Animal Viruses

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Animal Viruses
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Properties of Obligate Intracellular Parasites
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Definitions of Terms for Viruses and Viral
Infections

• Virion mature infectious virus particle
• Capsid protein shell that encloses and protects
  the viral nucleic acid
• Capsomer polymers of polypeptide chains which
  are the morphological units of icosahedral
  capsids
• Core internal part of a virus partciel, which
  consists of the nucleic acid and closely
  associated proteins
• Nucleocapsid structure composed of the capsid
  containing the nucleic acid or core

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Definitions continued

• Envelope viral membrane, consisting of a lipid
  bilayer, proteins and glycoproteins
• Peplomers (spike proteins) viral proteins or
  glycoproteins that project from the envolope

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Definitions continued

• ssRNA single stranded RNA of the same polarity
  as messenger RNA
• -ssRNA single stranded RNA complementary to
  messenger RNA
• vRNA RNA of an intact ssRNA virus particle
• cRNA RNA that is complementary to the RNA of an
  intact ssRNA virus particle
• cDNA complementary DNA made from a viral RNA by
  recombinant procedures

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Definitions continued

• DNA dependent DNA polymerase (DNA polymerase)
  an enzyme that uses DNA as a template for
  producing DNA
• DNA dependent RNA polymerase (RNA polymerase)
  an enzyme that uses DNA as a template for
  producing RNA
• RNA dependent RNA polymerase (reverse
  transcriptase) an enzyme that uses RNA as a
  template for producing DNA

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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 virus
• Permissive cells cells that support the
  complete virus life cycle, with production of
  infectious virus particles

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Definitions continued

• Nonpermissive cells cell which permit only part
  of the virus life cycle, usually transformed by
  viruses, especially DNA viruses
• Productive infection infection that results in
  th eproduction of infectious virus
• Nonproductive infection infection that has no
  infectious virus, cells may be transformed
• Defective 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

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Structure of Animal Viruses

• Size not seen under the microscope
• Nucleic Acid varies
• 12 codons -numerous codons
• Segmented genomes
• Either DNA or RNA, not both
• Capside
• Capsomeres protect nucleic acid

• Baltimores Classification
• Class I ds DNA
• Class II ss DNA
• Class III ds RNA
• Class IV ss RNA
• Class V ss- RNA
• Class VI RNA tumor viruses (retroviruses)

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Baltimores Classification System

• Class I ds DNA
• Class II ss DNA
• Class III ds RNA
• Class IV ss RNA
• Class V ss- RNA
• Class VI RNA tumor viruses (retroviruses)

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• Capside
• Capsomeres 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 envelopes
• Surrounded by nucleocapsid
• Formed by modified host cellular membrane
• Contain host derived phosphlipid bilayer
• Contains 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

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Viral envelopes

• Characteristics
• Surrounded by nucleocapsid
• Formed by modified host cellular membrane
• Contain host derived phospholipid bilayer
• Contains 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

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Viral Proteins

• Matrix 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 proteins
• Found 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

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Physiochemical Classification

• 6 DNA and 13 RNA virus families, with one
  unclassified virus, classification is based on
• Chemical nature of the nucleic acid
• Symmetry of the nucleocapsid
• Presence or absence of the envelope
• Number of capsomeres for isometric virions
• Diameter of the nucleocapsids for helical viruses
• Icosahedral viruses may be either DNA or RNA,
  enveloped or not
• Helical viruses are all RNA, enveloped

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Replication cycle, Productive Cycle

• Attachment
• Penetration
• Uncoating
• Transcription/translation of early mRNA
• Replication of viral nucleic acid
• Transcription/translation of late mRNA
• Assembly of virions
• Release

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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
  specificity
• Specie, tissue and physiological state of the
  cells determine the type of receptors present
• Viruses tend to be host specific

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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

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Replication cycle Productive Infection

• Uncoating the viral nucleic acid
• Attachment seems to lead to a conformation change
  in the capsid
• Sometimes
• cellular enzymes uncoat the viral nucleic acid
• Synthesis of virus products may take place
  without completely uncoating the viral nucleic
  acid

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Replication cycle Productive Infection

• Synthesis (Eclipse)
• Use host cell enzymes in replcation
• Synthesis of virus encoded macromolecules
  proceeds synthesis of
• early proteins
• Viral genome proteins
• Late proteins
• Requires viral RNA and host machinery

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Replication cycle Productive Infection

• Assembly
• Release
• Disintegration of infected cell (burst),
  especially for naked nucleocapsids
• Slow release, with aquision of the envolope as
  the nucleocapsid buds through a virus-modified
  cellular membrane
• Reverse phagocytosis

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Mixed Virally Infected Cells

• Mixed Viruses
• Interference with replication of second virus
• Complementation as some viruses are incomplete
  and can replicate only in the presence of another
  virus
• Multiple Same Viruses
• Recombination due to crossing over
• Reassortment in viruses with segmented genomes
• Reactivation 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 region
• Interference at a large multpilicity of
  infection, may have defective particles produced

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Cultivation of Viruses

• Cell systems
• Intact animals very expensive
• Embryonated 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

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Consequences of a Viral Infection

• As seen in tissue culture
• Cell death and lysis ? CPE
• Proliferation of host cell ? masses of cells
  piled on each other
• Fusion of membranes of adjacent cells leading to
  a multinucleate giant cell, thereby forming a
  syncytia
• Transformation 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 cell
• Steady-state persistent infection infected
  cells produce and release virus

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Measurement of Animal Viruses

• Infectious units
• Enumeration of total number of particles
• Observation of viruses products as antigens
• DNA probes
• PCR

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Measurement of Animal Viruses

• Infectious units
• Plaque formation in tissue culture
• Pock formation on chorioallantoic membrane in
  chick embryo
• Focus formation if virus causes proliferation of
  cells
• Serial dilution end point method
• Cytopathic effects in tissue culture
• Characteristic symptoms in experimental animals
  or eggs
• Enumeration of total number of virus particles
• Electron microscopy
• hemagglutionation

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Measurement of Animal Viruses

• Observation of viruses and virus products as
  antigens
• Complement fixation
• Direct flourescent antibody
• Gel immunodiffusion or immunoelectrophoresis
• Radioimmunoassay
• ELISA

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Measurement of Animal Viruses

• DNA Probes
• Dot 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 blood
• Other amplification methodologies, with HIV

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Antibody TiterResponse to Animal Viruses

• Types of tests
• Neutralization of infectivity
• Complement fixation
• Hemagglutination-inhibition
• Latex agglutination
• Indirect ELISA
• Indirect flourescent antibody
• Radioimmunoassay
• Western blot

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Uses of Tests

• Diagnosis
• Need four-fold rise in titer
• High concentrations on IgM
• Surveys
• Determine need for immunization
• Lookback

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Control of Virus Diseases

• Prevention of transmission
• Public health surveillance of the environment
• Education
• Isolation of cases of the disease
• Passive immunization of contracts
• Active immunization to creat an immune population
• Vaccines may be either
• Live attentuated
• Inactivated
• Subunit vaccines

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Live Attenuated vs Inactivated Vaccines
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Treatment of Viral Disease

• Symptomatic
• Immune serum
• Drugs
• Interferons
• Type 1 a and ß interferons
• Type 2 gamma interferons

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Treatment of Viral Disease

• Interferons
• Type 1 a and ß 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
• a interferons used to treat hairy cell leukemia,
  Kaposis sarcoma, Hepatitis B, genital warts
• ß interferons used to treat multiple sclerosis

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Treatment of Viral Disease

• Interferons
• Type 2 - gamma interferons
• Important cytokines for antiviral response with
  activated T cells
• gamma interferons used to treat chronic
  granulomatous disease

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Patterns of Pathogenesis

• Localized infections
• Disseminated infections
• Inapparent infections
• Persistent infections

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Patterns of Pathogenesis of Viral Infections

• Localized infections viral replication near
  site of entry
• Disseminated Infections
• local multiplication at site
• extension through lymphatics
• multiplication at second site
• secondary viremia
• Infectuon at target organ

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Patterns of Pathogenesis of Viral Infections

• Inapparent Infections
• Very common, result from infection by attenuated
  virus
• Considered to be very important since
• Represent an unrecognized source of dissemination
  of a virus
• Confer immunity to the host
• Persistent infections
• Latent infections - Herpes
• Chronic infections Hepatitis B

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Immunity to Virus Infectiuons

• Interferons
• Defective interference particle
• Viral neutralization
• Ig binding
• Antibody complement mediated cylolysis
• Antibody dependent cell mediated cytotoxicity
• Lysis by natural killer cells
• Cell mediated immunity

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Immunity to Virus Infections

• Interferons inhibit viral multiplication,
  temporary localized recurrences
• Defective interference particles allow for
  temporary localized protection
• Viral neutralization by antibody prevents viral
  infection from entering susceptable cells
• Ig binding to virus can enhance defense host
  mechanisms by promoting phagocytosis

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Immunity to Virus Infections

• Antibody complement mediated cylolysis
• Antibody dependent cell mediated cytotoxicity
• Lysis by natural killer cells
• Cell mediated immunity
• Important for recovery from host viral infection,
  especially when host cells are not killed

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Unusual immune reactions to viral diseases

• Immunological tolerance
• Examples of exposure in utero include
• Lymphocytic choriomeningitis (LCM)
• Rubella
• Diseases from virally-induced immunological
  response
• Cell mediated response hep B
• Enhancing antibodies flavovirus
• Inactivated virus vaccine respiratory syncytial
  virus

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Dynamics of Parasitism

• Transmissibilty
• Mobility of the host, coupled with loss of
  virulence
• Resistence to the effects of the parasite, so
  that the host lives long enough for viral
  transmission
• Infects a variety of animals, although specific
  to species
• If infect new species, tend to be more virulent

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New virus diseases, zoonoses

• Environmental changes increase human contact with
  vector
• Genetic changes in virus
• Point mutations
• Intramolecular recombination
• Genetic reassortment

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How do we eliminate a virus from a population?

• Limited antigenic types
• Lifelong immunity
• Limited subclinical cases
• No carrier state
• No animal reservoir
• Good vaccine