Viruses
Viruses
• Small size(nanometer-nm)
• Filtrable agents
• Obligate intracellular parasites:
Can not make energy or proteins independently of
a host cell
Viral genome
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RNA or DNA
Never both !!!!!!
A naked capsid or +envelope
Do not replicate by division=binary fission
Viral components are produced in the host cell and
assembled.
Consequences of viral properties
• Viruses are not living: acellular
• Viruses must be infectious to endure in nature.
• Viruses must be able to use host cell processes to
produce their components
(viral messenger RNA, protein and identical copies of
the genome)
Consequences of viral properties
• Viruses must encode any required processes not
provided by the cell.
• Viral components must self-assemble.
Knowledge of
• the structural (size and morphology)
• Genetic (type and structure of nucleic acid)
Provides insight how the virus replicates, spreads and
causes disease.
Viruses
• Very small
• Nanometers (nm)
• Clinically important viruses range from 18nm
(parvoviruses) to 300 nm (poxviruses).
Viruses
• Range from small and simple (parvoviruses and
picornaviruses) to
• Large and complex viruses (pox viruses and
herpesviruses)
Naming of viruses
• Structure: size, morphology and nucleic acid
(picornavirus (small RNA) togavirus (cloak)
• Members of its family : papovavirus (papilloma,
polyoma, vacuolating viruses)
• The disease they cause:smallpox(poxviruses)
Naming of viruses
• Tissue or organ tropism: adenovirus, enterovirus,
reovirus(respiratory,enteric, orphan)
• Place of isolation: Norwalk, Coxsackie and many
toga,arena and bunyaviruses are named after African
places where they were first isolated.
Means of classification
• Biochemical characteristics: structure and mode of
replication: current means
• Host cell (host range): animal (human, mouse, bird),
plant, bacteria
• Means of transmission: arboviruses by insects
• Disease: ensephalites, hepatitis viruses
Viruses
• DNA viruses: 6 families
• Poxviridae
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Herpesviridae
Adenoviridae
Hepadnaviridae
Papovaviridae
Parvoviridae
RNA viruses
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Paramyxoviridae
Orthomyxoviridae
Coronaviridae
Arenaviridae
Rhabdoviridae
Filoviridae
Bunyaviridae
Retroviridae
Reoviridae
Picornaviridae
Togaviridae
Flaviviridae
Caliciviridae
Delta
Naked viruses
• DNA or RNA +structural proteins(capsid)=
Nucleocapsid
Enveloped viruses
Nucleocapsid+envelope
Virus capsid
• Helical (rod)
• Icosahedral (spherical)
DNA genome
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Double stranded
Single stranded
Linear
Circular
• Enveloped:
• Pox
• Herpes
• Hepadna
• Naked:
• Papova
• Adeno
• Parvo (ss)
DNA viruses
RNA viruses
• Mostly single stranded
• Reoviruses ds
• Segmented: orthomyxoviruses, reoviruses,
arenaviruses
Naked viruses
• Stable to environmental conditions.
• Temperature, acid, proteases, detergents, drying
Naked viruses
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Can be spread easily(on fomites, from hand to hand)
Can dry out but retain infectivity
Many of them are transmitted by fecal-oral route
Resistant to acid and bile of the enteric tract
Enveloped viruses
• Environmentally labile
• Must stay wet
• Spreades in large droplets, secretions, respiratory
droplets, organ transplants and blood transfusion
Steps in viral replication (I)
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Recognition of the target cell
Attachment
Penetration
Uncoating
Macromolecular synthesis
Assembly of virus
Buddding of enveloped viruses
Release of virus
Steps in viral replication (II)
• Macromolecular synthesis:
-early mRNA and nonstructural protein synthesis
-replication of the genome
-late mRNA and structural protein synthesis
-posttranslational modification of protein
Viral attachment proteins
(VAP)
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Rhinovirus
VP1-VP2-VP3 complex
Adenovirus
Fiber protein
Rotavirus
VP7
Rabies
G protein
Influenza A
Hemaglutinin
HIV
gp120
Viral receptors
• Epstein-Barr virus:
Target cell: B cell (C3d complement receptor)
• HIV:
Target cell: Helper T cell(CD4 molecule and
chemokine coreceptor)
• Rhinovirus:
Target cell: Epitelial cell (ICAM-1)
Viral receptors
• Rabies virus:
Target cell: Neuron(Acatylcoline receptor)
• Influenza A virus:
Target cell: Epitelial cells(sialic acid)
• B19 parvovirus:
Target cell: Erythroid precursors ( Erythrocyte P
antigen-globoside)
Host range
• Viruses may only bind to receptors only on spesific
cell types on certain species
Human, mouse
• Susceptible target cell defines the tissue tropism
neurotropic, lymphotropic
Penetration
• Viropexis (receptor-mediated endocytosis):
naked viruses
• Fusion
enveloped viruses
Release
• Budding (enveloped)
• Lysis (naked)
Mechanisms of Viral Transmission
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Aerosols
Food, water
Fomites (e.g., tissues, clothes)
Direct contact with secretions (e.g., saliva, semen)
Sexual contact, birth
Blood transfusion or organ transplant
Zoonoses (animals, insects [arboviruses])
Disease and Viral Factors That
Promote Transmission
• Stability of virion in response to the environment
(e.g., drying, detergents, temperature)
• Replication and secretion of virus into transmissible
aerosols and secretions (e.g., saliva, semen)
• Asymptomatic transmission
• Transience or ineffectiveness of immune response to
control reinfection or recurrence
Risk Factors
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Age
Health
Immune status
Occupation: contact with agent or vector
Travel history
Lifestyle
Children in daycare centers
Sexual activity
Geography and Season
• Presence of cofactors or vectors in the environment
• Habitat and season for arthropod vectors
(mosquitoes)
• School session: close proximity and crowding
• Home-heating season
Modes of Control
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Quarantine
Elimination of the vector
Pasive Immunization
Vaccination
Treatment