Lesson 4.1

• Virus General Characteristics

  1. Acellular - neither prokaryotic nor eukaryotic
  2. Obligate intracellular pathogens
  3. Smallest infectious agent with nucleic acid
  4. NO ribosomes, enzymes for energy production, lack many enzymes for reproduction
  5. NOT susceptible to antibiotics

  Are viruses cell? No, they are acellular and doesn't have nucleus, nucleoid, ribosomes, cell membrane but they have genome. They are not also living things as they are not capable of respiration.
• Basic Structures of Viruses

  • Nucleic Acid (Genome)
  • Proteins
  • Lipid Envelope
• Nucleic Acid (Genome)

  • contains genetic information for replication
  • either DNA or RNA but not both
  • double-stranded or single-stranded
  • segmented or non-segmented
  • Polarity (for RNA): (+) or (-) sense
  • Virion - complete virus particle (at least nucelocapsid/ nucleocapsid + envelope)
  • Viroid - defective/ incomplete virus
• Proteins

  • Structural Proteins: Capsid / Protein Coat

  -Facilitates genome transfer/attachment

  -Protection vs. nuclease and solvents (ether-resistant)

  -Provides symmetry

  • Enzymes

  -Not present in all viruses, if present, in low amounts

  -Usually RNA polymerase; reverse transcriptase

  • Antigenic Characteristics
  • Hemagglutinin
  • Transcription Factors
• Lipid Envelope

  • Acquired by some viruses during maturation
  • Sensitive to treatment with ether and organic solvents (ether-susceptible)

  -Loss of lipid coat = loss of infectivity

  • Contain glycoproteins

  -virus-encoded proteins, host-derived sugars

  -surface glycoproteins mediate attachment
• Virus Morphology/Structure

  Capsid symmetry seen under electron microscopy

  1. Cubic - Icosahedral (20 faces) and spherical; DNA viruses except Poxvirus; (+) sense RNA viruses
  2. Helical - (-) sense RNA viruses
  3. Complex - Poxvirus (brick-shaped); bacteriophage
• Viral Replication Cycle

  1. Adsorption/Attachment
  2. Penetration/Engulfment - entry
  3. Uncoating - nucleic acid release
  4. Synthesis/Eclipse - protein synthesis; intact virions
  5. Assembly/Maturation/Morphogenesis - virion formation
  6. Release - exit
• Pathogenesis Definition

  • Viral disease - abnormally caused by viral disease
  • Clinical disease - symptomatic
  • Syndrome - group of signs and symptoms
  • Subclinical infection - asymptomatic
  • Viral pathogenesis - process of disease on cellular level
  • Disease pathogenesis - process on the host level as the whole
  • Pathogenic - can cause disease
  • Virulent - can cause more severe disease
  • Incubation Period: Clinical - time interval between exposure and appearance of symptoms vs Biologic - period between contact and manifestation of symptoms
• Pathogenesis Concepts

  • Many viral infections are subclinical
  • Same syndrome may be produced by various viruses
  • Same virus may produce various disease
  • Outcome is determined: viral factors and host factors
  • Outcome is influenced by both virus' and host's: environment and genetics
• Cellular Response to Viral Infection

  1. Cell Lysis
  2. Discernible Effect - inclusion body, cell transformation, cell dysfunction
  3. Undiscernible Effect - viral multiplication, exposure without attachment or entry
• Host Responses to Viral Infection

  1. Death
  2. Discernible Effect - severe disease, classic disease, mild/moderate disease
  3. Undiscernible Effect - infection without disease, exposure without infection
• Disease Pathogenesis

  1. Entry and Primary Replication
  2. Viral Spread and Tropism

  • Portal of entry or systemic
  • Viral Tropism - associated with cell surface receptors

  3. Cell injury and Clinical Illness

  • Destruction of cells/tissues
  • Immune response

  4. Outcome

  • Recovery/ immunity
  • Chronic infection
  • Death

  5. Viral Shedding - usually involves surface of entry