10. virology

Question 1

what are the routes by which viruses can spread from one person to another

entry into new host

Answer 1

skin- resistant if intact

• Mucosal surfaces – oro-, naso-pharynx, conjunctivae, genital tract
• Respiratory tract
• Gastrointestinal tract
• Placenta

Question 2

four possible outcomes of virus infection

Answer 2

1. cell death: Cytolytic or cytocidal infection (kill host cell) (rhinovirus)
2. chronic infection: continuous viral replication but survival (hep B)
3. Latency: no virus replication, can be reactivated, always infected , primary and secondary infections (herpes)
4. transformation: immortalisation of cell (Epstein Barr)

viruses can cause cancer

Question 3

define cytolytic infection

Answer 3

kill host cell

Question 4

what is the cytopathic effect

Answer 4

structural changes in host cell caused by viral invasion

Question 5

define viral latency

Question 7

spread of virus within host

Answer 7

• Local ie cell-to-cell
• Lymphatics
• Bloodstream – viraemia (provides access to placenta)
• Axonal pathways

Question 8

viral shedding

Answer 8

• Skin – vesicles
• Respiratory tract – droplets
• Gastrointestinal tract – saliva, faeces
• Urogenital tract – urine, semen, female genital tract secretions
• Blood – BBV incl HBV, HCV, HIV
• Breast milk

Question 9

what are viruses in terms of metabolism

Answer 9

obligate intracellular parasites

metabolically inert- need to infect host to be active

Question 10

viral components: nucleic acids

Answer 10

Dna: ds, ss, circular or linear

rna: ds, ss (+ve/-ve polarity), linear or segmented

Question 11

viral components: proteins

Answer 11

• Capsid formation
  
  • Symmetry – icosahedral or helical
• Attachment
  
  • Specific interaction between viral ligand and cellular receptor
  • Determines viral tropism: specificity of a virus for a particular host tissue, determined by interaction of surface with receptors
• Enzymes
  
  • Mostly concerned with macromolecular synthesis
• Interference with cell function
  
  • E.g. to stop apoptosis, to avoid immune recognition

Question 12

Viral components: Lipid envelope

Answer 12

• Derived from host cell membranes
• Must contain external attachment proteins
• Stripping of envelope leads to loss of infectivity
• Enveloped viruses generally more fragile

Question 13

virus classification

what is the baltimore classification

Answer 13

• Nature of genetic material
• Nature of capsid
• Enveloped or not?
• Size and shape
• Families, genera, strains

groups viruses into families, depending on their type of genome (DNA, RNA, single-stranded (ss), double-stranded (ds), etc..) and their method of replication.

Question 14

viral replication

Answer 14

1. attachment: receptor/ ligand
2. entry: endocytosis/ fusion
3. uncoating
4. macromolecular synthesis
5. assembly
6. release: budding/ cell lysis

Question 15

define virion

Answer 15

mature virus particle

nucleic acid, protein coat (capsid), lipid (envelope)

Question 16

postive and negative polarity of RNA

Answer 16

classification of ss RNA

+ve can be used directly as a sense strand (coding)- can serve as mRNA

-ve must be transcripted first then used as sense

Question 17

define viral tropism

Answer 17

infection specifity of viral infection to particular host tissues

explains why some viruses can only infect a limited range of host organisms

Question 18

reverse transcriptase

Answer 18

an enzyme used to generate complementary DNA (cDNA) from an RNA template, a process termed reverse transcription.

Question 19

retrovirus

Answer 19

type of RNA virus that inserts a copy of its genome into the DNA of a host cell that it invades, thus changing the genome of that cell.