Viruses

• Viruses


  Infectious disease-causing agents that reproduce only in living host cells and are inert outside host cell
• Viruses


  • Very small particles: 20-250 nm — need electron microscope to see
  • Simplest virus has a core with nucleic acid RNA or DNA within a protein or "capsid" shell
  • Carry only one or two enzymes which decode genetic instructions: they're important for taking over the host cell
  • Need a host cell (bacteria, plant or animal) in which to live and replicate

  
• Virus classification


  • Based on: Morphology (size and shape), Chemical composition and structure of genome, Mode of replication

  
• Virus function as parasites

  Obligate intracellular parasites that use the host's biochemical machinery to grow and reproduce. During replication, they fully depend on the biochemistry of the host cell.
• Virus function


  1. Virus infects host and commands the cells to produce and replicate the virus itself

  2. Their main purpose is delivery of genome into host cell to allow its expression (transcription and translation) by the host cell

  
• Virion


  A fully assembled infectious virus, consisting of nucleic acid and a protein coat (capsid)

  
• Virus structure


  • Some virus families have an outer membrane layer or "envelope" derived from the cell membrane of the host cells and incorporates both host and viral-specific antigens
  • Envelope consists of lipid bilayer with glycosylated membrane proteins (spikes) that are important for attachment and penetration into the cell, enabling the virus to bypass the host's defence mechanism

  
• Virus envelope


  • An additional protective coat to protect the virus from being damaged, but the virus can still survive if the envelope is removed/destroyed
• Viral morphology


  • Helical - identical subunits self-assemble into helical array surrounding nucleic acid
  • Icosahedral - polyhedron have 20 equilateral triangular faces and 12 vertices

  
• Viral pathogenesis


  • Includes: Implantation/attachment of virus at the entry site, Local replication once entered the host cell, Spread to target organs, Shedding of virus into environment

  
• Factors affecting viral pathogenesis


  • Accessibility of virus to tissue, Cell susceptibility to virus multiplication, Virus susceptibility to host defences

  
• Virus replication


  1. Virus particle attaches to host cell

  2. Virus coat breaks down (uncoating), releasing genetic instructions/genome into host cell

  3. Injected genetic material recruits host cell's enzymes to take over the biochemical machinery

  4. Enzymes make constituents for more new virus particles

  5. New particles assemble into new viruses (fully assembled virions)

  6. New viruses break free from host cell

  
• Vaccines


  Weakened virus or a fragment of a virus that is injected into the body, allowing the immune system to produce antibodies to be ready to attack the virus if it infects the body in the future
• Viral vaccines and cell lines used for production

  • Measles Virus - Chick Embryo Cells
  • German Measles Virus - Monkey Kidney Cells
  • Mumps Virus - Embryonated Chicken Eggs
  • Pollio Virus - Monkey Kidney
  • Yellow Fever Virus - Chick Embryo Eggs
  • Varicella-Zosta Virus - Human Embryo Eggs
• Viral diseases


  • Varicella-Zosta (Chicken Pox), Cold Sores, Genital Warts, Measles, HIV, Hepatitis B
• Antiviral chemotherapy


  Targets the replication process of the virus inside the cell, including: Virucidal agents (directly inactivate viruses), Antiviral agents (inhibit viral replication at cellular level), Immunomodulators (alter host immune response, e.g. vaccines)

  
• Potential sites for pharmacological treatment of viruses


  • Prevent uncoating of virus (fusion inhibitor), Nucleotide analogues (inhibit DNA replication), Blocking/inhibiting reverse transcriptase (prevent change from ssRNA to dsDNA), Blocking protease activity (causing newly produced viruses to be inactive)

  
• Types of antiviral drugs


  • Fusion inhibitors, Ion channel inhibitors, Polymerase inhibitors, Protease inhibitors, Neuraminidase inhibitors
• Examples of antiviral drugs


  • Amantadine, Acyclovir, Ribavirin, Zidovudine, Oseltamivir, Zanamivir
• Swine flu (s-OIV)


  • Triple-reassortment virus with genes from human, swine & avian influenza A viruses
  • Transmitted by indirect, direct contact, inhalation of infectious droplets
  • Incubation 2-14 days, infectious for 1-7 days from onset of illness
  • Frequent symptoms: Fever, cough, sore throat, diarrhoea & vomiting, myalgia and joint pain
  • 26% need hospitalization, complications include pneumonia, dehydration, respiratory failure, renal or multi-organ failure
  • Antiviral agents used: Oseltamivir (neuraminidase inhibitor), Zanamivir (similar to oseltamivir)
  • Resistance already detected, vaccines have been developed
• Virus definition


  Sub microscopic entity consisting of a single nucleic (RNA or DNA) acid surrounded by a protein coat (capsid) and capable of replication only within the living cells of bacteria, animals or plants