Viruses

Question 1

Incubation period

Answer 1

Period between start of infection and onset of symptoms

Question 2

Basic reproduction number (R0)

• Definition
• Rates: avian flu, plague, ebola, influenza, SARS, Measles

Answer 2

The transmission rate of a disease, from one case.

• The number of new infections that arise from one person their infective period.
• Affected by type of infection, mode of transmission and location

Avian flu- 0.7
Plague- 1.3
Ebola=
Influenza= 1-3
SARS- 3
Measles= 16

Question 3

Size of viruses

Answer 3

20-400nm

Question 4

Structure of virus

Answer 4

Genetic material
- Nucleic acid= DNA or RNA

Protein coat

Envelope/ undeveloped

Question 5

Viral pathogenesis journey

Answer 5

1. Encounter
   - Virus encounters the host
2. Entry
3. Mulitplication
4. Spread
5. Damage

Question 6

Viral entry into the body

Answer 6

Skin

• Abrasions
• Bites
• Needle punctures

Alimentary tract
- Movement of GIT facilitates entry

Urogenital tract
- Abrasions facilitate entry

Eye

Question 7

Baltimore system for viral classification 1-7

• Definition
• Types

Answer 7

Classification of viruses based on the type of genome and method of replication.

1- dsDNA

2- ssDNA

3- dsRNA

4- positive ssRNA

5- negative ssRNA

6- positive RNA with DNA intermediate

7- gapped dsDNA

Question 8

Type 1 virus examples

Answer 8

dsDNA

• Adenovirus
• Herpes simplex virus
• PoxVirus

Question 9

Type 2 virus examples

Answer 9

ssDNA

- Parvovirus

Question 10

Type 3 virus examples

Answer 10

dsRNA

- Reovirus

Question 11

Type 4 virus examples

Answer 11

positive ssRNA

- Poliovirus

Question 12

Type 5 virus examples

Answer 12

negative ssRNA

- Influenza virus (orthomyoxvirus), rhabdoviruses

Question 13

Type 6 virus examples

Answer 13

positive RNA with DNA intermediate

- Retrovirus (HIV)

Question 14

Type 7 virus examples

Answer 14

Gapped dsDNA

- Hep B virus (Hepadnavirus)

Question 15

Viral spread and release mechanisms

Answer 15

Localised replication
- Within epithelium/ system

Dissemination/ systemic infection

• Haematogenous
• Neurological

Viral release

• Apical (i.e flu)
• Basolateral (i.e rabies), csn invade underlying spread.

Question 16

Haematogenous spread

- Entry into blood

Answer 16

Entry via

• Capillaries
• Endothelial replication
• Vector bite
• Lymphatic capillaries

Question 17

Viraemia

• Defintion
• Passive/ active
• Primary/ secondary

Answer 17

Presence of virus in blood

• Can be calculated as %
• Measures viral replication

Passive vs active

• Passive= introduction of virus into the blood stream
• Active= replication of virus in blood stream

Primary
- Spread to blood from primary site of infection

Secondary
- Further infection of other tissues from primary site of infection, via bloodstream

Question 18

CNS infection of virus

• Neurotrophic
• Neuroinvasive
• Neurovirulent

Answer 18

Neurotropic
- Infection of neural cells

Neuroinvasive
- Viral infection of CNS from peripheral site

Neurovirulent
- Virus that can cause disease of nervous tissue

Question 19

Determinants of viral tissue tropism

Answer 19

Presence of cell receptors on hosts/ virus allowing entry
- e.g HIV/ CD4

Cell proteases

Cellular proteins that regulate viral transcription

Question 20

Viral virulence

Answer 20

Determined by genes

• Ability to replicate
• Modification of host’s defence mechanisms (virokines/ viroceptors)
• Ability to spread
• Mechanisms to kill cells

Question 21

Viral mechanism of cell injury

Answer 21

Cytolytic
- Inhibit host protein/RNA synthesis= loss of membrane integrity= syncytium= apoptosis

Non cytolytic

• Lymphocyte mediated (CD4, CD8, B)
• Free radicals

Question 22

Examples of acute viral infections

Answer 22

Acute infection
- Non equilibrium between virus and host response

Rhinovirus

Rotavirus

Influenza virus

Question 23

Examples of persistent viral infections

Answer 23

Lymphocytic choriomeningitis virus

Question 24

Examples of latent, reactivating viral infections

Answer 24

Herpes simplex

Question 25

Examples of slow viral infections

Answer 25

Measles virus SSPE

HIV

Question 26

Latent infections

Answer 26

Seen in DNA or retroviruses

Viral genome is replicated along with host cell chromosome in cell growth

Examples

• HSV: fever, blisters, cold-sores
• Retrovirus: transformation into cancer

Question 27

Mechanisms of viruses evade the immune system (5)

Answer 27

Antigenic variation

• Influenza: can change its surface antigens (neuraminidase, hemagglutinin)
• HIV
• Rhinovirus

Inhibition of antigen processing

• Block transported associated with antigen processing: HSV
• Removed MCH-1 from endoplasmic reticulum: CMV

Production of cytokine receptor homologues

• IL-1, IFN-G: poxvirus
• Chemokine: CMV

Production of immunosuppressive cytokines
- IL-10: EBV

Infection of immunocompetent cells
- HIV