Viruses

Small size(nanometer-nm) Filtrable agents Obligate intracellular parasites: Can not make energy or proteins independently of a host cell

Viral genome 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 Herpesviridae Adenoviridae Hepadnaviridae Papovaviridae Parvoviridae

RNA viruses Paramyxoviridae Orthomyxoviridae Coronaviridae Arenaviridae Rhabdoviridae Picornaviridae Filoviridae Togaviridae Bunyaviridae Flaviviridae Retroviridae Caliciviridae Reoviridae Delta

Naked viruses DNA or RNA +structural proteins(capsid)= Nucleocapsid

Enveloped viruses Nucleocapsid+envelope

Virus capsid Helical (rod) Icosahedral (spherical)

DNA genome Double stranded Single stranded Linear Circular

DNA viruses Enveloped: Pox Herpes Hepadna Naked: Papova Adeno Parvo (ss)

RNA viruses Mostly single stranded Reoviruses ds Segmented: orthomyxoviruses, reoviruses, arenaviruses

Naked viruses Stable to environmental conditions. Temperature, acid, proteases, detergents, drying

Naked viruses 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) 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) RhinovirusVP1-VP2-VP3 complex AdenovirusFiber protein RotavirusVP7 RabiesG protein Influenza AHemaglutinin HIVgp120

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 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 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 Quarantine Elimination of the vector Pasive Immunization Vaccination Treatment