Other Viral Infections

• Herpes• Hepatitis• Rhinovirus• HIV• Ebola

• Measles • Mumps• Smallpox• Chickenpox

Point mutation of HA and NAgenes

ANTIGENIC DRIFT

ANTIGENIC SHIFT

Genetic Reassortment

Human H3N2

Avian H3N8

Human H2N2

Generation of Novel Influenza A Viruses

Antiviral Targets

• Attachment/Entry

• Nucleic acid replication

• Virus protein processing

• Virus maturation

Problems with Antivirals

• Identification of virus-specific target.

• Generation of resistant variants.