Transcript Viruses
Viruses 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 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 • • • • 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 • • • • 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) • • • • • • 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 • • • • • • • 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