Viruses 1


Viruses 1

Viruses 1

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Viral Acquisition and Transmission

  • Airborne
    • Chickenpox (varicella)
    • Influenza
    • Measles (Rubeola)
    • Mumps
    • Rubella (German measles)
    • Smallpox
  • Food, water
    • Viral gastroenteritis
    • Norovirus
    • Hepatitis A
    • Poliomyelitis
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Viral Acquisition and Transmission

  • Direct contact
    • AIDS (HIV)
    • Cold sores - Herpes Simplex Virus type 1
    • Human papillomavirus - cervical cancer
    • Genital herpes - Herpes Simplex Virus type 2
    • Leukaemia - retroviruses
    • Hepatitis B, C
  • Arthropod
    • Yellow fever
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What are Viruses?

  • Not cells
    • acellular
  • Carriers of genetic material
    • DNA or RNA
  • Requires a host for replication
  • Obligatory intracellular parasites
    • self assembly
  • Cause diseases (human, animals, plants, insects, bacteria)
  • Difficult to treat
  • Viruses differ greatly is size and morphology
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Viral Envelope

  • It surrounds the viral capsid in some but not all viruses
    • e.g. HIV, herpes, influenza
  • It is derived from portions of the host cell membranes (phospholipids and proteins) during viral replication
    • cytoplasmic membrane
    • cell nuclear envelope
    • organelles (Golgi apparatus, endoplasmic reticulum)
  • Enveloped viruses are susceptible to desiccation (low survival in the environment) and physical (heat) and chemical (disinfectant) challanges 
    • they can survive longer when mixed with fomites, e.g. blood, mucous
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Viral Envelope

  • It contains host proteins but also virus-encoded proteins
    • immune system - evade or antigens
  • It is responsible for viral entry into the host cells
    • recognition of host cell receptor
    • initiation of entry by fusion or endocytosis
  • It is responsible for viral exit from the cells
    • budding
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Viral Capsid/Nucleocapsid

  • Virus building blocks, proteins, are assembled to form a tight 'shell' - capsid
  • Inside capsid nucleic acid genome lodges for protection
  • Capsid may take the form of a polyhedron (usually icosahedral) or it may be spiral (helical symmetry), or it may be more complex
  • Structure units are the smallest functional equivalent building units of the capsid
  • Individual proteins fit together like the pieces of a puzzle to form a building block called a capsomere
  • The capsid denotes the protein shell that encloses the nucleic acid. It is built of structure units
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Viral Genome, Tegument and Receptors

  • Viral genome:
    • Either RNA or DNA but can be single or double stranded
    • Circular, linear
    • Dictate viral genome expression - translation
  • Viral tegument (viral matrix)
    • A cluster of proteins that lines the space between the envelope and nucleocapsid of all herpes viruses
      • aid in viral DNA replication
      • evasion of the immune response (inhibition of signalling in the immune system and activation of interferons)
  • Receptors - Glycoproteins (virus encoded) on the surface of the envelope or protruding from the capsid - identify/bidn to receptor sites of host's membrane
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Virus Classification

  • Order, family, genus, strain/type, species
  • Families end in viridae, generaend in virus
  • Classification is based on:
    • Morphology - size, shape, enveloped?
    • Physiochemical properties - molecular mass, thermal and ionic stability
    • Genome - RNA/DNA
    • Macromolecules - protein composition and function
    • Antigenic properties
    • Biological properties - host range
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Genome Classification

  • Group 1 - Double stranded DNA
  • Group 2 - Single stranded DNA
  • Group 3 - Double stranded RNA
  • Group 4 and 5 - Single stranded RNA
  • Group 6 - Single stranded RNA
  • Group 7 - Double stranded DNA
  • Subviral agents: satellites, viroids spongiform encephalopathies
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Genome Classification: dsDNA viruses

  • Adenoviruses
    • Upper repiratory tract infections
  • Poxviruses
    • Variola - smallpox virus
    • Vaccinia - derived from cowpox virus, immunise against smallpox
  • Herpes viruses
    • Herpes simplex: 1 - cold sores, 2 - genital herpes
    • Cytomegalovirus - unborn baby during pregnancy, people who work with children, immunocompromised patients, organ transplant recipients
  • Papillomavirus - cervical cancer
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Genome Classification: ssDNA Viruses

  • Parvoviruses
    • Parvovirus B-19 - fifth disease - slapped child disease
  • Symptoms: low-grade fever, headache, cold-like symptoms, then a few days later bright red mild rash, most commonly on face (mainly cheeks)
  • Incubation time: 4-21 days
  • Transmission: respiratory secretions (saliva, mucus)
  • Problems: complications may arise during pregnancy and with immunocompromised patients
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Genome Classification: dsRNA Viruses

  • Rotaviruses
    • Group A - severe gastroenteritis in infants and young children
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Genome Classification: ssRNA Viruses

  • Togaviruses
    • Rubella - Congenital rubella infection (CRI) intrauterine rubella infection, miscarriage, stillbirth, abortion
  • Orthomyxoviruses
    • Influenza A, B and C
  • Rhinoviruses
    • Common cold (50%)
  • Paramyxoviruses
    • Mumps virus
  • Picornaviruses
    • Hepatitis A
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Genome Classification: Retroviruses

  • Retroviruses (Group 6: Single stranded RNA) - e.g. HIV
  • Hepadnaviruses (Group 7: Double stranded DNA) - e.g. hepatitis B
  • These are called reverse transcriptase viruses as there is an RNA intermediate before viral proteins can be manufactured
  • Group 7 can first transcribe their DNA into RNA, then transcribe it back to DNA using reverse transcriptase, before it is inserted into the host DNA
  • Reverse transcriptases are enzymes encoded in retroviruses viral genome. The enzyme responsible for transcription of the viral RNA to produce a dsDNA that can be inserted into the host genome
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Influenza Virus

Influenza Virus: Reassortment

  • Antigenic shift
    • Process by which two or more different strains of a virus, or strains of two or more different viruses, combine to form a new subtype having a mixture of the surface antigens of the two or more original strains
  • Antigenic drift
    • Small changes (mutations) in the virus that happen continually over time
    • Produces new virus strains that may not be recognised by the body's immune system
    • Difficult to immunise/vaccinate for long term protection
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Herpes Simplex Virus 1

Herpes Simplex Virus 1 - Latent Infection

  • Herpes Simplex Viruses persist in a quiescent but persistent form known as latent infection, notable in neural ganglia
  • Infection of epithelial cells in the mucousal surface
    • Productive replication, production of progeny virions and virions spread to infect additional epithelial cells
    • Virus enters innervating sensory neurons, and nucleocapsids are transported to the neuronal cell body
    • Viral DNA is released into the neuronal nucleus and circulatises
    • Circular viral DNA persists in the neuronal cell nucleus
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Herpes Simplex Virus 1 - Latent Infection

  • Reactivation in the neuronal cell body
    • Initiation of viral lytic gene expression
    • Newly formed capids are transported to the axonal termini
    • Infectious virus is released from the axon and infects epithelial cells, resulting in recurrent infection and virus shedding
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