Virus

Question 1

Infection mechanism of a virus

Answer 1

Virus invades mucosal barrier in the body

1. Implantation:
   Proteins of the outer membrane of the virus binds to a specific receptor of the host’s cell.
2. Penetration:
   Activation of the receptor allows virus to enter the host’s cell through endocytosis.
3. Uncoating
   If the virus contains a capsule, this is shed. The genetic material of the virus is released to into the host’s cell.
4. Replication
   Virus forms it’s viral mRNA using either it’s own or the host’s RNA polymerase.
   This forms proteins which allow the formation new viruses and also new genetic material.
5. Release:
   New viruses are released in an envelope made from the host’s cell material.

Question 2

General structure of a virus.

Answer 2

Genetic material, DNA or RNA, enclosed in a capsid.

DNA and RNA can be double or single stranded. Can also be positive or negative.

Nucleocapsid, composed of nucleic acid and capsid, is surrounded in an enveloped derived from its host’s cell.

Question 3

Positive/ negative genetic material.

Answer 3

Positive- when the nucleotide sequence of DNA or RNA has the same sequence as mRNA.

Negative- when the sequence is complementary to mRNA. Therefore it has to be transcribed to mRNA first before creating proteins.

Question 4

Virulence

Answer 4

The severity of disease caused by a virus. This can be affected by:

Cell tropism
The response from the host
The efficiency of its replication
Its interaction with the host

Question 5

Cell tropism

Answer 5

These are the features of a virus that allows it to target specific tissue- adaptations that allow increased virulence.

Viruses can only affect the cells that have receptors that bind to its surface proteins- Susceptibility.

Host cells must also have the correct enzymes to allow the production of viral proteins- Permissivity.

Question 6

Susceptibility

Answer 6

Virus requires its host cell to have the correct receptors that allows viral implantation.

Question 7

Permissivity

Answer 7

Host cells must obtain the correct enzymes and transcription factors to allow the successful production of viral proteins.

Question 8

The outcome of viral infections

Answer 8

Causes cells to be dysfunctional; affects metabolic and regulatory processes.

Can release toxic productions which cause:
Cell necrosis
Further viral replication.

Cell lysis

Question 9

Latency

Answer 9

When a virus is present in a host but is not active.

Does not replicate genetic material until triggered by a specific factor.

Includes:
HSV (herpes simplex virus type 1/2) reactivation- causes fever, blisters.
Retrovirus can cause cell to be cancerous.

Question 10

Different methods of viral transmission

Answer 10

Through blood into organ systems

Through neurones into organs in the CNS

Vector (yellow fever)

Droplet (Influenza)

Transplacental (Zika)

Oral contact (Hep A)

Sexually transmitted (HIV)

Question 11

Exogenous infection

Answer 11

When the virus from outside the host infects the host by breaking through physical barrier..

Question 12

Endogenous infection

Answer 12

When there is a re-activation of a latent virus present in the host.

Usually occurs when the immune system has been suppressed.

Includes
Shingles virus (VZN)
HSV
CMV (Cytomegalovirus)

Question 13

Acute vs Chronic viral infections.

Answer 13

Acute: Immediate onset of infection. When there is no equilibrium between the virus and the host cells. There is a constant change in both factors.

Chronic: Persistent infection. When there is an equilibrium between the viral and host cells.

Question 14

Different methods of showing that a virus is the causative agent. (5)

Answer 14

• Antigen detection
• Serology (antibody response)
• Using cell cultures
• Detection of the viral nucleic acid
• Microscopy

Question 15

Using microscopy to test for viruses

Answer 15

Allows viruses to be clearly seen but:

Electron microscopes are very expensive
Require expertise
Requires a high viral copy in sample.

Question 16

Using cell cultures to test for viruses

Answer 16

Viruses are grown in different cell lines, not all viruses can be grown in a cell culture.

A culture of inoculated and uninoculated cells are grown and compared under a microscope.
The virus is recognised by its cytopathic effects.

HOWEVER:
The method is slow for diagnosis and has limited use.

Question 17

Using immunofluorescence to detect viruses.

Answer 17

Detects the antigen on the virus:

Specific monoclonal antibodies are created for a known antigen on the virus.

Antibody is attached to a fluorescent marker and placed in sample.

A fluorescent microscope is used to detect the antigen present.

HOWEVER:
This method is limited to only a certain number of antigens.

Requires expert personnel to carry out.

Question 18

Serology

Answer 18

Detects the presence of antibodies in the serum.

ELISA test:
A substrate that changes colour when cleaved by the enzyme attached to the second antibody.

2nd antibody is attached to an enzymes. If patient’s serum contains the antibodies, colour change occurs.

Question 19

Viral nucleic acid detection.

Answer 19

Uses PCR to amplify viral sample.

PCR uses a primer that has a specific known sequence of a virus’s genetic material.

Question 20

Methods of viral prevention

Answer 20

General measures: gloves, masks

Specific measures:
Vaccines
Chemoprophylaxis
Immunoprophylaxis

Question 21

Methods of treatment of viral infections.

Answer 21

General: antipyretics
Treats fevers by decreasing body temperature. Less hospitable for viruses.

Specific: antiviral medication.

Question 22

Antiviral drug for HIV

Answer 22

HAART, Highly active antiretroviral therapy, does not cure HIV. Only reduces viral load.

Highly active antiretroviral therapy.

Question 23

Antiviral drugs for Influenza

Answer 23

Tamiflu and others