introduction to viruses

Question 1

what is a virus

Answer 1

• Viruses are small (20-400nm diameter)
• Non cellular
• Obligate intracellular pathogens (can only replicate inside of host cell)
• Most viruses have a specific host range and only infect specific host cell types (tissue tropism) e.g. hepatitis only infects hepatic tissue
  -

Question 2

what is a virion

Answer 2

Virion - extracellular form of a virus
○ Exits outside host and facilitates transmission from one host cell to another
○ Contains nucleic acid genome surrounded by a protein coat and in some cases other layers of material (protein coat is sometimes made by the virus itself or is stolen from the host cell as it leaves)

Question 3

classification of viruses

Answer 3

- Order, Family, Genus, Species
Virus families can be classified according to: 
	- Virion shape/symmetry 
	- Presence/absence of an envelope 
	- Genome structure 
	- Mode of replication

Question 4

lipid envelopes in viruses

Answer 4

make the virus less robust, dont survive well outside of the body

Question 5

virus replication

Answer 5

Virion required for transmission form one host cell to another – (basically wrapping it in a protein coating: a capsid) The capsid is the infective form. You can also sometimes get a lipid coating derived from the host cell.

Question 6

transmission of viral infections

Answer 6

airborne, blood-borne, close-contact, droplet, faecal-oral, sexual, vertical, vector-borne, zoonotic

Question 7

host ranges

Answer 7

• Some viruses may only infect humans - these are the ones targeted for eradication e.g. smallpox, measles
• Some also infect other animals/birds
  ○ Transmission of a novel virus to humans
  ○ Coinfection of human and animal/bird strains in one organism may lead to recombination and generation of a new strain e.g. flu

Question 8

consequences of a viral infection

Answer 8

1. Clearance of virus (w/ no/short/long term immunity)
   ○ Hep C (no lasting immunity)
   ○ Measles (long term immunity)
   
   2. Chronic infection - most favourable from the viruses point of view
      ○ HIV, hep B, hep C
   3. Latent infection: herpes virus
   4. Transformation: long term infection w/ altered cellular gene expression
      ○ Epstein-Barr virus, human papillomavirus (HPV)
      Leads to malignancy

Question 9

viral latency

Answer 9

• Following 1y infection some viruses lay dormant in the cell
• The full viral genome is retained in the host cell but its expression is restricted, such that few viral antigen and no viral particles are produced - evades immune detection
• Reactivation of viral replication can occur and may or may not cause apparent disease, reactivation more likely to occur and more severe in immunocompromised
E.g. herpes simplex virus, varicella zoster virus

Question 10

viruses and cancer

Answer 10

• A number of viral infections can lead to cancer (often requires some other event ot also occur)
  
  • Mechanisms:
    ○ Modulation of cell cycle control (driving cell proliferation)
    ○ Modulation of apoptosis (prevention of programmed cell death)
    Reactive oxygen species mediated damage (some persistent viral infections can cause persistent inflammatory processes which lead to cancer via reactive oxygen species)

Question 11

examples of viruses which are linked to cancer

Answer 11

• EBV: Burkitt’s lymphoma, Hodgkin’s lymphoma, nasopharyngeal carcinoma, post-transplant lymphoproliferative disease (PTLD)
• Human herpes virus 8: Kaposi’s sarcoma, primary effusion lymphoma, Castleman’s disease
• Human T-cell Lymphotropic Virus (HTLV): Adult T-cell leukaemia/lymphoma
• Human Papillomavirus (HPV): cervical, anal, oropharyngeal cancers
  Hepatitis B and Hepatitis C: hepatocellular carcinoma

Question 12

viral detection methods

Answer 12

• whole organism
• part of an organism
• immune response to pathogen

Question 13

antiviral therapy

Answer 13

• All antiviral agents are virustatic, none are virucidal
• As viruses utilise host cell enzymes in order to replicate, there are limited viral proteins that are potential targets for antiviral drugs
• Toxicity to the host cell isn’t uncommon –> side effects
• Only used in a minority of viral infections due to the side effects

Question 14

prevention of viral infection

Answer 14

• Immunisation
  
  • Vaccination
  • Passive immunisation with immunoglobulin
• Prophylactic treatment post exposure
• Infection prevention and control measures (very important)
  
  • Isolation of symptomatic patients
  • PPE
  • Safe use and disposal of sharps
• Blood/tissue/organ screening
• Antenatal screening

Question 15

eradication of viruses

Answer 15

• Viruses with the following properties can be potentially eradicated:
  
  • No animal reservoir or ability to amplify in the environment i.e. it only affects humans
  • Clearly identifiable with accurate diagnostic tool
  • No chronic carrier state
  • Efficient and practical intervention e.g. vaccination
  • Political/social support
• E.g. small pox (eradicated 1979)
  Measles and polio - could be eradicated

Question 16

what are the 3 basic structures of viruses

Answer 16

○ Icosahedral
○ Helical
○ Complex

Question 17

icosahedral

Answer 17

20 faces, each an equilateral triangle

Question 18

helical

Answer 18

protein binds around DNA/RNA in a helical fashion

Question 19

complex

Answer 19

neither icosahedral or helical

Question 20

use of antivirals

Answer 20

• Prophylaxis
• Pre-emptive therapy
• Overt disease

Question 21

prophylaxis and antivirals

Answer 21

(to prevent infection) e.g. in transplanted patients

Question 22

pre-emptive therapy with antivirals

Answer 22

(when evidence of infection/replication detected but before symptoms are apparent) e.g. in sub sets of patients where they’ve had an exposure rather than working out whether they are immune or not

Question 23

antivirals and overt disease

Answer 23

e.g. flu, HIV

Question 24

suppressive therapy and antivirals

Answer 24

(to keep viral replication below the rate that causes tissue damage in an asymptomatic infected patient) e.g. Hep B