Animal Viruses Powerpoint
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13-Sep-2014 -
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Transcript of Animal Viruses Powerpoint
Animal Viruses
Properties of Obligate Intracellular Parasites
Property Rickettsiae Chlamydiae Viruses
Generation of metabolic energy
+ - -
Independent protein synthesis
+ + -
Rigid cell envelope + + Variable
Antibiotic susceptibility
+ + -
Mode of replication Fission Fission Host cell synthesis
Nucleic acids in mature particle
DNA/RNA DNA/RNA DNA or RNA, not usually both
Definitions of Terms for Viruses and Viral Infections
Virion – mature infectious virus particleCapsid – protein shell that encloses and protects the viral nucleic acidCapsomer – polymers of polypeptide chains which are the morphological units of icosahedral capsidsCore – internal part of a virus partciel, which consists of the nucleic acid and closely associated proteinsNucleocapsid – structure composed of the capsid containing the nucleic acid or core
Definitions continued
Envelope – viral membrane, consisting of a lipid bilayer, proteins and glycoproteinsPeplomers (spike proteins)– viral proteins or glycoproteins that project from the envolope
Definitions continued+ssRNA – single stranded RNA of the same polarity as messenger RNA-ssRNA – single stranded RNA complementary to messenger RNAvRNA – RNA of an intact ssRNA virus particlecRNA – RNA that is complementary to the RNA of an intact ssRNA virus particlecDNA – complementary DNA made from a viral RNA by recombinant procedures
Definitions continued
DNA dependent DNA polymerase (DNA polymerase) – an enzyme that uses DNA as a template for producing DNADNA dependent RNA polymerase (RNA polymerase) – an enzyme that uses DNA as a template for producing RNARNA dependent RNA polymerase (reverse transcriptase) – an enzyme that uses RNA as a template for producing DNA
Definitions continued
Transfection – in infection of mammalian or bacterial cells by vare viral nucleic acid Transformation – stable hereitable change in the genetic makeup and phenotype of a cell resulting from the infection of that cell by a virusPermissive cells – cells that support the complete virus life cycle, with production of infectious virus particles
Definitions continued:Nonpermissive cells – cell which permit only part of the virus life cycle, usually transformed by viruses, especially DNA virusesProductive infection – infection that results in th eproduction of infectious virusNonproductive infection – infection that has no infectious virus, cells may be transformedDefective virus – virus that is not capable of going through its entire replicative cycle unless the cell is infected with a complete virus particle as well.Cytopathic effect – observable damage to a cell resulting from virus infection
Structure of Animal Viruses
Size – not seen under the microscopeNucleic Acid – varies
12 codons -numerous codonsSegmented genomesEither DNA or RNA, not both
CapsideCapsomeres protect nucleic acid
Baltimore’s Classification
Class I – ds DNAClass II – ss DNAClass III – ds RNAClass IV – ss+ RNAClass V – ss- RNAClass VI – RNA tumor viruses (retroviruses)
Baltimore’s Classification SystemClass I – ds DNAClass II – ss DNAClass III – ds RNAClass IV – ss+ RNAClass V – ss- RNAClass VI – RNA tumor viruses (retroviruses)
CapsideCapsomeres protect nucleic acid
• Complex – found in pox viruses
• Isometric/Icosahedral– 20 facets with 12 vertices– Each hexomer has six
neighbors– Number of capsomeres per
capsid is used to classify icosahedral animal viruses
• Helical – Naked helical viruses are all
resistent– No naked human helical
viruses
Viral envelopesSurrounded by nucleocapsidFormed by modified host cellular membraneContain host derived phosphlipid bilayerContains virus derived proteins and glycoproteins
• Some are enzymes• Some provide attachment to
cellsEnvelopes are more fragile than naked viruses and are often inactivated by lipid solvents
Viral envelopesCharacteristics
Surrounded by nucleocapsidFormed by modified host cellular membraneContain host derived phospholipid bilayerContains virus derived proteins and glycoproteins
• Some are enzymes• Some provide attachment to cells
Envelopes are more fragile than naked viruses and are often inactivated by lipid solvents
• Ether sensitive – have membranes• Ether resistent – have no membranes
Viral ProteinsMatrix proteins – M proteins
Found associated with the inner layer of the envelope, seem to make the envelope more rigid and help with organization of the virus particle
Fusion proteins – F proteinsFound on the envelope surface in some virus groups, cause viruses and virus-infected cells to fuse with uninfected cells
Nonstructural viral proteins Enzymes found in the core of the virions of some virus types
Physiochemical Classification6 DNA and 13 RNA virus families, with one unclassified virus, classification is based on:
Chemical nature of the nucleic acidSymmetry of the nucleocapsidPresence or absence of the envelopeNumber of capsomeres for isometric virionsDiameter of the nucleocapsids for helical viruses
Icosahedral viruses may be either DNA or RNA, enveloped or notHelical viruses are all RNA, enveloped
Replication cycle, Productive Cycle
AttachmentPenetrationUncoatingTranscription/translation of early mRNAReplication of viral nucleic acidTranscription/translation of late mRNAAssembly of virionsRelease
Replication cycle; Productive Infection
Attachment to specific receptor sites on the host cell membrane
Presence of specific receptor sites on the cell is the most important determinant of host specificitySpecie, tissue and physiological state of the cells determine the type of receptors presentViruses tend to be host specific
Replication cycle; Productive Infection
Penetration by one of several methods:Nonenveloped
• Naked viruses may have rearrangement of the capsid protein after binding to cells, virus slips through by direct penetration of the membrane
• Most are engulfed by receptor mediated endocytosis, with partial breakup of the capsid in the vacoule, followed by migration into the cyctplasm
Enveloped • Engulfement of virions by receptor mediated
endocytosis via coated pits to coated vesicles which then fuse with lysosomes to form phagosomes
• Fusion of the viral envelope and cell membrane, leaves the nucleocapsid inside the cell
Replication cycle; Productive Infection
Uncoating the viral nucleic acidAttachment seems to lead to a conformation change in the capsidSometimes• cellular enzymes uncoat the viral nucleic
acid• Synthesis of virus products may take
place without completely uncoating the viral nucleic acid
Replication cycle; Productive Infection
Synthesis (Eclipse)Use host cell enzymes in replcationSynthesis of virus encoded macromolecules proceeds synthesis of:• early proteins• Viral genome proteins• Late proteins
Requires viral RNA and host machinery
Replication cycle; Productive Infection
AssemblyRelease
Disintegration of infected cell (burst), especially for naked nucleocapsidsSlow release, with aquision of the envolope as the nucleocapsid buds through a virus-modified cellular membraneReverse phagocytosis
Mixed Virally Infected CellsMixed Viruses
Interference with replication of second virusComplementation as some viruses are incomplete and can replicate only in the presence of another virus
Multiple Same VirusesRecombination due to crossing overReassortment in viruses with segmented genomesReactivation can lead to rescue of a marker on an inactivated particle by superinfection of a live viris particle or virus particle inactivated by a different regionInterference at a large multpilicity of infection, may have defective particles produced
Cultivation of Viruses
Cell systemsIntact animals – very expensiveEmbryonated egg Tissue culture
• Primary cell lines edrived directly from animals• Diploid cell strains from embryonic tissues, such
as from a chick embryo can grow for 40-50 generations
• Permament (cancerous) cell lines that may divide indefinately, such as Hela cells
Consequences of a Viral InfectionAs seen in tissue culture
Cell death and lysis CPEProliferation of host cell masses of cells piled on each otherFusion of membranes of adjacent cells leading to a multinucleate giant cell, thereby forming a syncytiaTransformation into malignant cancer cells
• Viral nucleic acid integrated into host DNA• Viral nucleid acid arranged in circular duplex (similar
to a nucleosome)Silent infection (latency) with no morphologogical change in the cellSteady-state persistent infection – infected cells produce and release virus
Measurement of Animal Viruses
Infectious unitsEnumeration of total number of particlesObservation of viruses & products as antigensDNA probesPCR
Measurement of Animal VirusesInfectious units
Plaque formation in tissue culturePock formation on chorioallantoic membrane in chick embryoFocus formation if virus causes proliferation of cellsSerial dilution end point method
• Cytopathic effects in tissue culture• Characteristic symptoms in experimental animals or eggs
Enumeration of total number of virus particlesElectron microscopyhemagglutionation
Measurement of Animal Viruses
Observation of viruses and virus products as antigens
Complement fixationDirect flourescent antibodyGel immunodiffusion or immunoelectrophoresisRadioimmunoassayELISA
Measurement of Animal Viruses
DNA ProbesDot hybridization with autoradiography or ELISA readout
• HPV 16,18 most common with cervical cancer
PCR gene amplification using small primer pairs of ssDNA
Nucleic Acid Amplification Testing (NAT) for calculation of viral load, such as HIV in bloodOther amplification methodologies, with HIV
Antibody Titer:Response to Animal Viruses
Types of testsNeutralization of infectivityComplement fixationHemagglutination-inhibitionLatex agglutinationIndirect ELISAIndirect flourescent antibodyRadioimmunoassayWestern blot
Uses of Tests
DiagnosisNeed four-fold rise in titerHigh concentrations on IgM
SurveysDetermine need for immunizationLookback
Control of Virus Diseases
Prevention of transmissionPublic health surveillance of the environmentEducationIsolation of cases of the diseasePassive immunization of contractsActive immunization to creat an immune population
• Vaccines may be either:– Live attentuated– Inactivated– Subunit vaccines
Live Attenuated vs Inactivated Vaccines
Property Attenuated Inactivated
Duration of Immunity
Many years Usually less
Antibody response IgG, IgA IgG only
Cell mediated response
Good Poor
Interference Occasionally No
Reversion to Virulence
Rarely No
Treatment of Viral Disease
SymptomaticImmune serum DrugsInterferons
Type 1 – α and β interferonsType 2 – gamma interferons
Treatment of Viral Disease
InterferonsType 1 – α and β interferons
• Important cytokines for antiviral response• Synthesis induced by an infected cell with either a active or
inactive virus, ds RNA or other compounds• Species – specific, not virus specific• Binds to recipient cell, activates a protein transcription
factor• Have both a + and – effect on cells of the immune system,
such as flu-like symptoms– α interferons used to treat hairy cell leukemia, Kaposi’s
sarcoma, Hepatitis B, genital warts– β interferons used to treat multiple sclerosis
Treatment of Viral Disease
InterferonsType 2 - gamma interferons• Important cytokines for antiviral response
with activated T cells– gamma interferons used to treat chronic
granulomatous disease
Patterns of Pathogenesis
Localized infectionsDisseminated infectionsInapparent infectionsPersistent infections
Patterns of Pathogenesis of Viral Infections
Localized infections – viral replication near site of entryDisseminated Infections
local multiplication at siteextension through lymphaticsmultiplication at second sitesecondary viremiaInfectuon at target organ
Patterns of Pathogenesis of Viral Infections
Inapparent InfectionsVery common, result from infection by attenuated virusConsidered to be very important since:
• Represent an unrecognized source of dissemination of a virus
• Confer immunity to the host
Persistent infectionsLatent infections - HerpesChronic infections – Hepatitis B
Immunity to Virus Infectiuons
InterferonsDefective interference particleViral neutralizationIg bindingAntibody complement mediated cylolysisAntibody dependent cell mediated cytotoxicityLysis by natural killer cellsCell mediated immunity
Immunity to Virus Infections
Interferons inhibit viral multiplication, temporary localized recurrencesDefective interference particles allow for temporary localized protectionViral neutralization by antibody prevents viral infection from entering susceptable cellsIg binding to virus can enhance defense host mechanisms by promoting phagocytosis
Immunity to Virus Infections
Antibody complement mediated cylolysisAntibody dependent cell mediated cytotoxicityLysis by natural killer cellsCell mediated immunity
Important for recovery from host viral infection, especially when host cells are not killed
Unusual immune reactions to viral diseases
Immunological toleranceExamples of exposure in utero include:
• Lymphocytic choriomeningitis (LCM)• Rubella
Diseases from virally-induced immunological response
Cell mediated response – hep BEnhancing antibodies – flavovirusInactivated virus vaccine – respiratory syncytial virus
Dynamics of Parasitism
TransmissibiltyMobility of the host, coupled with loss of virulenceResistence to the effects of the parasite, so that the host lives long enough for viral transmissionInfects a variety of animals, although specific to speciesIf infect new species, tend to be more virulent
New virus diseases, zoonoses
Environmental changes increase human contact with vectorGenetic changes in virus
Point mutationsIntramolecular recombinationGenetic reassortment
How do we eliminate a virus from a population?
Limited antigenic typesLifelong immunityLimited subclinical casesNo carrier stateNo animal reservoirGood vaccine