An introduction to Viruses

Question 1

Basic classification of viruses

Answer 1

Family; genus; species.

Classified according to shape/symmetry, presence/absence of envelope, genome structure and mode of replication.

Question 2

Outline the principles of viral culture

Answer 2

Used to recognise the whole organism. A sample of body fluid/tissue is added to certain cells used to grow a virus. If no virus infects the cells, the culture is negative and the opposite would be positive. It may take several weeks to show results.

Question 3

HIV

Answer 3

Human immunodeficiency virus type 1 is a global infection. Type 2 is largely restricted to western Africa or contact with. It is a retrovirus and both cause AIDS.

Question 4

Hepatitis B

Answer 4

Acute infection often sub-clinical and chronic infection is associated with increased risk of cirrhosis, hepatoma and extra-hepatic manifestations.

Question 5

Hepatitis C

Answer 5

Acute infection is often sub-clinical but chronic infection follows in around 80%. This is associated with cirrhosis and hepatoma.

Question 6

Hepatitis A

Answer 6

Sub-clinical in children, prolonged symptoms in adults but chronic infection or death are rare. No carrier state.

Question 7

Influenza

Answer 7

A and B cause respiratory illness, C causes a mild respiratory infection.

Question 8

Structure of viruses

Answer 8

Icosahedral; 20 faces, each an equilateral triangle.
or
Helical; protein binds around DNA/RNA in a helical fashion.
or
Complex
See diagram in notes, Book 1, pg 5

Question 9

Virus replication

Answer 9

Viruses only replicate inside the host cell (obligate intracellular pathogens). Viral replication involves six steps: attachment, penetration, uncoating, replication, assembly, and release. See diagram bk 1, pg5.

Question 10

Host range

Answer 10

Most viruses have a specific host range and only infect specific host cell types. Some will only infect humans, others infect animals/birds. There may be transmission f a novel (animal) virus to humans. Coinfection of human and animal/bird strains in one organism may lead to recombination and generation of a new strain.

Question 11

Consequences of viral infection

Answer 11

1. Clearance of virus (with possible immunity)
2. Chronic infection
3. Latent infection
4. Transformation; long term infection with altered cellular gene expression.

Question 12

Viral latency

Answer 12

Following a primary infection some viruses lay dormant in the cell. The full genome is retained in the host cell, but its expression is restricted such that few viral antigens and no viral particles are produced. Reactivation of viral replication may cause apparent disease which is more likely and more severe in immunocompromised.

Question 13

Describe the link between viruses and cancer and explain the mechanism through which this results.

Answer 13

A number of viral infections can lead to cancer through modulation of cell cycle control (drives cell proliferation), modulation of apoptosis (prevention), reactive O2 species mediated damage (persistent viral infection leads to persistent inflammation leading to cancer via reactive O2 species).

Question 14

Indications for and principles of anti-viral therapy.

Answer 14

All antiviral drugs are virustatic (stop replication) which target limited viral proteins. May be used for prophylaxis, preemptive therapy, overt disease or suppressive therapy.

Question 15

Describe how to prevent viral infection and explain how viruses can be eradicated.

Answer 15

1. Immunisation/vaccination
2. Prophylactic treatment post exposure
3. Infection prevention and control measures (isolation, sharps disposal, PPE)
4. Blood/tissue/organ screening
5. Antenatal screening
   The potential for eradication is dependant upon the following factors;
   - no animal reservoir or ability to amplify in the environment.
   - Clearly identifiable, accurate diagnostics.
   - No chronic carrier state
   - Efficient and practical intervention e.g. vaccination
   - political/social support