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Medical Virology
Introduction to Basics
Dr.T.V.Rao MD
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History Virology
• Smallpox was endemic in
China by 1000BC. In response,
the practice of variolation was
developed. Recognizing that
survivors of smallpox
outbreaks were protected from
subsequent infection,
variolation involved inhalation
of the dried crusts from
smallpox lesions like snuff, or
in later modifications,
inoculation of the pus from a
lesion into a scratch on the
forearm of a child.
Dr.T.V.Rao MD
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Virus infections are Universal …….
Introduction to Virology
• A virus is an obligate intracellular
parasite containing genetic material
surrounded by protein
• Virus particles can only be
observed by an electron
microscope
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Introduction to Virology
• Recognizing the shape, size, and
structure of different viruses is critical to
the study of disease
– Viruses have an inner core of nucleic acid
surrounded by protein coat known as an
envelope
– Most viruses range in sizes from 20 – 250
nanometers
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Viral Properties
• Viruses are inert (nucleoprotein ) filterable Agents
• Viruses are obligate intracellular parasites
• Viruses cannot make energy or proteins independent of
a host cell
• Viral genome are RNA or DNA but not both.
• Viruses have a naked capsid or envelope with attached
proteins
• Viruses do not have the genetic capability to multiply
by division.
• Viruses are non-living entities
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Viruses are Ultramicroscopic
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The size of viruses
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VIRAL STRUCTURE – SOME
TERMINOLOGY
• virus particle = virion
• protein which coats the genome =
capsid
• capsid usually symmetrical
• capsid + genome = nucleocapsid
• may have an envelope
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Virion
• The complete
infectious unit
of virus particle
• Structurally
mature,
extracellular
virus particles.
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Virion
envelope
Capsid
Viral core
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Virion Structure
Lipid Envelope
Nucleic Acid
Protein
Capsid
Virion
Associated
Polymerase
Spike
Projections
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Distinguishing characteristics of viruses
• Obligate intracellular parasites
• Extreme genetic simplicity
• Contain DNA or RNA
• Replication involves disassembly
and reassembly
• Replicate by "one-step growth”
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How are viruses named?
• Based on:
- the disease they cause
poliovirus, rabies virus
- the type of disease
murine leukemia virus
- geographic locations
Sendai virus, Coxsackie virus
- their discovers
Epstein-Barr virus
- how they were originally thought to be contracted
dengue virus (“evil spirit”), influenza virus (the “influence” of bad
air)
- combinations of the above
Rous Sarcoma virus
Virus particle = virion
White, DO and Fenner, FJ.
Medical Virology, 4th Ed. 1994
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5 BASIC TYPES OF VIRAL STRUCTURE
icosahedral nucleocapsid
nucleocapsid
lipid bilayer
ICOSAHEDRAL
ENVELOPED ICOSAHEDRAL
helical nucleocapsid
COMPLEX
nucleocapsid
lipid bilayer
glycoprotein spikes
= peplomers
HELICAL
ENVELOPED HELICAL
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Dr.T.V.Rao MD
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Icosahedral
• Adeno-associated Virus (AAV) • Herpes Simplex Virus 1 (HHV1)
Adenovirus
Herpes Simplex Virus 2 (HHV2)
Human Immunodeficiency Virus (HIV)
B19
Human T-lymphotrophic Virus (HTLV)
Coxsackievirus - A
Norwalk Virus
Coxsackievirus - B
Papilloma Virus (HPV)
Cytomegalovirus (CMV)
Polio virus
Eastern Equine Encephalitis
Rhinovirus
Rubella Virus
Virus (EEEV)
Saint Louis Encephalitis Virus
Echovirus
Varicella-Zoster Virus (HHV3)
Epstein-Barr Virus (EBV)
Western Equine Encephalitis Virus
Hepatitis A Virus (HAV)
(WEEV)
Hepatitis B Virus (HBV)
Yellow Fever Virus
Hepatitis C Virus (HCV)
Hepatitis Delta Virus (HDV)
Hepatitis E Virus (HEV)
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Viral Structure
• Varies in size, shape and symmetry
• VIP for classification
• 3 types of capsid symmetry:
– Cubic
(icosahedral)
• Has 20 faces, each an equilateral triangle. Eg. adenovirus
– Helical
• Protein binds around DNA/RNA in a helical fashion eg. Coronavirus
– Complex
• Is neither cubic nor helical eg. poxvirus
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The Baltimore classification system
Based on genetic contents and replication strategies of
viruses. According to the Baltimore classification, viruses are
divided into the following seven classes:
1. dsDNA viruses
2. ssDNA viruses
3. dsRNA viruses
4. (+) sense ssRNA viruses (codes
5. (-) sense ssRNA viruses
6. RNA reverse transcribing viruses
7. DNA reverse transcribing viruses
directly for protein)
where "ds" represents "double strand" and "ss" denotes "single
strand".
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Virus Classification I
- the Baltimore classification
• All viruses must produce mRNA, or (+) sense RNA
• A complementary strand of nucleic acid is (–) sense
• The Baltimore classification has + RNA as its central point
• Its principles are fundamental to an understanding of virus
classification and genome replication, but it is rarely used
as a classification system in its own right
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From Principles of Virology Flint et al ASM Press
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Virus classification II the Classical system
• This is a based on three principles -
– 1) that we are classifying the virus itself,
not the host
– 2) the nucleic acid genome
– 3) the shared physical properties of the infectious agent
(e.g capsid symmetry, dimensions, lipid envelope)
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Virus classification III the genomic system
• More recently a precise ordering of viruses
within and between families is possible based
on DNA/RNA sequence
• By the year 2000 there were over 4000 viruses
of plants, animals and bacteria - in 71 families,
9 subfamilies and 164 genera
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Viral Structure - Overview
Nucleic acid
Nucleocapsid
Capsid
Envelope protein
Membrane protein
Spike protein
Viral envelope**
Fig 1. Schematic overview of the structure of animal viruses
** does not exist in all viruses
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Icosahedral capsids
a) Crystallographic structure of a
simple icosahedral virus.
b) The axes of symmetry
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Cubic or icosahedral symmetry
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ICOSAHEDRAL SYMMETRY
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ICOSAHEDRAL SYMMETRY
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ICOSAHEDRAL SYMMETRY
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ICOSAHEDRAL SYMMETRY
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Adenovirus
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Adenovirus
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Helical symmetry
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Helical
• California Encephalitis Virus
Coronavirus
Hantavirus
Influenza Virus (Flu Virus)
Measles Virus ( Rubeola)
Mumps Virus
Para influenza Virus
Rabies Virus
Respiratory Syncytial Virus(RSV)
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• Helical symmetry
How to
assemble
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Helical symmetry
In 1955, Fraenkel,
Conrat, and Williams
demonstrated that
tobacco mosaic virus
(TMV) spontaneously
formed when mixtures
of purified coat
protein and its
genomic RNA were
incubated together.
TMV, a filamentous virus
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Enveloped helical virus
Enveloped icosahedral virus
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Properties of naked viruses
•
•
•
•
•
•
•
•
Stable in hostile environment
Not damaged by drying, acid, detergent, and heat
Released by lysis of host cells
Can sustain in dry environment
Can infect the GI tract and survive the acid and bile
Can spread easily via hands, dust, fomites, etc
Can stay dry and still retain infectivity
Neutralizing mucosal and systemic antibodies are
needed to control the establishment of infection
Naked viruses( Non Enveloped )
• Adeno-associated Virus (AAV)
Adenovirus
B19
Coxsackievirus - A
Coxsackievirus - B
Echovirus
Hepatitis A Virus (HAV)
Hepatitis E Virus (HEV)
Norwalk Virus
COMPLEX SYMMETRY
surface view
White, DO and Fenner, FJ.
Medical Virology, 4th Ed. 1994
cross section
POXVIRUS FAMILY
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ENVELOPE
• OBTAINED BY BUDDING THROUGH A
CELLULAR MEMBRANE (except poxviruses)
• POSSIBILITY OF EXITING CELL WITHOUT
KILLING IT
• CONTAINS AT LEAST ONE VIRALLY CODED
PROTEIN
– ATTACHMENT PROTEIN
• LOSS OF ENVELOPE RESULTS IN LOSS OF
INFECTIVITY
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CLASSIFICATION
NUCLEIC ACID
• RNA or DNA
•
•
•
•
segmented or non-segmented
linear or circular
single-stranded or double-stranded
if single-stranded RNA
– is genome mRNA (+) sense or
complementary to mRNA (-) sense
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Genome
• The genome of a virus can be either DNA or
RNA
• DNA-double stranded (ds): linear or circular
Single stranded (ss) : linear or circular
• RNA- ss:segmented or non-segmented
ss:polarity+(sense) or polarity –(nonsense)
ds: linear (only reovirus family)
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DNA
double-stranded
line
ar
circular
singl sing
e
le
RNA
single-stranded
line
ar
circular
multip singl sing
le
e
le
multip
le
doublestranded
single-stranded
linear
linear (circular)*
singl
e
multipl
e
(+)sense
sing
le
multip
le
(-)sense
sing
le
multip
le
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Viral genome strategies
•
•
•
•
•
•
•
•
dsDNA (herpes, papova, adeno, pox)
•ssDNA (parvo)
•dsRNA (reo, rota)
•ssRNA (+) (picorna, toga, flavi, corona)
•ssRNA (-) (rhabdo, paramyxo, orthomyxo,
bunya, filo)
•ssRNA (+/-) (arena, bunya)
•ssRNA (+RTase) (retro, lenti)
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DNA VIRUSES
DOUBLE STRANDED
ENVELOPED
HERPESVIRIDAE
HEPADNAVIRIDAE
NON-ENVELOPED
CIRCULAR
PAPILLOMAVIRIDAE
POLYOMAVIRIDAE
(formerly grouped together as the
PAPOVAVIRIDAE)
SINGLE STRANDED
NON-ENVELOPED
COMPLEX
ENVELOPED
PARVOVIRIDAE
POXVIRIDAE
LINEAR
ADENOVIRIDAE
All families shown are
icosahedral except for
poxviruses
Modified from Volk et al., Essentials of Medical Microbiology, 4th Ed. 1991
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DNA viruses
From Principles of
Virology Flint et al
ASM Press
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RNA VIRUSES
SINGLE STRANDED
positive sense
ENVELOPED
ICOSAHEDRAL
FLAVIVIRIDAE
TOGAVIRIDAE
RETROVIRIDAE
HELICAL
CORONAVIRIDAE
SINGLE STRANDED
negative sense
DOUBLE STRANDED
NONENVELOPED
ENVELOPED
NONENVELOPED
ICOSAHEDRAL
HELICAL
ICOSAHEDRAL
PICORNAVIRIDAE
CALICIVIRIDAE
ASTROVIRIDAE
ORTHOMYXOVIRIDAE
PARAMYXOVIRIDAE
RHABDOVIRIDAE
FILOVIRIDAE
BUNYAVIRIDAE
ARENAVIRIDAE
REOVIRIDAE
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RNA viruses
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BASIC STEPS IN VIRAL LIFE CYCLE
•
•
•
•
ADSORPTION
PENETRATION
UNCOATING AND ECLIPSE
SYNTHESIS OF VIRAL NUCLEIC ACID AND
PROTEIN
• ASSEMBLY (maturation)
• RELEASE
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RECEPTOR
VIRUS
ICAM-1
polio
CD4
HIV
acetylcholine
rabies
EGF
vaccinia
CR2/CD21
EpsteinBarr
herpes
HVEM
Sialic acid
Influenza,
reo, corona
Virus Replication
1 Virus attachment
2
3
1
5
4
5
6
4
2
3
7
8
7
6
and entry
Uncoating of virion
Migration of
genome nucleic
acid to nucleus
Transcription
Genome replication
Translation of virus
mRNAs
Virion assembly
Release of new
virus particles
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ADSORPTION
• TEMPERATURE INDEPENDENT
• REQUIRES VIRAL ATTACHMENT
PROTEIN
• CELLULAR RECEPTORS
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PENETRATION
- ENVELOPED VIRUSES
•FUSION WITH PLASMA MEMBRANE
•ENTRY VIA ENDOSOMES
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PENETRATION
herpesviruses, paramyxoviruses, HIV
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PENETRATION
- ENVELOPED VIRUSES
•FUSION WITH PLASMA MEMBRANE
•ENTRY VIA ENDOSOMES, FUSION WITH
ACIDIC ENDOSOME MEMBRANE
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VIRUS UPTAKE VIA ENDOSOMES
• CALLED
–VIROPEXIS / ENDOCYTOSIS /
PINOCYTOSIS
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PENETRATION
NON-ENVELOPED VIRUSES
entry directly across
plasma membrane:
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Replicative cycle
• As obligate intracellular parasites, Virus must
enter and replicate in living cells in order to
“reproduce” themselves. This “growth cycle”
involves specific attachment of virus,
penetration and uncoating, nucleic acid
transcription, protein synthesis, maturation
and assembly of the virions and their
subsequent release from the cell by budding
or lysis
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UNCOATING
• NEED TO MAKE GENOME AVAILABLE
• ONCE UNCOATING OCCURS, ENTER ECLIPSE
PHASE
• ECLIPSE PHASE LASTS UNTIL FIRST NEW
VIRUS PARTICLE FORMED
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SYNTHESIS OF VIRAL NUCLEIC ACID
AND PROTEIN
• MANY STRATEGIES
• NUCLEIC ACID MAY BE MADE IN
NUCLEUS OR CYTOPLASM
• PROTEIN SYNTHESIS IS ALWAYS IN
THE CYTOPLASM
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ASSEMBLY AND MATURATION
• NUCLEUS
• CYTOPLASM
• AT MEMBRANE
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RELEASE
• LYSIS
• BUDDING THROUGH PLASMA MEMBRANE
• NOT EVERY RELEASED VIRION IS INFECTIOUS
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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
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Viruses enter the body of the host
in a variety of ways, for example...
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The commonest forms of
transmission are via...
INHALED DROPLETS
in sneezing of coughing
for example the COMMON COLD
or INFLUENZA VIRUSES.
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or by
...
drinking water or
eating raw food, for example,
HEPATITIS A and POLIOVIRUS.
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The commonest forms of
transmission are also via...
sexual intercourse for example
HIV and HEPATITIS B and...
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also...
vertical transmission from mother to baby for example
HIV, HEPATITIS B and RUBELLA...
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also...
bites of vector arthropods such as
mosquitoes for example YELLOW FEVER,
RIFT VALLEY FEVER and DENGUE.
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Most viral infections...
do not lead to such serious
complications and the host...
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get well after a period of sickness
to be immune for the rest of their lives.
Examples are MEASLES INFECTION,
RUBELLA or German measles,
MUMPS and many others...
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A bacteriophage
• A bacteriophage is any one of a number of
viruses that infect bacteria. They do this by
injecting genetic material, which they carry
enclosed in an outer protein capsid. The
genetic material can be ssRNA, dsRNA, ssDNA,
or dsDNA ('ss-' or 'ds-' prefix denotes singlestrand or double-strand) along with either
circular or linear arrangement.
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Structure of Bacteriophage
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Classification of Bacteriophages
• The dsDNA tailed phages, or Caudovirales,
account for 95% of all the phages reported in
the scientific literature, and possibly make up
the majority of phages on the planet.
However, other phages occur abundantly in
the biosphere, with different virions, genomes
and lifestyles. Phages are classified by the
International Committee on Taxonomy of
Viruses (ICTV) according to morphology and
nucleic acid.
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Sub-viral agents
• Satellites
–
–
–
–
–
Contain nucleic acid
Depend on co-infection with a helper virus
May be encapsidated (satellite virus)
Mostly in plants, can be human e.g. hepatitis delta virus
If nucleic acid only = virusoid
• Viroids
– Unencapsidated, small circular ssRNA molecules that replicate
autonomously
– Only in plants, e.g. potato spindle tuber viroid
– Depend on host cell polII for replication, no protein or mRNA
• Prions
– No nucleic acid
– Infectious protein e.g. BSE
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Viroids & Prions
• Viroids
– ss RNA genome and the smallest known pathogens.
– Affects plants
• Prions
–
–
–
–
–
Infectious particles that are entirely protein.
No nucleic acid
Highly heat resistant
Animal disease that affects nervous tissue
Affects nervous tissue and results in
• Bovine spongiform encepahltits (BSE) “mad cow disease”,
• scrapie in sheep
• kuru & Creutzfeld-Jakob Disease (CJD) in humans
Dr.T.V.Rao MD
84
Viroids
• Viroids are small (200-400nt),
circular RNA molecules with a rodlike secondary structure which
possess no capsid or envelope which
are associated with certain plant
diseases. Their replication strategy
like that of viruses - they are obligate
intracellular parasites.
Dependovirus /Virusoids
• Viroids are small (200-400nt), circular
RNA molecules with a rod-like secondary
structure which possess no capsid or
envelope which are associated with
certain plant diseases. Their replication
strategy like that of viruses - they are
obligate intracellular parasites.
(Prions)
• Prions are rather ill-defined infectious
agents believed to consist of a single type of
protein molecule with no nucleic acid
component. Confusion arises from the fact
that the prion protein & the gene which
encodes it are also found in normal
'uninfected' cells. These agents are
associated with diseases such as
Creutzfeldt-Jakob disease in humans,
scrapie in sheep & bovine spongiform
encephalopathy (BSE) in cattle.
THANK YOU
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