Virions,%20Prions,%20and%20Viroids:%20Infectious%20Agents%20of%20Animals%20and%20Plants

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Virions, Prions, and Viroids Infectious Agents
of Animals and Plants

• Topics
• Structure
• Classification
• Methods of study
• Viral infections
• Viral tumors
• Human tumors

Viral host range Prions
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Structure

• Analogous to bacteriophages
• Capsid (protein coat) made up of capsomeres
• Naked vs. enveloped viruses
• Attachment proteins or spikes in enveloped
  viruses
• Distinct from bacteriophages
• Segmented viruses (contain more than one RNA
  molecule)
• Plant viruses do not enter via receptor attachment

Shapes Isometric Helical Pleomorphic
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Classification

• Taxonomy in constant flux
• Most common taxonomic criteria for animal
  viruses
• Genome structure (DNA or RNA, ss vs. ds,
  segmented or not)
• Virus particle structure (isometric, helical,
  pleomorphic)
• Presence or absence of viral envelope
• Viruses infecting vertebrates are divided into 14
  RNA families and 7 DNA families (Ending -viridae)

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Coronavirus

• Classification
• Coronaviridae (Family)
• Coronavirus (Genus)
• Common cold virus (Species) (together with
  rhinoviruses)
• Structure
• non-seg., lin., ssRNA, helical, env.

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Herpesvirus

• Classification
• Herpesviridae (Family)
• Herpesvirus (Genus)
• Herpes simplex type 1 / type 2 (Species)
• Structure
• non-seg., lin., dsDNA, helical, env.

http//darwin.bio.uci.edu/faculty/wagner/movieind
ex.html
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Cells Infected with a Herpes Virus
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Groupings Based on Routes of Transmission

• Not a taxonomic grouping more than one family
  may be included in one transmission grouping

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Methods of Study

• Much more expensive and difficult to study animal
  viruses than bacteriophages
• Cultivation in host cells
• Living animal
• Embryonated chicken eggs
• Cell or tissue culture ( in vitro)

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Methods of Study cont Quantitation

• Plaque assay (useful for infective and lytic
  viruses)
• Virion counting with EM
• Quantal assay (ID50 or LD50)
• Hemagglutination (e.g.influenza virus)

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Acute Viral Infections

• Productive infection of relative short duration
• Naked viruses usually cause cell lysis, while
  enveloped viruses do not
• Symptoms due to localized or widespread tissue
  damage
• Host defense mechanisms gradually eliminate virus
• Examples of acute infections
• Time course

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Mumps, Measles, Influenza, and Poliomyelitis
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Time Course of Acute Viral Infection
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HEV
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Reproductive Cycle

1. Attachment
2. Entry
3. Uncoating
4. Replication of NA and protein
5. Maturation of viral particles
6. Cell lysis
7. Spreading and shedding
8. Transmission to next host

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Transcription Strategy
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Mechanisms of Release

1. Cytopathic effect Unlike virulent phages most
   animal viruses do not encode for cell lytic
   enzymes. Instead degenerative changes associated
   with the virus lead to cell death.
2. Budding from plasma membrane (most common) or
   from Golgi apparatus. May or may not kill cell.
   Enveloped viruses

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Persistent Viral Infections

• Virus continually present in body. Released by
  budding.
• May or may not cause disease
• Carrier able to spread disease
• Four categories (if more than one applies
  complex infections)
• Late complications from acute infections
• Latent infections
• Chronic infections
• Slow infections

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Late Complications from Acute Infections

1. Subactue sclerosing panencephalitis (SSPE) -
   years following measles in 1 in 300,000. Virus
   persistent in brain tissue with decreased
   transcription of viral genome. Lack of envelope
   protein production prevents immune response to
   eliminate infected cells. High Ab, low CMI. Exact
   mechanism unknown.
2. Progressive (pan)encephalitis following rubella

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Latent Infections

• Acute infection ? symptomless period ?
  reactivation of disease
• Symptoms of reactivation may differ from original
  disease
• No measurable viral particles during symptomless
  period
• Examples HSV-1 and HSV-2 varicella

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Chickenpox- Varicella

• Blister-like rash on surface of skin and mucous
  membranes. Blisters usually appear first on trunk
  and face, then spread to almost everywhere else.

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Shingles or Herpes Zoster
About 20 of those people who have had chicken
pox will get zoster at some time during their
lives. Most people will get zoster only once.
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Shingles comes from latin cingulum, which means
girdle or belt. It occurs in an area of the skin
that is supplied by the sensory fibers of a
single nerve- dermatome. Rash appears as
well-defined band on one side of body, or on one
side of face, arms or legs.
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Chronic Infections

• Virus can be demonstrated at all times
• Disease may or may not be present for extended
  time periods or show up late (carriers!)

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Slow Infections

• Gradual increase of infectious agent over long
  time period often no apparent symptoms for long
  time (preclinical phase)
• Usually slowly progressive lethal diseases
• Examples
• AIDS
• Lentivirus
• prions

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HIV Example of Complex Infection

• Retrovirus ssRNA, envelope
• RNA ? DNA (with the help of reverse
  transcriptase) ? permanent integration into host
  genome (provirus)
• Polyprotein is cleaved into individual proteins
  with viral protease ? assembly of virions ?
  budding

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Viral Tumors (Neoplasms)

• Benign
• Malignant ? cancer, metastasizes
• Proto-oncogenes and oncogenes are regulatory
  genes
• Properties of normal and transformed cells
• Only about 15 of human tumors are due to viruses
• Examples of human tumors
• Kaposis sarcoma (herpes virus)
• Squamous cell carcinomas (HPV)
• Hepatocellular carcinoma (HBV and HCV)

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Kaposis Sarcoma
Purplish lesions of a skin cancer not usually
seen in young men
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Viral Host Range

• Mostly species and even cell type specific
• Exception Zoonotic viruses are transmissible
  from animals (arthropods, vertebrates) to man
  (zoonosis)
• Arboviruses (West Nile virus), rabies etc.
• Modification of host range due to
• Phenotypic mixing
• Genetic reassortment

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Genetic Reassortent

• In segmented viruses
• Simultaneous infection of one cell with 2
  different types of viruses leads to exchange of
  genetic information
• Creation of major new influenza strains ?
  resulting in pandemics
• Antigenic shift vs. antigenic drift

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Other Infectious Agents Prions

• Small proteinaceous infectious particles (resist
  inactivation by procedures that modify nucleic
  acids)
• Prion diseases are often called spongiform
  encephalopathies because of the post mortem
  appearance of the brain with large vacuoles in
  the cortex and cerebellum
• Human prion diseases
• CJD Creutzfeld-Jacob Disease
• BSE Mad cow disease (BSE)
• GSS Gerstmann-Straussler-Scheinker syndrome
• Kuru

The End