Introduction to viruses

Question 1

classification of viruses

Answer 1

virus families can be classified according to:

• virion shape/symmetry
• presence/absence of envelope
• genome structure
• mode of replication

Question 2

virion

Answer 2

complete infective form of a virus OUTSIDE the host

Question 3

principles of viral culture

Answer 3

cells are inoculated with virus in vitro, the virus repicates within these cells.

Question 4

virus structure

Answer 4

• lipid envelope
• spike projections from envelope
• protein capsid inside lipid envelope
• nucleic acid inside protein capsid
• virion associted polymerase attached to DNA

Question 5

virus replication

Answer 5

1. virus binds membrane of host cell
2. virus is uncoated and the viral nucleic acid enters the cell
3. this nucleic acid is replicated to be incorporated into new virions
4. the viral nucleic acid is also transcribed into mRNA which then form proteins for the production of new virions.
5. the replicated viral nucleic acid and viral proteins then combine to form virions which bud off from host cell and mature.

Question 6

Host range

Answer 6

the range of organisms that a virus can infect; some viruses may only infect humans, some may infect animals/birds too.

Question 7

consequences of a viral infection

Answer 7

-Clearance of virus (with no, short or long term immunity)
Hepatitis C (no lasting immunity)
Measles (long term immunity)
- Chronic infection
HIV, hepatitis B, hepatitis C
- Latent infection 
- Transformation (long term infection with altered cellular gene expression)
Epstein-Barr Virus, Human Papillomavirus

Question 8

viral latency

Answer 8

following primary infection viruses will lie dormant in host cells. Viral genome will remain in cells but expression will be limited. Reactivation may occur.

Question 9

mechanisms by which viruses can cause cancers

Answer 9

• modulation of cell cycle
• modulation of apoptosis
• reactive oxygen species mediated damage

Question 10

modulation of cell cycle

Answer 10

drives cell proliferation

Question 11

modulation of apoptosis

Answer 11

prevents programmed cell death

Question 12

Reactive oxygen species mediated damage

Answer 12

persistent viral infections can cause inflammatory processes which lead to cancer via reactive oxygen species)

Question 13

3 ways in which viruses can be detected

Answer 13

• detecting whole organism (microscopy and culture)
• detecting part of virus (antigen, nucleic acid)
• immune response to a pathogen (antibodies)

Question 14

antiviral therapy

Answer 14

all antiviral agents are virustatic, none are virucidal.

Question 15

when is antiviral therapy used?

Answer 15

• prophyaxis (prevent infection)
• pre-emptive (before symptoms occur but when there is evidence of infection/replication detected)
• overt (plain) disease
• suppressive therapy

Question 16

prevention of viral infection

Answer 16

• Immunisation
• Prophylactic treatment post exposure
• Infection prevention & control measures
• Blood / tissue / organ screening
• Antenatal screening

Question 17

factors that allow a virus to be eradicated

Answer 17

• No animal reservoir or ability to amplify in the environment
• Clearly identifiable, with accurate diagnostic tool
• No chronic carrier state
• Efficient and practical intervention, e.g. vaccination
• Political / social support

Question 18

human virus infections of major importance

Answer 18

HIV
hepatitis (A,B,C,D and E)
Polio
influenza
measles
chickenpox
herpes simplex
Dengue
Yellow fever