Mechanisms of viral infection and pathogenesis

Question 1

What factors can affect the rate of infection?

Answer 1

• Immunodeficiency
• Number of sexual partners (eg hpv)
• age extremities (eg VZV)

Question 2

What are the two routes that virus may take after first infection?

Answer 2

• Replicate at the site of entry and spread

- Remain at the site of entry

Question 3

How do viruses evade the immune system after replicating at the site of entry?

Answer 3

• Virus penetrates the skin and replicates at point of entry
• Migrates down the neurones and stays there
• Reactivation can lead to the symptoms coming back

Question 4

When may we get a secondary infection?

Answer 4

• infection with a second organism
• Following treatment - candida albicans (thrush)
• Following an infection that compromises immunity - HIV or following RTIs

Question 5

What viruses are we likely to get reinfected by?

Answer 5

• Influenza

- Rhinovirus

Question 6

How do viruses get in to our body?

Answer 6

• Resp tract (influenza, rhinovirus)
• Faecal oral (norovirus, rotavirus)
• Blood (HIV, HepB,C)
• Cuts (HPV, molluscum)
• Sex (hpv, HSV, HIV)
• Animal bites (rabies)
• Insect bites (haemorrhagic fever)

Question 7

What determines the pathogenesis of a virus?

Answer 7

• Nature of the virus
• Site of entry
• Tissue tropism
• Cell damage caused
• Ability of immune response to clear the virus

Question 8

What is vertical transmission?

Answer 8

• Virus moves from mother to foetus via the placenta
• Foetus is unable to mount a protective immune response
• eg Rubella

Question 9

How does cytomegalovirus evade the immune response?

Answer 9

• Downregulates the MHC class I - hinders antigen presentation on cell surface
• Replicates in privileged sites and destroys leukocytes

Question 10

What are decoy particles?

Answer 10

• Hep B virus releases decoy particles to divert the attention of the immune system away from itself

Question 11

How does HPV cause cancer?

Answer 11

• Virus infects the dividing epithelial cells (basal layer), but has to stop their senescence
• HPV has circular dsDNA with early and late genes
• When it infects the cell and inserts its genome into the host DNA, it has to become linear
• Cuts at E2, allowing the upstream regulatory region to act, which causes the transcription of E6 and E7
• E6 inhibits p53, which
  would normally trigger apoptosis; also activates telomerase
• E7 binds to pRB, which means that E2F can keep the cycle going
• This means that the cells will keep growing -> cancer

Question 12

What is zika virus?

Answer 12

• Flaviviridae
• Enveloped, ssRNA +ve sense
• Transmited via mosquitos
• Causes microcephaly in foetus