Introduction to Virology

Tony Mazzulli, MD, FRCPC

September 21, 2009

VirologyObjectives1. To understand the characteristics/properties

of viruses which make them unique microbial pathogens

2. To examine the structure and composition of viruses

3. To understand the classification of viruses

4. To understand the replication strategies of viruses

Virus Properties

• Virus is defined as a nucleoprotein complex which infects cells and uses their metabolic processes to replicate

• Smallest known infective agents• Metabolically inert - no metabolic activity

outside host cell; must enter host cell to replicate

• Most are highly species specific

Virus versus Virion

• Virus is a broad general term for any aspect of the infectious agent and includes:

• the infectious or inactivated virus particle

• viral nucleic acid and protein in the infected cell

• Virion is the physical particle in the extra-cellular phase which is able to spread to new host cells; complete intact virus particle

Characteristics of Infectious Microorganisms

Property Bacteria Viruses Fungi Protozoa

Size (nm)

Nuclear structure

Obligate intracellular

Nucleic acids

Culture on Artificial media

100 -10,000

Prokaryotes

No

DNA/RNAHaploid

Yes

30 - 300

----

Yes

DNA or RNA

No

4,000 - 40,000

Eukaryotes

No

DNA/RNA

Yes

4,000 - 40,000

Eukaryotes

No

DNA/RNA

Yes

Relative Sizes of MicroorganismsLimits ofresolution

50,000-Velectron microscope0.003 m

Light microscope0.2 m

Unaidedhuman eye40 m

Microscopicprotozoa & fungi4 - 40 m

Classes of organisms

Bacteria0.1 - 10 mViruses

0.03 - 0.3 m

0.001 0.01 0.1 1 10 100

Size (m)

Virus Structure

• consist of a core of nucleic acid surrounded

by a protein coat +/- envelope• components of virus particle include:

i) nucleic acid - DNA or RNA

- single - or double - stranded

- intact / fragmented;

linear / circular

- encodes very few proteins

Virus Structure

ii) proteins:

a) structural - capsid made of capsomeres

- serve as antigens which elicit an immune response

b) enzymes - differ from host cell

- targets of antiviral therapy

iii) envelope - found in some viruses;

- lipoprotein envelope containing viral and host cell components

- destroyed by lipid solvents

Virus Structure

Virus particles exhibit 3 types of capsid symmetry:1. helical - tubular: most helical viruses possess an

outer envelope (eg. measles)

2. icosahedral - isometric or cubic; may (eg. herpes) or may not (eg. adenovirus) possess an outer envelope

3. complex - does not conform to either of above (eg. vaccinia)

Adenovirus

Herpesviridae

Influenza Virus

Smallpox Virus

Virus Classification

• Historically based on:– Host preference: Plant, insect, animal, human– Target organ: respiratory, hepatic, enteric, etc.– Vector: arboviruses

• Overlapping, inconsistent

• Currently based on molecular biology of genome and biophysical structure

Virus Classification

• Viruses with similar structural, genomic & replication properties are grouped into families (suffix: viridae) e.g. Herpesviridae

• Families subdivided into genera (suffix: virus) e.g. Herpes simplex virus, Cytomegalovirus, Varicella zoster virus

• Subtypes based on nucleotide sequence and antigenic reactivities e.g. Herpes simplex virus type 1, Herpes simplex virus type 2

Virus Classification

Viruses

Nucleic acid: DNA RNA

Envelope: Yes No

Symmetry: Cubic Helical (Icosahedral) (Cylindrical)

Classification of Some Common Viruses

Family VirusesType of

Nucleic Acid EnvelopeCapsid

SymmetryPicornaviridae Enteroviruses,

polio, hep. Ass (+) RNA No I

Caliciviridae Norwalk virus ss (+) RNA No ITogaviridae Rubella ss (+) RNA Yes IRhabodoviridae Rabies ss (+) RNA Yes HParamyxoviridae Parainfluenza,

RSV, measles,mumps

ss (-) RNA Yes H

Orthomyxoviridae Influenza ss (-) RNA Yes HRetroviridae HIV 1,2, HTL I,II ss (+) RNA Yes IHepadnaviridae Hepatitis B ds DNA Yes UnknownParvoviridae Parovirus B - 19 ss (+) or (-) DNA No IAdenoviridae Adenovirus ds DNA No IHerpesviridae HSV, CMV, EBV,

VZV, HHV6 ds DNA Yes I

I = icosahedral, H = helical

Virus Classification (Common)

DNA RNA Hepatitis B

Human Papilloma Virus Parvovirus B19

Adenovirus Herpesviridae

Polyomaviruses

Influenza RSV

Parainfluenza Hepatitis A, C, D, E

Enteroviruses Encephalitis viruses

Measles, Mumps, Rubella Norwalk, Rotavirus Virtually all others

Viral Replication

i) adsorption (attachment)

ii) entry

iii) uncoating

iv) transcription

v) synthesis of virus components

vi) assembly

vii) release

Viral Replication

i) Adsorption (attachment):• random collision• interaction between specific proteins on viral surface

and specific receptors on target cell membrane (tropism)

• not all cells carrying a receptor for a particular virus can be productively infected by that virus

Viral Replication

i) Adsorption (attachment):

– some viruses may use more than one host cell receptor (e.g. HIV)

– able to infect a limited spectrum of cell types (host range)

– most neutralizing antibodies are specific for virion attachment proteins

Viral Replication

ii) Entry (penetration):• 2 mechanisms - endocytosis

- fusion of virus envelope with cell membrane

iii) Uncoating:• release of viral genome• cell enzymes (lysosomes) strip off the virus

protein coat• virion can no longer be detected; known as the

“eclipse period”

Viral Replication

iv) Transcription/Translation/Synthesis:a) DNA viruses:

• replicate their DNA in host cell nucleus mediated by

viral enzymes

• synthesize capsid and other proteins in cytoplasm

using host cell enzymes

• new viral proteins move to nucleus where they

combine with new DNA to form new viruses

• Exception - Poxviruses synthesize their parts in host

cell’s cytoplasm

Viral Replication

iv) Transcription/Translation/Synthesis:b) RNA viruses:

–“+” sense RNA acts as mRNA - viral proteins are made immediately in cytoplasm mediated by viral enzymes

–“-” sense RNA (e.g. influenza) - lst makes a “+” sense RNA copy via viral enzyme

Viral Replicationiv) Transcription/Translation/Synthesis:

• Retroviridae (e.g. HIV)

• Contain enzyme “Reverse transcriptase”

• “+” sense Viral RNA cDNA integrated into host cell

chromosone

• mRNA (for viral proteins) and progeny virion RNA are synthesized from

integrated viral DNA by host cell enzymes (RNA polymerases)

Viral Replication

v) Synthesis:• Protein synthesis - 2 types

• structural • non-structural (enzymes for replication)

• Nucleic acid synthesis• new virus genome• most often by a virus - coded polymerase or

replicase; with some DNA viruses a cell enzyme carries this out

Viral Replication

vi) Assembly:• may take place in cell nucleus, cytoplasm or

(with most enveloped viruses) at the plasma membrane

vii) Release:• sudden rupture of cell• gradual extrusion (budding) of enveloped

viruses through the cell membrane• may occur together with assembly

Enveloped Virus Entry via Fusion

Non-enveloped Virus Entry via Endocytosis

Outcome of Viral Infections

Adsorption (attachment)

Entry

Replication Latency Transformation

ReleaseCell Fusion

Outcome of Viral Infections

Virus-host cell interaction may result in:

1. Cell death (lytic) - due to cytopathic effect of virus

2. Cell transformation - cell converted to malignant or cancerous cell

3. Latent infection (occult) - persistent infection in quiescent state which may reactive anytime to produce disease; continuous or intermittent shedding

4. Cell fusion to form multinucleated cells

Persistent Viral Infections

3 types of persistent viral infection (some overlap):

1. Chronic carrier - eg. Hepatitis B; results in chronic illness

2. Latent infection - eg. Herpesviridae; result in symptomatic or asymptomatic shedding

3. Slow virus infections - due to prolonged incubation period (eg. Measles virus and SSPE)

Host - Organism Relationship

• Interaction between host and organism affecting the development and outcome of an infection includes:

– Host’s primary physical barriers– Host’s immunologic ability to control and eliminate

the invading organisms– Organism’s ability to evade destruction/virulence– Ability of organism to spread in the body

Virulence of Viruses & Evasion of the Immune Response

• Poorly understood processes:– Antigenic variation– Some viruses encode receptors for various mediators of

immunity (eg. IL1 & TNF) thus blocking their ability to interact with receptors on their intended targets

– Some viruses (eg. HIV) reduce expression of class I MHC proteins, thus reducing ability of cytotoxic T cells to kill the virus-infected cells

– Direct cell-to-cell propagation– Attenuated viruses (eg. Vaccine strains)

Definitions

• Exposure: contact with a potentially infectious agent

• Infection: persistence on or within another living organism

• Disease: end product (damage) resulting from an infectious process

• Incubation: time from infection to development of symptoms / disease

Virus: Incubation Times

Hours to 1-2 days:• Respiratory viruses• GI viruses1 to 3 weeks:• Measles/Mumps/Rubella• VZV, HSV• Chlamydia• Enteroviruses, Polio• WNV

Weeks to months:

• Hepatitis viruses

• HIV

• EBV

• Rabies

Months to years:

• Prions

Routes of Transmission

• Horizontal transmission:– Direct contract (secretions, blood etc.)– Respiratory (aerosol)– Contaminated inanimate objects– Insect vector (mosquitoes, ticks, etc.)– Zoonoses

• Vertical transmission:– Mother to fetus [Transplacental

(Congenital), Perinatally]

Viruses - Transmission

• Can occur - with or without disease- during asymptomatic

shedding- during incubation period

• Transmission results in primary infection disease; reactivation results in secondary disease

Viruses - Epidemiology

• mode of transmission

• age

• gender

• ethnic background / country of origin

• travel history

• occupation

• season

• underlying medical condition(s)

Thank you for your attention!