Introduction to viruses

Introduction
Introduction
to
to
viruses
viruses

Lecture topics - 1
Lecture topics - 1
•
What is a virus?

Definition

Structure and replication
•
Human virus infections
•
Treatment

Antivirals

Vaccines

Lecture topics – 2
Lecture topics – 2
•
Immunity to viruses

Cell-mediated


Humoral
•
Role of Complement
•
Vaccination against viruses

Inactivated vaccines

Live vaccines
•
Interferon

Definition of a Virus
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
Definition of a Virus
Obligate
Intracellular
Parasite

Virion Structure
Virion Structure

Nucleic Acid
Spike
Projections
Protein
Capsid
Lipid Envelope
Virion
Associated
Polymerase

Virion Morphology
Virion Morphology
•
Simple Structure
•
Repetitive Structure
•
High Level of Redundancy

Virus Morphology
Virus Morphology
Helical Icosahedral

Virus Replication
Virus Replication
1
Virus attachment
and entry
1
1

2
Uncoating of virion
2
2
3
Migration of
genome nucleic
acid to nucleus
3
3
4
Transcription
5
Genome replication
4
4
5
5
6
Translation of virus
mRNAs
6
6
7
Virion assembly
7
7
8
Release of new
virus particles

8
8

Cytopathic Effect (cpe)
Cytopathic Effect (cpe)
Adenovirus Herpes virus

Transmission of Viruses
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

Virus Tissue Tropism
Virus Tissue Tropism
•
Targeting of the virus to specific tissue
and cell types
•
Receptor Recognition

CD4+ cells infected by HIV

CD155 acts as the receptor for
poliovirus

In vivo
In vivo
Disease Processes
Disease Processes
•
Cell destruction
•
Virus-induced changes to gene
expression
•
Immunopathogenic disease

Acute Virus Infection
Acute Virus Infection
Amount of virus
Time
Symptoms

Virus

Acute Virus Infections
Acute Virus Infections
•
Localised to specific site of
body
•
Development of viraemia
with widespread infection of
tissues

Poliovirus
Poliovirus

Poliovirus
Poliovirus
•
Enterovirus.
•
Possesses a RNA
genome.
•
Transmitted by the
faecal oral route.
•
Cause of
gastrointestinal illness
and poliomyelitis.
Properties of the virus

Poliovirus Infection
Poliovirus Infection
Gut
Virus
Infection
Virus excretion
in the faeces
Viraemia
Non-neuronal
tissues
Neuronal
tissues
Paralysis

Incidence of Poliomyelitis
Incidence of Poliomyelitis
40
30
20
10
0
1950 1960 1970 1980
Number of cases (in thousands)
A
B
Poliovirus vaccines
A: Salk – killed inactivated
vaccine.
B: Sabin – live attenuated

vaccine

Influenza A 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
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
Spread of influenza virus

Respiratory Tract
Respiratory Tract

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999
0
100
200
300
400
500
600
Rate per 100 000 population
Year
Epidemic activity
Higher than expected
seasonal activity
Normal seasonal activity
Baseline activity

Weekly consultation rates for influenza and influenza-like illness: Weekly
Weekly consultation rates for influenza and influenza-like illness: Weekly
Returns Service of the Royal College of General Practitioners, 1988 to
Returns Service of the Royal College of General Practitioners, 1988 to
1999
1999
CDR Weekly Report: 5th November 1999

Point mutation of HA and NA
genes
ANTIGENIC DRIFT
ANTIGENIC SHIFT
Genetic Reassortment
Human H3N2
Avian H3N8
Human H2N2
Generation of Novel Influenza A Viruses
Generation of Novel Influenza A Viruses