How Viruses Cause Disease

Question 1

simply, what is a virus?

Answer 1

an infective, simple micro-organism, not capable of independent existence, therefore requiring a host cell to reproduce

Question 2

basic structure of a virus

Answer 2

DNA or RNA
capsid protein
lipoprotein envelope

Question 3

how are viruses classified?

Answer 3

• the type of nucleic acid they carry
  and
• the presence/absence of an envelope

Question 4

give the basic principles of the virus life cycle in human cells (5)

Answer 4

1. attachment
2. entry
3. replication
4. assembly
5. release

Question 5

describe the attachment phase for viruses.

Answer 5

• virus enters body
• randomly collides with host cell
• VIRAL TROPISM - determines if virus can attach and enter via appropriate receptors

Question 6

what are the different types of tropism?

Answer 6

host tropism
- the difference between human and dog

tissue tropism
- difference between lung and kidney

cellular tropism
- difference between the receptors on cell

Question 7

describe the entry phase for virus’.

Answer 7

enveloped viruses
- enter via membrane fusion (bind with receptors) or endocytosis

non-enveloped viruses
- enter via endocytosis or penetration

Question 8

what is endocytosis?

Answer 8

the substance is brought into the cell and internalised

Question 9

in normal human cells, how do cells replicate?

Answer 9

• proteins produced by transcription of DNA into mRNA
• mRNA translated into amino acids

Question 10

describe the replication phase of virus’

Answer 10

• virus removes capsid
• exposes genome
• viral mRNA mimics host mRNA
• host cell makes viral proteins
• viral proteins make virions, structural proteins, enzymes - proteases and ones needed for replication

Question 11

what does the replication process depend on?

Answer 11

RNA or DNA
single or double stranded

HAS TO BE VIRAL mRNA

Question 12

what virus can immediately use host cell processes for translation of their proteins and why?

Answer 12

hep C

• single stranded and their genome is already mRNA, 5’ to 3’

Question 13

how do other virus’ create mRNA in the replication phase?

Answer 13

• single stranded negative sense RNA virus
• uses RNA dependent RNA polymerase
• creates positive sense mRNA via complementary base pairing
• uses positive sense mRNA for translation

Question 14

describe the assembly phase of virus’

Answer 14

• all viral components transported to a site and put together, forming a bud

Question 15

describe the 3 ways in which a virus can be released.

Answer 15

lysis
- the cell splits apart, dies, virus bye

exocytosis
- host cells transport system releases virions at surface

budding
- virions enclose themselves in host cell membrane and leave

Question 16

describe 4 major mechanisms by which viruses cause disease in humans

Answer 16

disruptive cell function

inhibit host cell protein manufacture
- reduced cell function and cell death

forming inclusion bodies inside the cell
- accumulation of toxic proteins and metabolites that damage the cell

damage caused by the host immune response

Question 17

what are the possible routes of entry for virus into a human body?

Answer 17

• mucosal contact
• parental - injection
• respiratory droplet/aerosol
• faecal-oral
• direct contact
• vertical - mother to child

Question 18

what must there be in the entry phase, for a virus to establish effective entry?

Answer 18

• sufficient dosage of the virus
• host cell must be: acceptable, susceptible and permissive
• a host immune system which is inadequate or absent

Question 19

how can the host immune response damage and cause further disease?

Answer 19

may be excessive release of antibodies, interferons and pro inflam cytokines

Question 20

the extent of infection may be localised or systemic, expand on these.

Answer 20

localised
- shorter incubation period
- virus remains at the site of entry
- may have focal lesions and affect large areas of the same mucous membrane

generalised
- longer incubation period
- initial replication
- generalised spread via blood to lymph notes or via nerves
- virus can reach distant organs

Question 21

give examples of viral infections in the skin, respiratory tract, liver and CNS.

Answer 21

skin rashes
- varicella zoster

respiratory tract
- localised infection - influenza
- generalised infection - measles

liver
- hepatitis virus’ have tropism for hepatocytes in the liver

CNS
- rabies via nerves
- polio via bloodstream

Question 22

illness may be acute or chronic, describe both.

Answer 22

acute
- present quickly
- mild with spontaneous recovery OR rapid decline and death
- either easily transmitted or very infectious

chronic
- infection persists
- immune system cant fully control
- damages host cells over time
e.g. hep c and HIV

Question 23

what is latency? where is it established?

Answer 23

when the virus infects the cell but doesn’t replicate or make progeny

• it periodically replicates
  e.g. herpes simplex cold sores

established in the neurone of the dorsal root ganglia

Question 24

what is viral transformation?

Answer 24

some viruses have potential to induce malignant change by disrupting normal expression of host cell response

Question 25

SO, what are the clinical stages of viral infection?

Answer 25

1. incubation period - no symptoms
   - virus attaching
2. prodromal period
   - symptoms caused by early immune response
   - non-specific illness
3. specific-illness period
   - specific effects of virus on target cells
   - immune response immunologically attack
4. recovery period

Question 26

give examples of therapeutic interventions that counteract viral pathogenic mechanisms

Answer 26

• antiviral drugs - interrupt virus life cycle
• vaccination