Viruses 1

Viruses 1

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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• Rotaviruses
  • Group A - severe gastroenteritis in infants and young children

• 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

• 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