Viruses, Prions and Viroids Infectious Agents of Animals and Plants

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Viruses, Prions and ViroidsInfectious Agents of
Animals and Plants

• Chapter 14

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Structure and Classification of Animal Viruses

• Classification of animal viruses
• Taxonomic criteria based on
• Genomic structure
• DNA or RNA
• Single stranded or double stranded
• Virus particle structure
• Isometric
• Pleomorphic
• Helical
• Presence or absence of envelope

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Structure and Classification of Animal Viruses

• Groupings based on route of transmission
• Disease causing viruses often grouped by route of
  transmission
• Enteric viruses
• Generally transmitted via fecal-oral route
• Often cause gastroenteritis
• Some can cause systemic disease
• Respiratory viruses
• Usually inhaled via infected respiratory droplets
• Generally remain localized in respiratory tract
• Zoonotic viruses
• Transmitted from animal to human via animal
  vector
• Sexually transmitted viruses
• Can causes lesions on genitalia or cause systemic
  infections

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Interactions of Animal Viruses with Their Host

• Outcome of infection of eukaryotic cells depends
  on factors independent of cell
• Special importance are defense mechanisms of host
• Viruses may develop relationships with normal
  hosts
• No obvious disease or damage is caused to host
• State of balanced pathogenicity
• Relationships divided into two categories
• Acute
• persistent

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Interactions of Animal Viruses with Their Host

• Acute infections
• Usually short in duration
• Host may develop long lasting immunity
• Result in productive infections
• Produce large number of viruses during
  replication
• Disease symptoms result from tissue damage and
  infection of new cells

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Interactions of Animal Viruses with Their Host

• Acute infections
• Reproductive cycle of animal virus can be
  compared to virulent bacteriophage
• Essential steps include
• Attachment
• Entry into susceptible cell
• Targeting site of reproduction
• Uncoating of virion
• Removing protein coat exposing nucleic acid
• Replication nucleic acid and protein
• Maturation
• Cell lysis
• Spreading within host
• Shedding outside host
• Transmission to next host

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Interactions of Animal Viruses with Their Host

• Persistent infections
• Viruses continually present in host
• Releases from infected cell via budding
• Can be divided into three categories
• Latent infections
• Chronic infections
• Slow infections
• Categories distinguished by ability to detect the
  virus during period of persistence

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Interactions of Animal Viruses with Their Host

• Persistent infections
• Latent infections (presence of virus not always
  detectable)
• Infection is followed by symptomless period then
  reactivation
• Infectious particles not detected until
  reactivation
• Symptoms of reactivation and initial disease may
  differ
• Example
• Herpes simplex viruses 1 and 2 (HSV1 and HSV2)
• Shingles (zoster)

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Interactions of Animal Viruses with Their Host

• Chronic infections
• Infectious virus can be detected at all times
• Disease may be present or absent during extended
  times or may develop late
• Best know example
• Hepatitis B
• A.k.a serum hepatitis

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Interactions of Animal Viruses with Their Host

• Slow infections
• Infectious agent gradually increases in amount
  over long period of time
• No significant symptoms apparent during this time
• Two groups of infectious agents cause slow
  infections
• Retroviruses which includes HIV Prions

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Viruses and Human Tumors

• Double stranded DNA viruses responsible for most
  virus induced tumors in humans
• Tumor viruses interact with host cells on one of
  two ways
• Virus can go through productive cycle and lyse
  cell
• Virus can transform cell without killing it
• Cancers caused by DNA viruses result from
  integration of viral genome into host DNA
• Transformed genes are expressed
• Uncontrolled growth results

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Viral Genetic Alterations

• Genome exchange in segmented viruses
• Viruses can alter properties via
• Mutation
• Genetic reassortment
• Genetic reassortment of viruses results from two
  viruses infecting the same cell
• Each virus incorporates segments of viral DNA
• One segment comes from one virion
• Rest of segment come from other virion
• Reassortment responsible fro antigenic shift and
  antigenic drift in influenza virus

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Methods of Studying Viruses

• First Cultivate a host
• Viruses multiply only inside host cell
• Viruses are obligate intercellular parasites
• Host cells are cultivated in the laboratory in
  cell culture or tissue culture
• Tissue culture prepared directly from an animal
  host is termed primary culture

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Methods of Studying Viruses

• Quantitation
• Plaque assay
• Determines number of viruses in solution
• Know volume of solution added to actively
  metabolizing cells
• Infection lyses cells and leads to clear zone or
  plaque surrounded by uninfected cells
• Each plaque represents one virion
• Plaques are only produced by infected cells

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Methods of Studying Viruses

• Quantitation
• Counting virions with electron microscope
• Used with pure preparations
• Concentration determined by counting number of
  virions in sample
• May distinguish infective from non-infective
  agents

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Methods of Studying Viruses

• Quantitation
• Quantal assays
• Provides and approximate concentration
• Dilutions of virus preparation administered into
  animal cells
• Chick embryos often used
• Endpoint is dilution at which 50 of inoculated
  host are infected or killed
• May be reported as either
• ID50 infective dose
• LD50 lethal dose

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Methods of Studying Viruses

• Hemagglutination
• Some animal viruses clump or agglutinate with red
  blood cells
• Termed hemagglutination
• The highest dilution showing maximum
  agglutination is titer of the virus
• i.e. Adding more virus does not increase the
  agglutingation

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Plant Viruses

• Number of plant diseases are caused by viruses
• Can be of major economic importance
• Infection may be recognized via outward signs
  including
• Pigment loss
• Marks on leafs and fruit
• Tumors
• Stunted growth
• Plants generally do not recover from viral
  infections

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Plant Viruses

• Spread of plant viruses
• Viruses infect plants through wound in plant cell
  wall
• Viruses do not attach to specific cell receptors
• Once started, infection spreads from cell to cell
  through plasmodesmata
• Many viruses resistant to inactivation
• Viruses can be transmitted through soil
  contaminated by prior growth
• Viruses spread through grafting healthy plants to
  infected plants

• Viruses can spread via parasitic vine called
  dodder
• Vine establishes simultaneous connection between
  two plants
• Serves as conduit of transfer

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Other Infectious Agents

• Prions
• Proteinaceous infectious agent
• Linked to a number of fatal human diseases
• All afflictions cause brain degeneration
• Brain tissue develops sponge like holes
• Disease termed transmissible spongiform
  encephalopathies
• Symptoms may not appear for years after infection

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Other Infectious Agents

• Prions
• Apparently arose following gene encoding normal
  prion protein
• Mutation caused protein to have different folding
  properties
• Mutated protein resistant to proteases
• Normal protein sensitive
• Resists UV light and nucleases
• Due to lack of nucleic acid
• Inactivated by chemicals that denature proteins

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Other Infectious Agents

• Viroids
• Defines group of pathogens much smaller and
  distinctly different from viruses
• Consist solely of small single-stranded RNA
  molecule
• Varies in size
• Have no protein coat
• Allows them to be resistant to proteases

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Other Infectious Agents

• Viroids
• Other viroid properties include
• Replicates autonomously in susceptible cells
• Single viroid capable of infecting a cell
• Viroid RNA is circular and resistant to nuclease
  digestion
• All identified viroids infect plants
• Diseases include
• Potato spindle tuber
• Chrysanthemum stunt
• Cadang-cadang